a side-by-side reference sheet

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

sheet two: file handles | files | directories | processes and environment | libraries and modules | objects | reflection
                net and web | unit tests | debugging and profiling | java interop | contact

sheet two: file handles | files | directories | processes and environment | libraries and modules | objects | reflection
                net and web | unit tests | debugging and profiling | deployment

General [link]

versions used [link]

The versions used for testing code in the reference sheet.

implicit prologue [link]

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

perl:

We adopt the convention that if an example uses a variable without declaring it, it should be taken to have been previously declared with my.

python:

To keep the examples short we assume that os, re, and sys are always imported.

show version [link]

How to get the version.

php:

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

perl:

Also available in the predefined variable $], or in a different format in $^V and $PERL_VERSION.

python:

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

import platform
platform.python_version()

ruby:

Also available in the global constant VERSION (Ruby 1.8) or RUBY_VERSION (Ruby 1.9).

Grammar and Invocation [link]

interpreter [link]

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 [link]

The customary name of the repl.

php:

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

php -a offers a different style of interactive mode. It collects input until EOF is encountered and then it executes it. Text inside <? code ?> and <?= code ?> is executed as PHP code. Text outside of PHP markup tags is echoed.

perl:

The Perl REPL perl -de 0 does not save or display the result of an expression. perl -d is the Perl debugger and perl -e runs code provided on the command line.

perl -de 0 does not by default have readline, but it can be added:

$ cpan -i Term::Readline::Perl

  <cpan output omitted>

$ perl -de 0

  DB<1> use Term::Readline::Perl;

  DB<2> print 1 + 1;
2

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 [link]

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

block delimiters [link]

How blocks are delimited.

perl:

Curly brackets {} delimit blocks. They are also used for:

• hash literal syntax which returns a reference to the hash: $rh = { 'true' => 1, 'false' => 0 }
• hash value lookup: $h{'true'}, $rh->{'true'}
• variable name delimiter: $s = "hello"; print "${s}goodbye";

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 dictionaries 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 [link]

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 ?>

perl:

In a script statements are separated by semicolons and never by newlines. However, when using perl -de 0 a newline terminates the statement.

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.

end-of-line comment [link]

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

multiple line comment [link]

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 [link]

local variable [link]

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.

perl:

Variables don't need to be declared unless use strict is in effect.

If not initialized, scalars are set to undef, arrays are set to an empty array, and hashes are set to an empty hash.

Perl can also declare variables with local. These replace the value of a global variable with the same name, if any, for the duration of the enclosing scope, after which the old value is restored.

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.

regions which define lexical scope [link]

A list of regions which define a lexical scope for the local variables they contain.

Local variables defined inside the region are only in scope while code within the region is executing. If the language does not have closures, then code outside the region has no access to local variables defined inside the region. If the language does have closures, then code inside the region can make local variables accessible to code outside the region by returning a reference.

A region which is top level hides local variables in the scope which contains it from the code it contains. A region can also be top level if the syntax requirements of the language prohibit it from being placed inside another scope defining region.

A region is nestable if it can be placed inside another scope defining region, and if code in the inner region can access local variables in the outer region.

php:

Only function bodies and method bodies define scope. Function definitions can be nested, but when this is done lexical variables in the outer function are not visible to code in the body of the inner function.

Braces can be used to set off blocks of codes in a manner similar to the anonymous blocks of Perl. However, these braces do not define a scope. Local variables created inside the braces will be visible to subsequent code outside of the braces.

Local variables cannot be created in class bodies.

perl:

A local variable can be defined outside of any function definition or anonymous block, in which case the scope of the variable is the file containing the source code. In this way Perl resembles Ruby and contrasts with PHP and Python. In PHP and Python, any variable defined outside a function definition is global.

In Perl, when a region which defines a scope is nested inside another, then the inner region has read and write access to local variables defined in the outer region.

Note that the blocks associated with the keywords if, unless, while, until, for, and foreach are anonymous blocks, and thus any my declarations in them create variables local to the block.

python:

Only functions and methods define scope. Function definitions can be nested. When this is done, inner scopes have read access to variables defined in outer scopes. Attempting to write (i.e. assign) to a variable defined in an outer scope will instead result in a variable getting created in the inner scope. Python trivia question: what would happen if the following code were executed?

def foo():
    v = 1
    def bar():
        print(v)
        v = 2
        print(v)
    bar()

foo()

ruby:

Note that though the keywords if, unless, case, while, and until each define a block which is terminated by an end keyword, none of these blocks have their own scope.

Anonymous functions can be created with the lambda keyword. Ruby anonymous blocks can be provided after a function invocation and are bounded by curly brackets { } or the do and end keywords. Both anonymous functions and anonymous blocks can have parameters which are specified at the start of the block within pipes. Here are some examples:

id = lambda { |x| x }

[3,1,2,4].sort { |a,b| a <=> b }

10.times do |i|
  print "#{i}..."
end

In Ruby 1.8, the scope of the parameter of an anonymous block or function or block is local to the block or function body if the name is not already bound to a variable in the containing scope. However, if it is, then the variable in the containing scope will be used. This behavior was changed in Ruby 1.9 so that parameters are always local to function body or block. Here is an example of code which behaves differently under Ruby 1.8 and Ruby 1.9:

x = 3
id = lambda { |x| x }
id.call(7)
puts x # 1.8 prints 7; 1.9 prints 3

Ruby 1.9 also adds the ability mark variables as local, even when they are already defined in the containing scope. All such variables are listed inside the parameter pipes, separated from the parameters by a semicolon:

x = 3
noop = lambda { |; x| x = 15 } # bad syntax under 1.8
noop.call
# x is still 3

global variable [link]

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.

perl:

Undeclared variables, which are permitted unless use strict is in effect, are global. If use strict is in effect, a global can be declared at the top level of a package (i.e. outside any blocks or functions) with the our keyword. A variable declared with my inside a function will hide a global with the same name, if there is one.

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 [link]

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 [link]

How to assign a value to a variable.

perl:

Assignment operators have right precedence and evaluate to the right argument, so assignments can be chained:

$a = $b = 3;

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 [link]

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 [link]

How to swap the values held by two variables.

compound assignment [link]

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 any 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 [link]

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--;

perl:

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 [link]

The null literal.

null test [link]

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"; }

perl:

$v == undef does not imply that $v is undef. Any comparison between undef and a falsehood will return true. The following comparisons are true:

$v = undef;
if ($v == undef) { print "true"; }

$v = 0;
if ($v == undef) { print "sadly true"; }

$v = '';
if ($v == undef) { print "sadly true"; }

undefined variable access [link]

The result of attempting to access an undefined variable.

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.

perl:

Perl does not distinguish between unset variables and variables that have been set to undef. In Perl, calling defined($a) does not result in a error if $a is undefined, even with the strict pragma.

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 [link]

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 [link]

true and false [link]

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 [link]

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 [link]

Logical and, or, and not.

php, perl, ruby:

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

relational operators [link]

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.

perl:

The operators: == != > < >= <= convert strings to numbers before performing a comparison. Many string evaluate as zero in a numeric context and are equal according to the == operator. To perform a lexicographic string comparison, use: eq, ne, gt, lt, ge, le.

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 … An opn An+1

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

    A1 op1 A2
    A2 op2 A3
    …
    An opn An+1

min and max [link]

How to get the min and max.

three value comparison [link]

Binary comparison operators which return -1, 0, or 1 depending upon whether the left argument is less than, equal to, or greater than the right argument.

The <=> symbol is called the spaceship operator.

arithmetic operators [link]

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

integer division [link]

How to get the integer quotient of two integers. How to get the integer quotient and remainder.

perl:

The integer pragma makes all arithmetic operations integer operations. Floating point numbers are truncated before they are used. Hence integer division could be performed with:

use integer;
my $a = 7 / 3;
no integer;

divmod [link]

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

integer division by zero [link]

What happens when an integer is divided by zero.

float division [link]

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

float division by zero [link]

What happens when a float is divided by zero.

power [link]

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

sqrt [link]

The square root function.

sqrt -1 [link]

The result of taking the square root of negative one.

transcendental functions [link]

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 [link]

Constants for π and Euler's constant.

float truncation [link]

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.

perl:

The CPAN module Number::Format provides a round function. The 2nd argument specifies the number of digits to keep to the right of the radix. The default is 2.

use Number::Format 'round';

round(3.14, 0);

absolute value [link]

How to get the absolute value of a number.

integer overflow [link]

What happens when the largest representable integer is exceeded.

float overflow [link]

What happens when the largest representable float is exceeded.

rational numbers [link]

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 [link]

python:

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

random integer, uniform float, normal float [link]

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 [link]

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 1.9 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 [link]

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

binary, octal, and hex literals [link]

Binary, octal, and hex integer literals

base conversion [link]

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

perl

Perl has the functions oct and hex which convert strings encoded in octal and hex and return the corresponding integer. The oct function will handle binary or hex encoded strings if they have "0b" or "0x" prefixes.

oct("60")
oct("060")
oct("0b101010")
oct("0x2a")

hex("2a")
hex("0x2a")

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 [link]

string literal [link]

The syntax for string literals.

perl:

When use strict is not in effect bareword strings are permitted.

Barewords are strings without quote delimiters. They are a feature of shells. Barewords cannot contain whitespace or any other character used by the tokenizer to distinguish words.

Before Perl 5 subroutines were invoked with an ampersand prefix & or the older do keyword. With Perl 5 neither is required, but this made it impossible to distinguish a bareword string from a subroutine without knowing all the subroutines which are in scope.

The following code illustrates the bareword ambiguity:

no strict;

print rich . "\n";  # prints "rich"; rich is a bareword string

sub rich { return "poor" }

print rich . "\n";  # prints "poor"; rich is now a subroutine

newline in literal [link]

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"""

character escapes [link]

Backslash escape sequences for inserting special characters into string literals.

perl:

In addition to the character escapes, Perl has the following translation escapes:

When use charnames is in effect the \N escape sequence is available:

binmode(STDOUT, ':utf8');

use charnames ':full';

print "lambda: \N{GREEK SMALL LETTER LAMDA}\n";

use charnames ':short';

print "lambda: \N{greek:lamda}\n";

use charnames qw(greek);

print "lambda: \N{lamda}\n";

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'

variable interpolation [link]

How to interpolate variables into strings.

python:

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()))

custom delimiters [link]

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.

sprintf [link]

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)

here document [link]

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

perl:

Put the custom identifer in single quotes to prevent variable interpolation and backslash escape interpretation:

s = <<'EOF';
Perl code uses variables with dollar
signs, e.g. $var
EOF

python:

Python lacks variable interpolation in strings. 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

concatenate [link]

The string concatenation operator.

replicate [link]

The string replication operator.

split [link]

How to split a string containing a separator into an array of substrings; how to split a string in two; how to split a string with the delimiters preserved as separate elements; how to split a string into an array of single character strings.

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)

join [link]

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

case manipulation [link]

How to put a string into all caps or all lower case letters. How to capitalize the first letter of a string.

trim [link]

How to remove whitespace from the ends of a string.

perl:

An example of how to trim a string without installing a library:

$s = " lorem ";
$s =~ s/^\s*(.*)\s*$/\1/;

The return value of the =~ operator is boolean, indicating whether a match occurred. Also the left hand side of the =~ operator must be a scalar variable that can be modified. Using the =~ operator is necessarily imperative, unlike the Text::Trim functions which can be used in expressions.

pad on right, on left [link]

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

convert from string, to string [link]

How to convert string data to numeric data and vice versa.

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.

perl:

Perl 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.

length [link]

How to get the length in characters of a string.

index of substring [link]

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

python:

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

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

extract substring [link]

How to extract a substring from a string by index.

extract character [link]

How to extract a character from a string by its index.

chr and ord [link]

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.

character translation [link]

How to apply a character mapping to a string.

Regular Expressions [link]

• PHP PCRE Regexes
• perlre and perlreref
• 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 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 hypen 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. In PHP and Perl use the m modifer. In Python use the re.M flag. In Ruby use the s modifer.

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 [link]

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

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')

character class abbreviations and anchors [link]

The supported character class abbreviations and anchors.

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

match test [link]

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 [link]

How to perform a case insensitive match test.

modifiers [link]

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

substitution [link]

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.

perl:

The =~ operator performs the substitution in place on the string and returns the number of substitutions performed.

The g modifiers specifies that all occurrences should be replaced. If omitted, only the first occurrence is 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 [link]

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

perl:

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

ruby:

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

group capture [link]

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]

scan [link]

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

backreference in match and substitution [link]

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

recursive regex [link]

Examples of recursive regexes.

The examples match substrings containing balanced parens.

Date and Time [link]

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 probably more useful than one which contains just date information or just time information; it is unfortunate that ISO 8601 doesn't provide a name for this entity. The word timestamp often gets used to denote a combined date and time. PHP and Python use the compound noun datetime for combined date and time values.

An useful property of ISO 8601 dates, times, and date/time combinations 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 fields 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 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 other method of representing dates 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 */
};

Perl and Python both use and expose the tm struct of the standard library. In the case of Perl, the first nine values of the struct (up to the member tm_isdst) are put into an array. Python, meanwhile, 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())

The tm struct is a low level entity, and interacting with it directly should be avoided. In the case of Python it is usually sufficient to use the datetime module instead. For Perl, one can use the Time::Piece module to wrap the tm struct in an object.

date/time type [link]

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

perl

Built in Perl functions work with either (1) scalars containing the Unix epoch as an integer or (2) arrays containing the first nine values of the standard C library tm struct. When use Time::Piece is in effect functions which work with tm arrays are replaced with variant that work with the Time::Piece wrapper.

The modules Time::Local and Date::Parse can create scalars containing the Unix epoch.

CPAN provides the DateTime module which provides objects with functionality comparable to the DateTime objects of PHP and Python.

current date/time [link]

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

to unix epoch, from unix epoch [link]

How to convert the native date/time 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 timezone 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.

current unix epoch [link]

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

strftime [link]

How to format a date/time as a string using the format notation of the strftime function from the standard C library. This same format notation is used by 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.

default format example [link]

Examples of how a date/time object appears when treated as a string such as when it is printed to standard out.

The formats are in all likelihood locale dependent. The provided examples come from a machine running Mac OS X in the Pacific time zone of the USA.

php:

It is a fatal error to treat a DateTime object as a string.

strptime [link]

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

parse date w/o format [link]

How to parse a date without providing a format string.

result date subtraction [link]

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

add time duration [link]

How to add a time duration to a date/time.

A time duration can easily be added to a date/time value when the value is a Unix epoch value.

ISO 8601 distinguishes between a time interval, which is defined by two date/time 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 timezone [link]

Do date/time values include timezone information. When a date/time value for the local time is created, how the local timezone is determined.

A date/time value can represent a local time but not have any timezone information associated with it.

On Unix systems processes determine the local timezone by inspecting the file /etc/localtime.

php:

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

date.timezone = "America/Los_Angeles"

Here is the list of timezones supported by PHP.

arbitrary timezone [link]

How to convert a timestamp to the equivalent timestamp in an arbitrary timezone.

timezone name, offset from UTC, is daylight savings? [link]

How to get time zone information: the name of the timezone, the offset in hours from UTC, and whether the timezone is currently in daylight savings.

Timezones are often identified by three or four letter abbreviations. As can be seen from the list, many of the abbreviations do not uniquely identify a timezone. Furthermore many of the timezones 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 and it gives these zones unique names.

perl:

It is not necessary to create a DateTime object to get the local timezone offset:

use Time::Piece;

$t = localtime(time);
$offset_hrs = $t->tzoffset / 3600;

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 timezone 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.

microseconds [link]

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 [link]

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);

perl:

The Perl standard library includes a version of sleep which supports fractional seconds:

use Time::HiRes qw(sleep);

sleep 0.5;

timeout [link]

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

Techniques relying on SIGALRM only work on Unix systems.

Arrays [link]

What the languages call their basic container types:

php:

PHP uses the same data structure for arrays and dictionaries.

perl:

array refers to a data type. list refers to a context.

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 [link]

Array literal syntax.

perl:

Square brackets create an array and return a reference to it:

$a = [1,2,3]

quote words [link]

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

size [link]

How to get the number of elements in an array.

empty test [link]

How to test whether an array is empty.

lookup [link]

How to access a value in an array by index.

perl:

A negative index refers to the length - index element.

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 [link]

How to update the value at an index.

out-of-bounds behavior [link]

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

index of array element [link]

perl:

Some techniques for getting the index of an array element.

slice by endpoints, by length [link]

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.

perl:

Perl arrays can take an array of indices as the index value. The range of values selected can be discontinuous and the order of the values can be manipulated:

@nums = (1,2,3,4,5,6);
@nums[(1,3,2,4)];

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 [link]

How to slice to the end of an array.

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

manipulate back [link]

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 [link]

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 [link]

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

replicate [link]

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

address copy, shallow copy, deep copy [link]

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.

perl:

Taking a reference is customary way to make an address copy in Perl, but the Perl example is not equivalent to the other languages in that different syntax has to be used to access the original array and the address copy: @a and @$a1. To make @a1 and @a refer to the same array, use typeglobs:

*a1 = *a;

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.

arrays as function arguments [link]

How arrays are passed as arguments.

iteration [link]

How to iterate through the elements of an array.

perl:

for and foreach are synonyms. Some use for exclusively for C-style for loops and foreach for array iteration.

indexed iteration [link]

How to iterate through the elements of an array while keeping track of the index of each element.

iterate over range [link]

Iterate over a range without instantiating it as a list.

perl:

With Perl 5.005 the for and foreach operators were optimized to not instantiate a range argument as a list.

instantiate range as array [link]

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 [link]

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 [link]

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.