a side-by-side reference sheet

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General Footnotes

versions 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.

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 1.8.7) or RUBY_VERSION (ruby 1.9.1).

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:

Submit each line of code to php -a as <?= php code ?> to have the line executed as php code and the result displayed.

perl:

The perl repl lacks readline and does not save or display the result of an expression. Actually, 'perl -d' runs the perl debugger, and 'perl -e' runs code provided on the command line.

python:

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

ruby:

irb saves the result of the last statement in _.

statement separator

How the parser determines the end of a statement.

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.

block delimiters

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 encouters 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.

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.

assignment

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 (=) in place of an equality test (==). 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 raises a TypeError:

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 value of the variable as an argument.

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

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.

local variable declarations

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. local declarations became obsolete with the introduction of the my declaration introduced in Perl 5.

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 local scope

A list of regions which define a 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 cope 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

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 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 individuality 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 declaration

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.

to-end-of-line comment

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

comment out multiple lines

How to comment out multiple lines.

python:

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

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"; }

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

The result of attempting to access an undefined variable.

undefined test

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.

Arithmetic and Logic Footnotes

true and false

Literals for the booleans.

These are the return values of the comparison 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__ instance method for the class.

logical operators

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.

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

comparison operators

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

Also known as the relational operators.

php:

Most of the comparison 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.

three way comparison

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.

convert from string, to string

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.

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. 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;

float division

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

arithmetic functions

Some arithmetic 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)

arithmetic 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.

perl:

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

use Number::Format 'round';

round(3.14, 0);

min and max

How to get the min and max.

division by zero

What happens when division by zero is performed.

integer overflow

What happens when the largest representable integer is exceeded.

float overflow

What happens when the largest representable float is exceeded.

sqrt -2

The result of taking the square root of negative two.

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.

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

base conversion

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)

String Footnotes

string literal

The syntax for string literals.

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"""

backslash escapes

Backslash escape sequences for inserting special characters into string literals.

python:

Before python 3, it was possible to enter a character by Unicode point using the following syntax:

u'\u03bb'

variable interpolation

How to interpolate variables into strings.

python:

Python lacks interpolating quotes. Except for the fact that they can contain single quotes, double quotes are identical to single quotes.

custom delimiters

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

How to create a string using a printf style format.

python:

Escape curly braces by doubling:

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

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.

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

The string concatenation operator.

replicate

The string replication operator.

split

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

ruby:

The argument to split can be a string or regular expression. A 2nd argument will put a limit on the number of elements in the array; the last element will contain the reaminder of the string. To split a string into an array of single character strings, use

"abcdefg".split('')

join

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

case manipulation

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

strip

How to remove whitespace from the ends of a string.

pad on right, on left

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

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.

extract substring

How to extract a substring from a string by index.

extract character

How to extract a character from a string by its 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.

character count

how to count the number times a character occurs in a string

perl:

The translate characters tr operator returns the number of characters replaced. Normally it performs a destructive operation on the string, but not if the second half of the translation specification is empty.

character translation

How to apply a character mapping to a string.

regex match

How to test whether a string matches a regular expression.

match, prematch, postmatch

substring matches

scan

single substitution

perl:

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

python:

The 3rd argument of sub indicates the maximum number of substitutions to be performed. The default value is zero, in which case there is no limit to the number of substitutions performed.

ruby:

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

global substitution

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

perl:

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

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.

Date and Time Footnotes

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. Avoiding the tm struct in Perl is a bit more difficult and might require downloading CPAN modules.

date/time type

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. The modules Time::Local and Date::Parse can both be used to 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

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

local timezone

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.

perl:

Although the tm struct from the standard library has a tm_zone member, the Perl tm array does not have a corresponding element.

strftime

How to format a timestamp 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.

strptime

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

parse date

How to parse a date without providing a format string.

date subtraction

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

perl

It is not useful to subtract arrays holding tm values as the arrays will first be converted to scalar values indicating their size.

add time duration

How to add a time duration to a timestamp.

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.

perl

The technique illustrated, namely using an arithmetic expression to calculate the length of a duration in seconds, is probably the most practical one when working in Perl. There is also the following CPAN module:

use Time::Interval;

$now = time;
$now += convertInterval( minutes=>10, seconds=>3, ConvertTo=>"seconds");

to unix epoch, from unix epoch

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

timezone name, offset from UTC, is daylight savings?

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.

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

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. Python and Ruby support sleeping for 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

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

Container Footnotes

Scripting languages have the same basic container types but the terminology is not universal:

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.

Array Footnotes

literal

Array literal syntax.

perl:

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

$a = [1,2,3]

quote words

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

For languages without this operator, the easiest way to specify such an array of strings is provided.

size

How to get the number of elements in an array.

perl:

The idiomatic way to test whether an array is empty in perl:

if ( not @a ) {
  print "@a is empty\n";
}

python:

The idiomatic way to test whether a list is empty in python:

if not a:
  print "a is empty"

ruby:

The empty list evaluates to true in a condition test. Use the empty? predicate to determine if an array is empty:

if a.empty?
  puts "a is empty"
end

lookup

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

How to update the value at an index.

out-of-bounds behavior

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

index of array element

perl:

Some techniques for getting the index of an array element.

slice

How to slice a subarray from an array.

perl:

Perl does not directly support slices. Rather Perl arrays can take an array of indices as the index value. As a result 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]

ruby:

There is notation for specifying the first index and the number of items in the slice

irb> nums = [1,2,3,4,5]
=> [1, 2, 3, 4, 5]
irb> nums[1,3]
=> [2, 3, 4]

slice to end

How to slice to the end of an 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.

address copy, shallow copy, deep 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.

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 because it makes @$b and @a refer to the same array. To make @b and @a refer to the same array, use typeglobs:

*b = *a;

python:

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

c = a[:]

arrays as function arguments

How arrays are passed as arguments.

iteration

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

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

iterate over range

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

How to convert a range to an array.

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.

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]))

membership

How to test for membership in an array.

intersection

How to compute an intersection.

python:

Python has a set data type which is built-in as of version 2.4. Furthermore, Python has literal syntax for sets: {1,2,3}.

The example computes the intersection by converting the lists to sets. The return value is a set; it can be converted to a list with the list constructor.

ruby:

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

union

python:

The python example computes the union by converting the lists to sets. The return value is a set; it can be converted to a list with the list constructor.

ruby:

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

map

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

ruby:

The map! method will apply the function to the elements of the source array in place.

filter

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

reduce

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

perl:

The reduce function was added in Perl 5.10.

universal test

How to test whether a condition holds for all members of an array. Always true for an empty array.

A universal test can be implemented with a filter: filter the set on the negation of the predicate and test whether the result is empty.

existential test

How to test whether an item in an array exists for which a condition holds. Always false for an empty array.

An existential test can be implemented with a filter: filter the set on the predicate and test whether the result is non-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 = array(1, 2, 3, 4);
$sample = array();
foreach (array_rand($a, 2) as $i) { array_push($sample, $a[$i]); }

zip