a side-by-side reference sheet

sheet one: version | grammar and execution | variables and expressions | arithmetic and logic | strings | regexes | dates and time | arrays | dictionaries | functions | execution control | exceptions | threads

sheet two: streams | asynchronous events | files | file formats | directories | processes and environment | option parsing | libraries and namespaces | objects | reflection | net and web | databases | unit tests | debugging

sheet two: streams | asynchronous events | files | directories | processes and environment | option parsing | libraries and namespaces | objects | inheritance and polymorphism | reflection | net and web | gui | databases | unit tests | logging | debugging

Version

version used

The versions used for testing code in the reference sheet.

show version

How to get the version.

php:

The function phpversion() will return the version number as a string.

python:

The following function will return the version number as a string:

import platform

platform.python_version()

ruby:

Also available in the global constant RUBY_VERSION.

implicit prologue

Code which examples in the sheet assume to have already been executed.

javascript:

underscore.js adds some convenience functions as attributes of an object which is normally stored in the underscore _ variable. E.g.:

_.map([1, 2, 3], function(n){ return n * n; });

cdnjs hosts underscore.js and other JavaScript libraries for situations where it is inconvenient to have the webserver host the libraries.

When using underscore.js with the Node REPL, there is a conflict, since the Node REPL uses the underscore _ variable to store the result of the last evaluation.

$ npm install underscore

$ node

> var us = require('underscore'); _

> us.keys({"one": 1, "two": 2});
[ 'one', 'two' ]

php:

The mbstring package adds UTF-8 aware string functions with mb_ prefixes.

python:

We assume that os, re, and sys are always imported.

Grammar and Execution

interpreter

The customary name of the interpreter and how to invoke it.

php:

php -f will only execute portions of the source file within a <?php php code ?> tag as php code. Portions of the source file outside of such tags is not treated as executable code and is echoed to standard out.

If short tags are enabled, then php code can also be placed inside <? php code ?> and <?= php code ?> tags.

<?= php code ?> is identical to <?php echo php code ?>.

repl

The customary name of the repl.

php:

The php -a REPL does not save or display the result of an expression.

python:

The python repl saves the result of the last statement in _.

ruby:

irb saves the result of the last statement in _.

command line program

How to pass the code to be executed to the interpreter as a command line argument.

block delimiters

How blocks are delimited.

python:

Python blocks begin with a line that ends in a colon. The block ends with the first line that is not indented further than the initial line. Python raises an IndentationError if the statements in the block that are not in a nested block are not all indented the same. Using tabs in Python source code is unrecommended and many editors replace them automatically with spaces. If the Python interpreter encounters a tab, it is treated as 8 spaces.

The python repl switches from a >>> prompt to a … prompt inside a block. A blank line terminates the block.

Colons are also used to separate keys from values in dictionary literals and in sequence slice notation.

ruby:

Curly brackets {} delimit blocks. A matched curly bracket pair can be replaced by the do and end keywords. By convention curly brackets are used for one line blocks.

The end keyword also terminates blocks started by def, class, or module.

Curly brackets are also used for hash literals, and the #{ } notation is used to interpolate expressions into strings.

statement separator

How the parser determines the end of a statement.

php:

Inside braces statements must be terminated by a semicolon. The following causes a parse error:

<? if (true) { echo "true" } ?>

The last statement inside <?= ?> or <? ?> tags does not need to be semicolon terminated, however. The following code is legal:

<?= $a = 1 ?>
<? echo $a ?>

python:

Newline does not terminate a statement when:

• inside parens
• inside list [] or dictionary {} literals

Python single quote '' and double quote "" strings cannot contain newlines except as the two character escaped form \n. Putting a newline in these strings results in a syntax error. There is however a multi-line string literal which starts and ends with three single quotes ''' or three double quotes: """.

A newline that would normally terminate a statement can be escaped with a backslash.

ruby:

Newline does not terminate a statement when:

• inside single quotes '', double quotes "", backticks ``, or parens ()
• after an operator such as + or , that expects another argument

Ruby permits newlines in array [] or hash literals, but only after a comma , or associator =>. Putting a newline before the comma or associator results in a syntax error.

A newline that would normally terminate a statement can be escaped with a backslash.

source code encoding

How to identify the character encoding for a source code file.

Setting the source code encoding makes it possible to safely use non-ASCII characters in string literals and regular expression literals.

end-of-line comment

How to create a comment that ends at the next newline.

multiple line comment

How to comment out multiple lines.

python:

The triple single quote ''' and triple double quote """ syntax is a syntax for string literals.

Variables and Expressions

local variable

How to declare variables which are local to the scope defining region which immediately contain them.

php:

Variables do not need to be declared and there is no syntax for declaring a local variable. If a variable with no previous reference is accessed, its value is NULL.

python:

A variable is created by assignment if one does not already exist. If the variable is inside a function or method, then its scope is the body of the function or method. Otherwise it is a global.

ruby:

Variables are created by assignment. If the variable does not have a dollar sign ($) or ampersand (@) as its first character then its scope is scope defining region which most immediately contains it.

A lower case name can refer to a local variable or method. If both are defined, the local variable takes precedence. To invoke the method make the receiver explicit: e.g. self.name. However, outside of class and modules local variables hide functions because functions are private methods in the class Object. Assignment to name will create a local variable if one with that name does not exist, even if there is a method name.

file scope variable

How to define a variable with scope bound by the source file.

global variable

How to declare and access a variable with global scope.

php:

A variable is global if it is used at the top level (i.e. outside any function definition) or if it is declared inside a function with the global keyword. A function must use the global keyword to access the global variable.

python:

A variable is global if it is defined at the top level of a file (i.e. outside any function definition). Although the variable is global, it must be imported individually or be prefixed with the module name prefix to be accessed from another file. To be accessed from inside a function or method it must be declared with the global keyword.

ruby:

A variable is global if it starts with a dollar sign: $.

constant

How to declare a constant.

php:

A constant can be declared inside a class:

class Math {
  const pi = 3.14;
}

Refer to a class constant like this:

Math::pi

ruby:

Capitalized variables contain constants and class/module names. By convention, constants are all caps and class/module names are camel case. The ruby interpreter does not prevent modification of constants, it only gives a warning. Capitalized variables are globally visible, but a full or relative namespace name must be used to reach them: e.g. Math::PI.

assignment

How to assign a value to a variable.

python:

If the variable on the left has not previously been defined in the current scope, then it is created. This may hide a variable in a containing scope.

Assignment does not return a value and cannot be used in an expression. Thus, assignment cannot be used in a conditional test, removing the possibility of using assignment (=) when an equality test (==) was intended. Assignments can nevertheless be chained to assign a value to multiple variables:

a = b = 3

ruby:

Assignment operators have right precedence and evaluate to the right argument, so they can be chained. If the variable on the left does not exist, then it is created.

parallel assignment

How to assign values to variables in parallel.

python:

The r-value can be a list or tuple:

nums = [1, 2, 3]
a, b, c = nums
more_nums = (6, 7, 8)
d, e, f = more_nums

Nested sequences of expression can be assigned to a nested sequences of l-values, provided the nesting matches. This assignment will set a to 1, b to 2, and c to 3:

(a,[b,c]) = [1,(2,3)]

This assignment will raise a TypeError:

(a,(b,c)) = ((1,2),3)

In Python 3 the splat operator * can be used to collect the remaining right side elements in a list:

x, y, *z = 1, 2        # assigns [] to z
x, y, *z = 1, 2, 3     # assigns [3] to z
x, y, *z = 1, 2, 3, 4  # assigns [3, 4] to z

ruby:

The r-value can be an array:

nums = [1, 2, 3]
a,b,c = nums

swap

How to swap the values held by two variables.

compound assignment

Compound assignment operators mutate a variable, setting it to the value of an operation which takes the previous value of the variable as an argument.

If <OP> is a binary operator and the language has the compound assignment operator <OP>=, then the following are equivalent:

x <OP>= y
x = x <OP> y

The compound assignment operators are displayed in this order:

First row: arithmetic operator assignment: addition, subtraction, multiplication, (float) division, integer division, modulus, and exponentiation.
Second row: string concatenation assignment and string replication assignment
Third row: logical operator assignment: and, or, xor
Fourth row: bit operator assignment: left shift, right shift, and, or, xor.

python:

Python compound assignment operators do not return a value and hence cannot be used in expressions.

increment and decrement

The C-style increment and decrement operators can be used to increment or decrement values. They return values and thus can be used in expressions. The prefix versions return the value in the variable after mutation, and the postfix version return the value before mutation.

Incrementing a value two or more times in an expression makes the order of evaluation significant:

x = 1;
foo(++x, ++x); // foo(2, 3) or foo(3, 2)?

x = 1;
y = ++x/++x;  // y = 2/3 or y = 3/2?

Python avoids the problem by not having an in-expression increment or decrement.

Ruby mostly avoids the problem by providing a non-mutating increment and decrement. However, here is a Ruby expression which is dependent on order of evaluation:

x = 1
y = (x += 1)/(x += 1)

php:

The increment and decrement operators also work on strings. There are postfix versions of these operators which evaluate to the value before mutation:

$x = 1;
$x++;
$x--;

ruby:

The Integer class defines succ, pred, and next, which is a synonym for succ.

The String class defines succ, succ!, next, and next!. succ! and next! mutate the string.

null

The null literal.

null test

How to test if a variable contains null.

php:

$v == NULL does not imply that $v is NULL, since any comparison between NULL and a falsehood will return true. In particular, the following comparisons are true:

$v = NULL;
if ($v == NULL) { echo "true"; }

$v = 0;
if ($v == NULL) { echo "sadly true"; }

$v = '';
if ($v == NULL) { echo "sadly true"; }

undefined variable

The result of attempting to access an undefined variable.

python:

Because a class can implement an eq method to change the implementation of v == None, the expression can be True when v is not None.

php:

PHP does not provide the programmer with a mechanism to distinguish an undefined variable from a variable which has been set to NULL.

A test showing that isset is the logical negation of is_null.

python:

How to test if a variable is defined:

not_defined = False
try: v
except NameError:
  not_defined = True

ruby:

How to test if a variable is defined:

! defined?(v)

conditional expression

How to write a conditional expression. A ternary operator is an operator which takes three arguments. Since

condition ? true value : false value

is the only ternary operator in C, it is unambiguous to refer to it as the ternary operator.

python:

The Python conditional expression comes from Algol.

ruby:

The Ruby if statement is also an expression:

x = if x > 0
  x
else
  -x
end

Arithmetic and Logic

true and false

Literals for the booleans.

These are the return values of the relational operators.

php:

Any identifier which matches TRUE case-insensitive can be used for the TRUE boolean. Similarly for FALSE.

In general, PHP variable names are case-sensitive, but function names are case-insensitive.

When converted to a string for display purposes, TRUE renders as "1" and FALSE as "". The equality tests TRUE == 1 and FALSE == "" evaluate as TRUE but the equality tests TRUE === 1 and FALSE === "" evaluate as FALSE.

falsehoods

Values which behave like the false boolean in a conditional context.

Examples of conditional contexts are the conditional clause of an if statement and the test of a while loop.

python:

Whether a object evaluates to True or False in a boolean context can be customized by implementing a __nonzero__ (Python 2) or __bool__ (Python 3) instance method for the class.

logical operators

Logical and, or, and not.

php, ruby:

&& and || have higher precedence than assignment, compound assignment, and the ternary operator (?:), which have higher precedence than and and or.

relational operators

Equality, inequality, greater than, less than, greater than or equal, less than or equal.

php:

Most of the relational operators will convert a string to a number if the other operand is a number. Thus 0 == "0" is true. The operators === and !== do not perform this conversion, so 0 === "0" is false.

python:

Relational operators can be chained. The following expressions evaluate to true:

1 < 2 < 3
1 == 1 != 2

In general if A_i are expressions and op_i are relational operators, then

    A₁ op₁ A₂ op₂ A₃ … A_n op_n A_n+1

is true if and only if each of the following is true

    A₁ op₁ A₂
    A₂ op₂ A₃
    …
    A_n op_n A_n+1

min and max

How to get the min and max.

arithmetic operators

The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation.

integer division

How to get the integer quotient of two integers.

divmod

How to get the quotient and remainder with single function call.

integer division by zero

What happens when an integer is divided by zero.

float division

How to perform floating point division, even if the operands might be integers.

float division by zero

What happens when a float is divided by zero.

power

How to get the value of a number raised to a power.

sqrt

The square root function.

sqrt -1

The result of taking the square root of negative one.

transcendental functions

Some mathematical functions. Trigonometric functions are in radians unless otherwise noted. Logarithms are natural unless otherwise noted.

python:

Python also has math.log10. To compute the log of x for base b, use:

math.log(x)/math.log(b)

ruby:

Ruby also has Math.log2, Math.log10. To compute the log of x for base b, use

Math.log(x)/Math.log(b)

transcendental constants

Constants for π and Euler's constant.

float truncation

How to truncate a float to the nearest integer towards zero; how to round a float to the nearest integer; how to find the nearest integer above a float; how to find the nearest integer below a float; how to take the absolute value.

absolute value

How to get the absolute value of a number.

integer overflow

What happens when the largest representable integer is exceeded.

float overflow

What happens when the largest representable float is exceeded.

rational numbers

How to create rational numbers and get the numerator and denominator.

ruby:

Require the library mathn and integer division will yield rationals instead of truncated integers.

complex numbers

python:

Most of the functions in math have analogues in cmath which will work correctly on complex numbers.

random integer, uniform float, normal float

How to generate a random integer between 0 and 99, include, float between zero and one in a uniform distribution, or a float in a normal distribution with mean zero and standard deviation one.

set random seed, get and restore seed

How to set the random seed; how to get the current random seed and later restore it.

All the languages in the sheet set the seed automatically to a value that is difficult to predict. The Ruby MRI interpreter uses the current time and process ID, for example. As a result there is usually no need to set the seed.

Setting the seed to a hardcoded value yields a random but repeatable sequence of numbers. This can be used to ensure that unit tests which cover code using random numbers doesn't intermittently fail.

The seed is global state. If multiple functions are generating random numbers then saving and restoring the seed may be necessary to produce a repeatable sequence.

bit operators

The bit operators for left shift, right shift, and, inclusive or, exclusive or, and negation.

binary, octal, and hex literals

Binary, octal, and hex integer literals

radix

How to convert integers to strings of digits of a given base. How to convert such strings into integers.

python

Python has the functions bin, oct, and hex which take an integer and return a string encoding the integer in base 2, 8, and 16.

bin(42)
oct(42)
hex(42)

Strings

string type

The type for a string of Unicode characters.

php:

PHP assumes all strings have single byte characters.

python:

In Python 2.7 the str type assumes single byte characters. A separate unicode type is available for working with Unicode strings.

In Python 3 the str type supports multibtye characters and the unicode type has been removed.

There is a mutable bytearray type and an immutable bytes type for working with sequences of bytes.

ruby:

The String type supports multibtye characters. All strings have an explicit Encoding.

string literal

The syntax for string literals.

python:

String literals may have a u prefix

u'lorem ipsum'
u"lorem ipsum"
u'''lorem
ipsum'''
u"""lorem
ipsum"""

In Python 3, these are identical to literals without the u prefix.

In Python 2, these create unicode strings instead of str strings. Since the Python 2 unicode type corresponds to the Python 3 str type, portable code will use the u prefix.

ruby:

How to specify custom delimiters for single and double quoted strings. These can be used to avoid backslash escaping. If the left delimiter is (, [, or { the right delimiter must be ), ], or }, respectively.

s1 = %q(lorem ipsum)
s2 = %Q(#{s1} dolor sit amet)

newline in literal

Whether newlines are permitted in string literals.

python:

Newlines are not permitted in single quote and double quote string literals. A string can continue onto the following line if the last character on the line is a backslash. In this case, neither the backslash nor the newline are taken to be part of the string.

Triple quote literals, which are string literals terminated by three single quotes or three double quotes, can contain newlines:

'''This is
two lines'''

"""This is also
two lines"""

literal escapes

Backslash escape sequences for inserting special characters into string literals.

python:

When string literals have an r or R prefix there are no backslash escape sequences and any backslashes thus appear in the created string. The delimiter can be inserted into a string if it is preceded by a backslash, but the backslash is also inserted. It is thus not possible to create a string with an r or R prefix that ends in a backslash. The r and R prefixes can be used with single or double quotes:

r'C:\Documents and Settings\Admin'
r"C:\Windows\System32"

The \uhhhh escapes are also available inside Python 2 Unicode literals. Unicode literals have a u prefiix:

u'lambda: \u03bb'

This syntax is also available in Python 3.3, but not Python 3.2. In Python 3.3 it creates a string of type str which has the same features as the unicode type of Python 2.7.

here document

Here documents are strings terminated by a custom identifier. They perform variable substitution and honor the same backslash escapes as double quoted strings.

python:

Triple quotes honor the same backslash escape sequences as regular quotes, so triple quotes can otherwise be used like here documents:

s = '''here document
there computer
'''

ruby:

Put the customer identifier in single quotes to prevent variable interpolation and backslash escape interpretation:

s = <<'EOF'
Ruby code uses #{var} type syntax
to interpolate variables into strings.
EOF

variable interpolation

How to interpolate variables into strings.

python:

The f'1 + 1 = {1 + 1}' and f"1 + 1 = {1 + 1}" literals, which support variable interpolation and expression interpolation, are new in Python 3.6.

str.format will take named or positional parameters. When used with named parameters str.format can mimic the variable interpolation feature of the other languages.

A selection of variables in scope can be passed explicitly:

count = 3
item = 'ball'
print('{count} {item}s'.format(
  count=count,
  item=item))

Python 3 has format_map which accepts a dict as an argument:

count = 3
item = 'ball'
print('{count} {item}s'.format_map(locals()))

expression interpolation

How to interpolate the result of evaluating an expression into a string.

format string

How to create a string using a printf style format.

python:

The % operator will interpolate arguments into printf-style format strings.

The str.format with positional parameters provides an alternative format using curly braces {0}, {1}, … for replacement fields.

The curly braces are escaped by doubling:

'to insert parameter {0} into a format, use {{{0}}}'.format(3)

If the replacement fields appear in sequential order and aren't repeated, the numbers can be omitted:

'lorem {} {} {}'.format('ipsum', 13, 3.7)

are strings mutable?

Are strings mutable?

copy string

How to copy a string such that changes to the original do not modify the copy.

concatenate

The string concatenation operator.

replicate

The string replication operator.

translate case

How to put a string into all caps or all lower case letters.

capitalize

How to capitalize a string and the words in a string.

The examples lowercase non-initial letters.

php:

How to define a UTF-8 aware version of ucfirst. This version also puts the rest of the string in lowercase:

function mb_ucfirst($string, $encoding = "UTF-8")
{
    $strlen = mb_strlen($string, $encoding);
    $firstChar = mb_substr($string, 0, 1, $encoding);
    $then = mb_substr(mb_strtolower($string), 1, $strlen - 1, $encoding);
    return mb_strtoupper($firstChar, $encoding) . $then;
}

ruby:

Rails monkey patches the String class with the titleize method for capitalizing the words in a string.

trim

How to remove whitespace from the ends of a string.

pad

How to pad the edge of a string with spaces so that it is a prescribed length.

number to string

How to convert numeric data to string data.

format float

How to control the number of digits in a float when converting it to a string.

python:

The number after the decimal controls the number of digits after the decimal:

>>> '%.2f' % math.pi
'3.14'

The number after the decimal controls the total number of digits:

>>> '{:.3}'.format(math.pi)
'3.14'

string to number

How to convert string data to numeric data.

php:

PHP converts a scalar to the desired type automatically and does not raise an error if the string contains non-numeric data. If the start of the string is not numeric, the string evaluates to zero in a numeric context.

python:

float and int raise an error if called on a string and any part of the string is not numeric.

ruby:

to_i and to_f always succeed on a string, returning the numeric value of the digits at the start of the string, or zero if there are no initial digits.

string join

How to concatenate the elements of an array into a string with a separator.

split

How to split a string containing a separator into an array of substrings.

See also scan.

python:

str.split() takes simple strings as delimiters; use re.split() to split on a regular expression:

re.split('\s+', 'do re mi fa')
re.split('\s+', 'do re mi fa', 1)

split in two

How to split a string in two.

javascript:

A regular expression is probably the best method for splitting a string in two:

var m = /^([^ ]+) (.+)/.exec("do re mi");
var first = m[1];
var rest = m[2];

This technique works when the delimiter is a fixed string:

var a = "do re mi".split(" ");
var first = a[0];
var rest = a.splice(1).join(" ");

python:

Methods for splitting a string into three parts using the first or last occurrence of a substring:

'do re mi'.partition(' ')         # returns ('do', ' ', 're mi')
'do re mi'.rpartition(' ')        # returns ('do re', ' ', 'mi')

split and keep delimiters

How to split a string with the delimiters preserved as separate elements.

prefix and suffix test

How to test whether a string begins or ends with a substring.

length

How to get the length in characters of a string.

index of substring

How to find the index of the leftmost occurrence of a substring in a string; how to find the index of the rightmost occurrence.

extract substring

How to extract a substring from a string by index.

byte array type

The type for an array of bytes.

byte array to string

How to convert an array of bytes to a string of Unicode characters.

string to byte array

How to convert a string of Unicode characters to an array of bytes.

character lookup

How to look up the character in a string at an index.

chr and ord

Converting characters to ASCII codes and back.

The languages in this reference sheet do not have character literals, so characters are represented by strings of length one.

to array of characters

How to split a string into an array of single character strings.

translate characters

How to apply a character mapping to a string.

python:

In Python 2, the string of lowercase letters is in string.lowercase instead of string.ascii_lowercase.

In Python 2, the maketrans function is in the module string instead of str.

delete characters

How to remove all specified characters from a string; how to remove all but the specified characters from a string.

squeeze characters

How to replace multiple adjacent occurrences of a character with a single occurrence.

Regular Expressions

• PHP PCRE Regexes
• Python re library: 2.7, 3.1
• Ruby Regexp

Regular expressions or regexes are a way of specifying sets of strings. If a string belongs to the set, the string and regex "match". Regexes can also be used to parse strings.

The modern notation for regexes was introduced by Unix command line tools in the 1970s. POSIX standardized the notation into two types: extended regexes and the more archaic basic regexes. Perl regexes are extended regexes augmented by new character class abbreviations and a few other features introduced by the Perl interpreter in the 1990s. All the languages in this sheet use Perl regexes.

Any string that doesn't contain regex metacharacters is a regex which matches itself. The regex metacharacters are: [ ] . | ( ) * + ? { } ^ $ \

character classes: [ ] .

A character class is a set of characters in brackets: [ ]. When used in a regex it matches any character it contains.

Character classes have their own set of metacharacters: ^ - \ ]

The ^ is only special when it is the first character in the character class. Such a character class matches its complement; that is, any character not inside the brackets. When not the first character the ^ refers to itself.

The hyphen is used to specify character ranges: e.g. 0-9 or A-Z. When the hyphen is first or last inside the brackets it matches itself.

The backslash can be used to escape the above characters or the terminal character class delimiter: ]. It can be used in character class abbreviations or string backslash escapes.

The period . is a character class abbreviation which matches any character except for newline. In all languages the period can be made to match all characters. PHP uses the m modifier. Python uses the re.M flag. Ruby uses the s modifier.

character class abbreviations:

alternation and grouping: | ( )

The vertical pipe | is used for alternation and parens () for grouping.

A vertical pipe takes as its arguments everything up to the next vertical pipe, enclosing paren, or end of string.

Parentheses control the scope of alternation and the quantifiers described below. They are also used for capturing groups, which are the substrings which matched parenthesized parts of the regular expression. Each language numbers the groups and provides a mechanism for extracting them when a match is made. A parenthesized subexpression can be removed from the groups with this syntax: (?:expr)

quantifiers: * + ? { }

As an argument quantifiers take the preceding regular character, character class, or group. The argument can itself be quantified, so that ^a{4}*$ matches strings with the letter a in multiples of 4.

When there is a choice, greedy quantifiers will match the maximum possible number of occurrences of the argument. Lazy quantifiers match the minimum possible number.

anchors: ^ $

escaping: \

To match a metacharacter, put a backslash in front of it. To match a backslash use two backslashes.

php:

PHP 5.3 still supports the EREG engine, though the functions which use it are deprecated. These include the split function and functions which start with ereg. The preferred functions are preg_split and the other functions with a preg prefix.

literal, custom delimited literal

The literal for a regular expression; the literal for a regular expression with a custom delimiter.

javascript:

The constructor for a regular expression is:

var rx = RegExp("lorem|ipsum");

php:

PHP regex literals are strings. The first character is the delimiter and it must also be the last character. If the start delimiter is (, {, or [ the end delimiter must be ), }, or ], respectively.

Here are the signatures from the PHP manual for the preg functions used in this sheet:

array preg_split ( string $pattern , string $subject [, int $limit = -1 [, int $flags = 0 ]] )

int preg_match ( string $pattern , string $subject [, array &$matches [, int $flags = 0 [, int $offset = 0 ]]] )

mixed preg_replace ( mixed $pattern , mixed $replacement , mixed $subject [, int $limit = -1 [, int &$count ]] )

int preg_match_all ( string $pattern , string $subject [, array &$matches [, int $flags = PREG_PATTERN_ORDER [, int $offset = 0 ]]] )

python:

Python does not have a regex literal, but the re.compile function can be used to create regex objects.

Compiling regexes can always be avoided:

re.compile('\d{4}').search('1999')
re.search('\d{4}', '1999')

re.compile('foo').sub('bar', 'foo bar')
re.sub('foo', 'bar', 'foo bar')

re.compile('\w+').findall('do re me')
re.findall('\w+', 'do re me')

ascii character class abbreviations

The supported character class abbreviations.

Note that \h refers to horizontal whitespace (i.e. a space or tab) in PHP and a hex digit in Ruby. Similarly \H refers to something that isn't horizontal whitespace in PHP and isn't a hex digit in Ruby.

unicode character class abbreviations

The supported character class abbreviations for sets of Unicode characters.

Each Unicode character belongs to one of these major categories:

Each major category is subdivided into multiple minor categories. Each minor category has a two letter code, where the first letter is the major category. For example, Nd is "Number, decimal digit".

Download UnicodeData.txt to find out which major and minor category and character belongs to.

anchors

The supported anchors.

match test

How to test whether a string matches a regular expression.

python:

The re.match function returns true only if the regular expression matches the beginning of the string. re.search returns true if the regular expression matches any substring of the of string.

ruby:

match is a method of both Regexp and String so can match with both

/1999/.match("1999")

and

"1999".match(/1999/)

When variables are involved it is safer to invoke the Regexp method because string variables are more likely to contain nil.

case insensitive match test

How to perform a case insensitive match test.

modifiers

Modifiers that can be used to adjust the behavior of a regular expression.

The lists are not comprehensive. For all languages except Ruby there are additional modifiers.

python:

Python modifiers are bit flags. To use more than one flag at the same time, join them with bit or: |

There are alternative identifiers for the modifiers:

substitution

How to replace all occurrences of a matching pattern in a string with the provided substitution string.

php:

The number of occurrences replaced can be controlled with a 4th argument to preg_replace:

$s = "foo bar bar";
preg_replace('/bar/', "baz", $s, 1);

If no 4th argument is provided, all occurrences are replaced.

python:

The 3rd argument to sub controls the number of occurrences which are replaced.

s = 'foo bar bar'
re.compile('bar').sub('baz', s, 1)

If there is no 3rd argument, all occurrences are replaced.

ruby:

The gsub operator returns a copy of the string with the substitution made, if any. The gsub! performs the substitution on the original string and returns the modified string.

The sub and sub! operators only replace the first occurrence of the match pattern.

match, prematch, postmatch

How to get the substring that matched the regular expression, as well as the part of the string before and after the matching substring.

ruby:

The special variables $&, $`, and $' also contain the match, prematch, and postmatch.

group capture

How to get the substrings which matched the parenthesized parts of a regular expression.

ruby:

Ruby has syntax for extracting a group from a match in a single expression. The following evaluates to "1999":

"1999-07-08"[/(\d{4})-(\d{2})-(\d{2})/, 1]

named group capture

How to get the substrings which matched the parenthesized parts of a regular expression and put them into a dictionary.

For reference, we call the (?P<foo>...) notation Python-style and the (?<foo>...) notation Perl-style.

php:

PHP originally supported Python-style named groups since that was the style that was added to the PCRE regex engine. Perl-style named groups were added to PHP 5.2.

python:

The Python interpreter was the first to support named groups.

scan

How to return all non-overlapping substrings which match a regular expression as an array.

backreference in match and substitution

How to use backreferences in a regex; how to use backreferences in the replacement string of substitution.

recursive regex

An examples of a recursive regex.

The example matches substrings containing balanced parens.

Date and Time

In ISO 8601 terminology, a date specifies a day in the Gregorian calendar and a time does not contain date information; it merely specifies a time of day. A data type which combines both date and time information is convenient, but ISO 8601 doesn't provide a name for such an entity. PHP, Python, and C# use the compound noun datetime for combined date and time values and we adopt it here as a generic term.

An useful property of ISO 8601 dates, times, and datetimes is that they are correctly ordered by a lexical sort on their string representations. This is because they are big-endian (the year is the leftmost element) and they used fixed-length, zero-padded fields with numerical values for each term in the string representation.

The C standard library provides two methods for representing dates. The first is the Unix epoch, which is the seconds since the beginning of January 1, 1970 in UTC. If such a time were stored in a 32-bit signed integer, the rollover would happen on January 18, 2038. The Unix epoch is an example of a serial datetime, in which the value is stored as a single numeric value representing the difference in time in some unit from a specially designated datetime called the epoch.

Another serial datetime is the Windows file time, which is the number of 100 nanosecond intervals since the beginning of January 1, 1601 UTC. It was introduced when journaling was added to NTFS as part of the Windows 2000 launch.

Some serial datetimes use days as the unit. The Excel serial number is the number of days since December 31, 1899. The Julian day number, used in astronomy, is the number of days since November 24, 4714 BCE in the proleptic Gregorian calendar. Julian days start at noon GMT.

A broken-down datetime uses multiple numeric values to represent the components of a calendar date and time. An example from the C standard library is the tm struct, a definition of which can be found on Unix systems in /usr/include/time.h:

struct tm {
        int     tm_sec;         /* seconds after the minute [0-60] */
        int     tm_min;         /* minutes after the hour [0-59] */
        int     tm_hour;        /* hours since midnight [0-23] */
        int     tm_mday;        /* day of the month [1-31] */
        int     tm_mon;         /* months since January [0-11] */
        int     tm_year;        /* years since 1900 */
        int     tm_wday;        /* days since Sunday [0-6] */
        int     tm_yday;        /* days since January 1 [0-365] */
        int     tm_isdst;       /* Daylight Savings Time flag */
        long    tm_gmtoff;      /* offset from CUT in seconds */
        char    *tm_zone;       /* timezone abbreviation */
};

The Linux man pages call the tm struct a "broken-down" date and time, whereas the BSD man pages call it a "broken-out" date and time.

The first day in the Gregorian calendar was 15 October 1582. The proleptic Gregorian calendar is an extension of the Gregorian calendar to earlier dates. When such dates are used, they should be called out to be precise. The epoch in the proleptic Gregorian calendar is the year 0001, also written 1 AD or 1 CE. The previous year is the year 0000, also written 1 BC or 1 BCE. The year before that is the year -0001, also written 2 BC or 2 BCE. The ISO 8601 standard recommends that years before 0000 or after 9999 be written with a plus or minus sign prefix.

An ordinal date is a broken-down date consisting of a year, followed by the day of the year. The ISO 8601 standard recommends that it be written in YYYY-DDD or YYYYDDD format. The corresponding strftime formats are %Y-%j and %Y%j.

A week date is a type of calendar which uses the year, week of the year, and day of the week to refer to to dates. In the ISO 8601 week date, the first week of the year is the week starting from Monday which contains January 4th. An ISO 8601 week date can thus have a different year number than the corresponding Gregorian date. The first week of the year is numbered 01, and the first day of the week, Monday, is numbered 1. Weeks are written in YYYY-Www-D or YYYYWwwD format, where the upper case W is literal. The corresponding strftime literals are %G-W%V-%u and %GW%V%u.

Common years have 365 days and leap years have 366 days. The extra day in February 29th. Leap years are years divisible by 4 but not 100, or years divisible by 400.

In 1967, the definition of a second was changed from 1/86,400 of a solar day to a value expressed in terms of radiation produced by ¹³³Cs. Because the length of a solar day is irregular, leap seconds are occasionally used to keep things in sync. This is accomplished by occasionally adding a leap second to the end of June 30th or December 31st. The system also allows for removing the last second of June 30th or December 31st, though as of 2014 this hasn't been done.

broken-down datetime type

The data type used to hold a combined date and time.

python:

Python uses and exposes the tm struct of the C standard library. Python has a module called time which is a thin wrapper to the standard library functions which operate on this struct. Here is how get a tm struct in Python:

import time

utc = time.gmtime(time.time())
t = time.localtime(time.time())

current datetime

How to get the combined date and time for the present moment in both local time and UTC.

current unix epoch

How to get the current time as a Unix epoch timestamp.

broken-down datetime to unix epoch

How to convert a datetime type to the Unix epoch which is the number of seconds since the start of January 1, 1970 UTC.

python:

The Python datetime object created by now() and utcnow() has no timezone information associated with it. The strftime() method assumes a receiver with no time zone information represents a local time. Thus it is an error to call strftime() on the return value of utcnow().

Here are two different ways to get the current Unix epoch. The second way is faster:

import calendar
import datetime

int(datetime.datetime.now().strftime('%s'))
calendar.timegm(datetime.datetime.utcnow().utctimetuple())

Replacing now() with utcnow() in the first way, or utcnow() with now() in the second way produces an incorrect value.

unix epoch to broken-down datetime

How to convert the Unix epoch to a broken-down datetime.

format datetime

How to format a datetime as a string using using a string of format specifiers.

The format specifiers used by the strftime function from the standard C library and the Unix date command:

php:

PHP supports strftime but it also has its own time formatting system used by date, DateTime::format, and DateTime::createFromFormat. The letters used in the PHP time formatting system are described here.

parse datetime

How to parse a datetime using the format notation of the strptime function from the standard C library.

parse datetime w/o format

How to parse a date without providing a format string.

date parts

How to get the year, month, and day of month from a datetime.

time parts

How to the hour, minute, and second from a datetime.

build broken-down datetime

How to build a broken-down datetime from the date parts and the time parts.

datetime subtraction

The data type that results when subtraction is performed on two combined date and time values.

add duration

How to add a duration to a datetime.

A duration can easily be added to a datetime value when the value is a Unix epoch value.

ISO 8601 distinguishes between a time interval, which is defined by two datetime endpoints, and a duration, which is the length of a time interval and can be defined by a unit of time such as '10 minutes'. A time interval can also be defined by date and time representing the start of the interval and a duration.

ISO 8601 defines notation for durations. This notation starts with a 'P' and uses a 'T' to separate the day and larger units from the hour and smaller units. Observing the location relative to the 'T' is important for interpreting the letter 'M', which is used for both months and minutes.

local time zone determination

Do datetime values include time zone information. When a datetime value for the local time is created, how the local time zone is determined.

On Unix systems processes determine the local time zone by inspecting the binary file /etc/localtime. To examine it from the command line use zdump:

$ zdump /etc/localtime
/etc/localtime  Tue Dec 30 10:03:27 2014 PST

On Windows the time zone name is stored in the registry at HKLM\SYSTEM\CurrentControlSet\Control\TimeZoneInformation\TimeZoneKeyName.

php:

The default time zone can also be set in the php.ini file.

date.timezone = "America/Los_Angeles"

Here is the list of timezones supported by PHP.

nonlocal time zone

How to convert a datetime to the equivalent datetime in an arbitrary time zone.

time zone info

How to get the name of the time zone and the offset in hours from UTC.

Timezones are often identified by three or four letter abbreviations. Many of the abbreviations do not uniquely identify a time zone. Furthermore many of the time zones have been altered in the past. The Olson database (aka Tz database) decomposes the world into zones in which the local clocks have all been set to the same time since 1970; it gives these zones unique names.

ruby:

The Time class has a zone method which returns the time zone abbreviation for the object. There is a tzinfo gem which can be used to create time zone objects using the Olson database name. This can in turn be used to convert between UTC times and local times which are daylight saving aware.

daylight savings test

Is a datetime in daylight savings time?

microseconds

How to get the microseconds component of a combined date and time value. The SI abbreviations for milliseconds and microseconds are ms and μs, respectively. The C standard library uses the letter u as an abbreviation for micro. Here is a struct defined in /usr/include/sys/time.h:

struct timeval {
  time_t       tv_sec;   /* seconds since Jan. 1, 1970 */
  suseconds_t  tv_usec;  /* and microseconds */
};

sleep

How to put the process to sleep for a specified number of seconds. In Python and Ruby the default version of sleep supports a fractional number of seconds.

php:

PHP provides usleep which takes an argument in microseconds:

usleep(500000);

timeout

How to cause a process to timeout if it takes too long.

Techniques relying on SIGALRM only work on Unix systems.

Arrays

What the languages call their basic container types:

javascript:

php:

PHP uses the same data structure for arrays and dictionaries.

python:

Python has the mutable list and the immutable tuple. Both are sequences. To be a sequence, a class must implement __getitem__, __setitem__, __delitem__, __len__, __contains__, __iter__, __add__, __mul__, __radd__, and __rmul__.

ruby:

Ruby provides an Array datatype. If a class defines an each iterator and a comparison operator <=>, then it can mix in the Enumerable module.

literal

Array literal syntax.

ruby:

The %w operator splits the following string on whitespace and creates an array of strings from the words. The character following the %w is the string delimiter. If the following character is (, [, or {, then the character which terminates the string must be ), ], or }.

The %W operator is like the %w operator, except that double-quote style #{ } expressions will be interpolated.

quote words

The quote words operator, which is a literal for arrays of strings where each string contains a single word.

size

How to get the number of elements in an array.

empty test

How to test whether an array is empty.

lookup

How to access a value in an array by index.

python:

A negative index refers to the length - index element.

>>> a = [1, 2, 3]
>>> a[-1]
3

ruby:

A negative index refers to to the length - index element.

update

How to update the value at an index.

out-of-bounds behavior

What happens when the value at an out-of-bounds index is referenced.

element index

How to get the index of an element in an array.

php:

Setting the 3rd argument of array_search to true makes the search use === for an equality test. Otherwise the == test is performed, which makes use of implicit type conversions.

slice

How to slice a subarray from an array by specifying a start index and an end index; how to slice a subarray from an array by specifying an offset index and a length index.

python:

Slices can leave the first or last index unspecified, in which case the first or last index of the sequence is used:

>>> a=[1, 2, 3, 4, 5]
>>> a[:3]
[1, 2, 3]

Python has notation for taking every nth element:

>>> a=[1, 2, 3, 4, 5]
>>> a[::2] 
[1, 3, 5]

The third argument in the colon-delimited slice argument can be negative, which reverses the order of the result:

>>> a = [1, 2, 3, 4]
>>> a[::-1]
[4, 3, 2, 1]

slice to end

How to slice to the end of an array.

The examples take all but the first element of the array.

manipulate back

How to add and remove elements from the back or high index end of an array.

These operations can be used to use the array as a stack.

manipulate front

How to add and remove elements from the front or low index end of an array.

These operations can be used to use the array as a stack. They can be used with the operations that manipulate the back of the array to use the array as a queue.

concatenate

How to create an array by concatenating two arrays; how to modify an array by concatenating another array to the end of it.

replicate

How to create an array containing the same value replicated n times.

copy

How to make an address copy, a shallow copy, and a deep copy of an array.

After an address copy is made, modifications to the copy also modify the original array.

After a shallow copy is made, the addition, removal, or replacement of elements in the copy does not modify of the original array. However, if elements in the copy are modified, those elements are also modified in the original array.

A deep copy is a recursive copy. The original array is copied and a deep copy is performed on all elements of the array. No change to the contents of the copy will modify the contents of the original array.

python:

The slice operator can be used to make a shallow copy:

a2 = a[:]

list(v) always returns a list, but v[:] returns a value of the same as v. The slice operator can be used in this manner on strings and tuples but there is little incentive to do so since both are immutable.

copy.copy can be used to make a shallow copy on types that don't support the slice operator such as a dictionary. Like the slice operator copy.copy returns a value with the same type as the argument.

array as function argument

How an array is passed to a function when provided as an argument.

iterate over elements

How to iterate over the elements of an array.

iterate over indices and elements

How to iterate over the element-index pairs.

iterate over range

Iterate over a range without instantiating it as a list.

instantiate range as array

How to convert a range to an array.

Python 3 ranges and Ruby ranges implement some of the functionality of arrays without allocating space to hold all the elements.

python:

In Python 2 range() returns a list.

In Python 3 range() returns an object which implements the immutable sequence API.

ruby:

The Range class includes the Enumerable module.

reverse

How to create a reversed copy of an array, and how to reverse an array in place.

python:

reversed returns an iterator which can be used in a for/in construct:

print("counting down:")
for i in reversed([1, 2, 3]):
  print(i)

reversed can be used to create a reversed list:

a = list(reversed([1, 2, 3]))

sort

How to create a sorted copy of an array, and how to sort an array in place. Also, how to set the comparison function when sorting.

php:

usort sorts an array in place and accepts a comparison function as a 2nd argument:

function cmp($x, $y) {
  $lx = strtolower($x);
  $ly = strtolower($y);
  if ( $lx < $ly ) { return -1; }
  if ( $lx == $ly ) { return 0; }
  return 1;
}

$a = ["b", "A", "a", "B"];

usort($a, "cmp");

python:

In Python 2 it is possible to specify a binary comparision function when calling sort:

a = [(1, 3), (2, 2), (3, 1)]

a.sort(cmp=lambda a, b: -1 if a[1] < b[1] else 1)

# a now contains:
[(3, 1), (2, 2), (1, 3)]

In Python 3 the cmp parameter was removed. One can achieve the same effect by defining cmp method on the class of the list element.

dedupe

How to remove extra occurrences of elements from an array.

python:

Python sets support the len, in, and for operators. It may be more efficient to work with the result of the set constructor directly rather than convert it back to a list.

membership

How to test for membership in an array.

intersection

How to compute an intersection.

python:

Python has literal notation for sets:

{1, 2, 3}

Use set and list to convert lists to sets and vice versa:

a = list({1, 2, 3})
ensemble = set([1, 2, 3])

ruby:

The intersect operator & always produces an array with no duplicates.

union

ruby:

The union operator | always produces an array with no duplicates.

relative complement, symmetric difference

How to compute the relative complement of two arrays or sets; how to compute the symmetric difference.

ruby:

If an element is in the right argument, then it will not be in the return value even if it is contained in the left argument multiple times.

map

Create an array by applying a function to each element of a source array.

ruby:

The map! method applies the function to the elements of the array in place.

collect and collect! are synonyms for map and map!.

filter

Create an array containing the elements of a source array which match a predicate.

ruby:

The in place version is select!.

reject returns the complement of select. reject! is the in place version.

The partition method returns two arrays:

a = [1, 2, 3]
lt2, ge2 = a.partition { |n| n < 2 }

reduce

Return the result of applying a binary operator to all the elements of the array.

python:

reduce is not needed to sum a list of numbers:

sum([1, 2, 3])

ruby:

The code for the reduction step can be provided by name. The name can be a symbol or a string:

[1, 2, 3].inject(:+)

[1, 2, 3].inject("+")

[1, 2, 3].inject(0, :+)

[1, 2, 3].inject(0, "+")

universal and existential tests

How to test whether a condition holds for all members of an array; how to test whether a condition holds for at least one member of any array.

A universal test is always true for an empty array. An existential test is always false for an empty array.

A existential test can readily be implemented with a filter. A universal test can also be implemented with a filter, but it is more work: one must set the condition of the filter to the negation of the predicate and test whether the result is empty.

shuffle and sample

How to shuffle an array. How to extract a random sample from an array.

php:

The array_rand function returns a random sample of the indices of an array. The result can easily be converted to a random sample of array values:

$a = [1, 2, 3, 4];
$sample = [];
foreach (array_rand($a, 2) as $i) { array_push($sample, $a[$i]); }

flatten

How to flatten nested arrays by one level or completely.

When nested arrays are flattened by one level, the depth of each element which is not in the top level array is reduced by one.

Flattening nested arrays completely leaves no nested arrays. This is equivalent to extracting the leaf nodes of a tree.

php, python:

To flatten by one level use reduce. Remember to handle the case where an element is not array.

To flatten completely write a recursive function.

zip

How to interleave arrays. In the case of two arrays the result is an array of pairs or an associative list.

Dictionaries

literal

The syntax for a dictionary literal.

size

How to get the number of dictionary keys in a dictionary.

lookup

How to lookup a dictionary value using a dictionary key.

missing key behavior

What happens when a lookup is performed on a key that is not in a dictionary.

python:

Use dict.get() to avoid handling KeyError exceptions:

d = {}
d.get('lorem')      # returns None
d.get('lorem', '')  # returns ''

is key present

How to check for the presence of a key in a dictionary without raising an exception. Distinguishes from the case where the key is present but mapped to null or a value which evaluates to false.

delete

How to remove a key/value pair from a dictionary.

from array of pairs, from even length array

How to create a dictionary from an array of pairs; how to create a dictionary from an even length array.

merge

How to merge the values of two dictionaries.

In the examples, if the dictionaries d1 and d2 share keys then the values from d2 will be used in the merged dictionary.

invert

How to turn a dictionary into its inverse. If a key 'foo' is mapped to value 'bar' by a dictionary, then its inverse will map the key 'bar' to the value 'foo'. However, if multiple keys are mapped to the same value in the original dictionary, then some of the keys will be discarded in the inverse.

iteration

How to iterate through the key/value pairs in a dictionary.

python:

In Python 2.7 dict.items() returns a list of pairs and dict.iteritems() returns an iterator on the list of pairs.

In Python 3 dict.items() returns an iterator and dict.iteritems() has been removed.

keys and values as arrays

How to convert the keys of a dictionary to an array; how to convert the values of a dictionary to an array.

python:

In Python 3 dict.keys() and dict.values() return read-only views into the dict. The following code illustrates the change in behavior:

d = {}
keys = d.keys()
d['foo'] = 'bar'

if 'foo' in keys:
  print('running Python 3')
else:
  print('running Python 2')

sort by values

How to iterate through the key-value pairs in the order of the values.

default value, computed value

How to create a dictionary with a default value for missing keys; how to compute and store the value on lookup.

php:

Extend ArrayObject to compute values on lookup:

class Factorial extends ArrayObject {

  public function offsetExists($i) {
    return true;
  }

  public function offsetGet($i) {
    if(!parent::offsetExists($i)) {
      if ( $i < 2 ) {
        parent::offsetSet($i, 1);
      }
      else {
        $n = $this->offsetGet($i-1);
        parent::offsetSet($i, $i*$n);
      }
    }
    return parent::offsetGet($i);
  }
}

$factorial = new Factorial();

Functions

Python has both functions and methods. Ruby only has methods: functions defined at the top level are in fact methods on a special main object. Perl subroutines can be invoked with a function syntax or a method syntax.

define

How to define a function.

invoke

How to invoke a function.

python:

Parens are mandatory, even for functions which take no arguments. Omitting the parens returns the function or method as an object. Whitespace can occur between the function name and the following left paren.

In Python 3 print is a function instead of a keyword; parens are mandatory around the argument.

ruby:

Ruby parens are optional. Leaving out the parens results in ambiguity when function invocations are nested. The interpreter resolves the ambiguity by assigning as many arguments as possible to the innermost function invocation, regardless of its actual arity. It is mandatory that the left paren not be separated from the method name by whitespace.

apply function to array

How to apply a function to an array.

perl:

Perl passes the elements of arrays as individual arguments. In the following invocation, the function foo() does not know which arguments came from which array. For that matter it does not know how many arrays were used in the invocation:

foo(@a, @b);

If the elements must be kept in their respective arrays the arrays must be passed by reference:

sub foo {
  my @a = @{$_[0]};
  my @b = @{$_[1]};
}

foo(\@a, \@b);

When hashes are used as arguments, each key and value becomes its own argument.

missing argument behavior

What happens when a function is invoked with too few arguments.

extra argument behavior

What happens when a function is invoked with too many arguments.

default argument

How to declare a default value for an argument.

variadic function

How to write a function which accepts a variable number of argument.

python:

This function accepts one or more arguments. Invoking it without any arguments raises a TypeError:

def poker(dealer, *players):
  ...

ruby:

This function accepts one or more arguments. Invoking it without any arguments raises an ArgumentError:

def poker(dealer, *players)
  ...
end

parameter alias

How to make a parameter an alias of a variable in the caller.

named parameters

How to write a function which uses named parameters and how to invoke it.

python:

The caller can use named parameter syntax at the point of invocation even if the function was defined using positional parameters.

The splat operator * collects the remaining arguments into a list. In a function invocation, the splat can be used to expand an array into separate arguments.

The double splat operator ** collects named parameters into a dictionary. In a function invocation, the double splat expands a dictionary into named parameters.

A double splat operator can be used to force the caller to use named parameter syntax. This method has the disadvantage that spelling errors in the parameter name are not caught:

def fequal(x, y, **kwargs):
  eps = opts.get('eps') or 0.01
  return abs(x - y) < eps

In Python 3 named parameters can be made mandatory:

def fequal(x, y, *, eps):
  return abs(x-y) < eps

fequal(1.0, 1.001, eps=0.01)  # True

fequal(1.0, 1.001)                 # raises TypeError

ruby:

In Ruby 2.1 named parameters can be made mandatory:

def fequals(x, y, eps:)
  (x - y).abs < eps
end

# false:
fequals(1.0, 1.001, eps: 0.1**10)
# ArgumentError:
fequals(1.0, 1.001)

return value

How the return value of a function is determined.

multiple return values

How to return multiple values from a function.

anonymous function literal

The syntax for an anonymous function literal; i.e. a lambda function.

python:

Python lambdas cannot contain newlines or semicolons, and thus are limited to a single statement or expression. Unlike named functions, the value of the last statement or expression is returned, and a return is not necessary or permitted. Lambdas are closures and can refer to local variables in scope, even if they are returned from that scope.

If a closure function is needed that contains more than one statement, use a nested function:

def make_nest(x):
    b = 37
    def nest(y):
        c = x*y
        c *= b
        return c
    return nest

n = make_nest(12*2)
print(n(23))

Python closures are read only.

A nested function can be returned and hence be invoked outside of its containing function, but it is not visible by its name outside of its containing function.

ruby:

The following lambda and Proc object behave identically:

sqr = lambda { |x| x * x }

sqr = Proc.new {|x| x * x }

With respect to control words, Proc objects behave like blocks and lambdas like functions. In particular, when the body of a Proc object contains a return or break statement, it acts like a return or break in the code which invoked the Proc object. A return in a lambda merely causes the lambda to exit, and a break inside a lambda must be inside an appropriate control structure contained with the lambda body.

Ruby are alternate syntax for defining lambdas and invoking them:

sqr = ->(x) {x*x}

sqr.(2)

invoke anonymous function

The syntax for invoking an anonymous function.

function as value

How to store a function in a variable and pass it as an argument.

php:

If a variable containing a string is used like a function then PHP will look for a function with the name in the string and attempt to invoke it.

python:

Python function are stored in variables by default. As a result a function and a variable with the same name cannot share the same scope. This is also the reason parens are mandatory when invoking Python functions.

function with private state

How to create a function with private state which persists between function invocations.

python:

Here is a technique for creating private state which exploits the fact that the expression for a default value is evaluated only once:

def counter(_state=[0]):
  _state[0] += 1
  return _state[0]

print(counter())

closure

How to create a first class function with access to the local variables of the local scope in which it was created.

python:

Python 2 has limited closures: access to local variables in the containing scope is read only and the bodies of anonymous functions must consist of a single expression.

Python 3 permits write access to local variables outside the immediate scope when declared with nonlocal.

generator