a side-by-side reference sheet

grammar and execution | variables and expressions | arithmetic and logic | strings | regexes | dates and time | arrays | dictionaries | functions | execution control | exceptions | streams | files | file formats | directories | processes and environment | option parsing | libraries and namespaces | objects | inheritance and polymorphism | reflection | net and web | unit tests | debugging and profiling

General

version used

The version used for verifying the examples in this cheat sheet.

show version

How to get the version.

perl:

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

implicit prologue

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.

Grammar and Execution

interpreter

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

unix:

On Unix, scripts are executing by passing the file containing the script to the interpreter as an argument:

bash ~/configure.sh

If the executable bit is set, the file can be run directly:

~/configure.sh

To determine the name of the interpreter that will process the script, Unix will look for the presence of a shebang (#!) at the start of the file. If the pathname to a command follows the shebang, it will be used to interpret the script. If no shebang is present, the script will be interpreted with sh, which is bash on modern Unix systems.

Arguments that follow the command will be passed to interpreter as command line arguments.

If it is undesirable to specify the pathname of the interpreter, the env command can be used to search the PATH directories:

#!/usr/bin/env lua

groovy:

Groovy does not in general have # style comments, but one can be used on the first line to create a shebang script:

#!/usr/bin/env groovy

println "hello world!"

repl

The customary name of the repl.

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

command line program

How to pass in a program to the interpreter on the command line.

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";

lua:

The function and if keywords open blocks which are terminated by end keywords. The repeat keyword opens a block which is terminated by until.

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.

are expressions statements

Whether an expression can be used where a statement is expected.

source code encoding

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

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

end-of-line comment

How to make the remainder of the line a comment.

multiple line comment

How to comment out multiple lines.

lua:

The double bracket notation [[ ]] is the syntax for a multiline string literal.

Variables and Expressions

local variable

How to declare a local variable.

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.

regions which define local scope

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

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.

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.

global variable

How to declare and access a 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.

constant

How to declare a constant.

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;

parallel assignment

Whether parallel assignment is supported, and if so how to do it.

swap

How to swap the values in two variables.

compound assignment

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

increment and decrement

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

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--;

null

The null literal.

null test

How to test if a value is null.

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

What happens when the value in an undefined variable is accessed.

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.

lua:

There is no distinction between a variable being defined and a variable having a nil value. Assigning nil to a variable frees it.

conditional expression

How to write a conditional expression.

lua:

notes on Lua and the ternary operator

Arithmetic and Logic

true and false

The literals for true and false.

falsehoods

Values which are false in conditional expressions.

logical operators

Logical and, or, and not.

perl:

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

relational expressions

How to write a relational expression.

relational operators

The available relational operators.

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.

min and max

How to find the min and max value in mix of values or variables.

three value 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.

arithmetic expression

How to evaluate an arithmetic expression.

arithmetic operators

The binary arithmetic operators.

The operators for addition, subtraction, multiplication, float division, integer division, modulus, and exponentiation. Some languages provide a function pow instead of an operator for exponentiation.

integer division

How to perform integer division.

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;

lua:

All Lua numbers are floating point.

integer division by zero

What happens when a float is divided by zero.

lua

As already noted there are no literals for inf and nan. Once could assign the correct values
to variables with the following code

inf = 1 / 0
nan = 0 / 0

inf can be compared to itself, but nan cannot. In other words:

# this prints 'true'
=(1 / 0) == (1 / 0)
# this prints 'false'
=(0 / 0) == (0 / 0)

float division

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

float division by zero

The result of dividing a float by zero.

power

How to compute exponentiation.

The examples calculate 2 to the 32 which is the number of distinct values that can be stored in 32 bits.

sqrt

How to get the square root of a number.

sqrt -1

The result of taking the square root of -1.

transcendental functions

Functions for computing the natural exponent, natural logarithm, sine, cosine, tangent, arcsine, arccosine, arctangent, and atan2.

The trigonometric functions are all in radians. atan2 takes two arguments which are the x and y co-ordinates of a vector in the Cartesian plane. It returns
the angle to the positive x-axis made by the vector.

transcendental constants

Approximate values for π and e.

float truncation

Ways to convert a float to a nearby integer: towards zero; to the nearest integer; towards positive infinity; towards negative infinity.

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

How to get the absolute value of a number.

integer overflow

What happens when an operation yields an integer larger than the largest representable value.

float overflow

What happens when an operation yields a float larger than the largest representable value.

rational construction

How to construct a rational number.

rational decomposition

How to decompose a rational number into a numerator and a denominator.

complex construction

How to construct a complex number.

complex decomposition

How to decompose a complex number into its real and imaginary part; how to get the polar decomposition of a complex number; how to get the complex conjugate.

random number

The examples show how to generate a uniform random integer in the range from 0 to 99, inclusive; how to generate a uniform float in the range 0.0 to 1.0; how to generate a float from a standard normal distribution

random seed

How to set the seed for the random number generator.

Setting the seed to a fixed number can be used for repeatable results.

bit operators

The available bit operators.

binary, octal, and hex literals

Binary, octal, and hex integer literals

radix

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

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")

Strings

string literal

The syntax for a string literal.

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

Perl allows custom delimiters to be used 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.

my $s1 = q(lorem ipsum);
my $s2 = qq($s1 dolor sit amet);

newline in literal

Are newlines permitted in a string literal?

lua:

One can avoid backslashes by using the double bracket string notation. The following are equivalent:

s = "one\
two\
three"

s = [[one
two
three]]

literal escapes

Backslash escape sequences that can be used in a string literal.

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";

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

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

variable interpolation

The syntax for interpolating variables into a string literal.

expression interpolation

Interpolating the result of evaluating an expression in a string literal.

concatenate

The string concatenation operator.

replicate

The string replication operator.

split

lua:

how to split a string in Lua

join

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

sprintf

How to create a string using a printf style format.

case manipulation

trim

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

How to pad a string to a given length on the right; how to pad on the left; how to center a string inside a larger string of a given length.

groovy

Provide a second argument to set the pad character:

"lorem".padRight(10, '_')
"lorem".padLeft(10, '_')
"lorem.center(10, '_')

string to number

How to convert a string to a number.

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.

lua:

Arithmetic operators will attempt to convert strings to numbers; if the string contains non-numeric data an error results. Note that relational operators do not perform conversion. Thus the following expression is false:

 0 == '0'

number to string

How to convert a number to a string.

lua:

print and the .. operator will convert all types to strings.

(8).toString()
x = 7
x.toString()

length

How to get the number of characters in a string.

index substring

How to get the index of the leftmost occurrence of a substring.

extract substring

How to extract a substring.

chr and ord

converting characters to ASCII codes and back

Regular Expressions

• perlre and perlreref

literal, custom delimited literal

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

character class abbreviations and anchors

The supported character class abbreviations and anchors

anchors

The supported anchors.

match test

How to test whether a regular expression matches a string.

groovy

The =~ operator succeeds if the regex matches part of the string. The ==~ succeeds only if the regular expression matches the entire string:

s = "it's 1999"

// true:
s =~ /1999/

// false:
s ==~ /1999/

case insensitive match test

How to test whether a regular expression matches a string, ignoring case.

modifiers

Available flags for modifying regular expression behavior.

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.

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

lua:

To specify the maximum number of pattern occurrences to be replaced, provide a fourth argument:

s = "do re mi mi mi"
s = string.gsub(s, "mi", "ma", 1)

match, prematch, postmatch

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

perl:

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

group capture

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

named group capture

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

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

perl:

Perl 5.10 added support for both Python-style and Perl-style named groups.

backreference

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

Perl uses and exposes the tm struct of the standard library. The first nine values of the struct (up to the member tm_isdst) are put into an array.

Date and Time

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. 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

How to get the current time.

to unix epoch, from unix epoch

How to convert a date/time object to the Unix epoch; how to convert a date/time object from the Unix epoch.

The Unix epoch is the number of seconds since January 1, 1970 UTC. In the case of Lua, the native date/time object is the Unix epoch, so no conversion is needed.

current unix epoch

strftime

How to use a strftime style format string to convert a date/time object to a string representation.

strftime is from the C standard library. The Unix date command uses the same style of format.

strptime

How to parse a string into a date/time object using a strftime style format string. strptime is from the C standard library.

parse date w/o format

How to parse a string into a date/time object without using a format string.

get date parts

How to get the year as a four digit integer; how to get the month as an integer from 1 to 12; how to get the day of the month as an integer from 1 to 31.

get time parts

How to get the hour as an integer from 0 to 23; how to get the minute as an integer from 0 to 59; how to get the second as an integer from 0 to 59.

build date/time from parts

How to assemble a date/time object from the 4 digit year, month (1-12), day (1-31), hour (0-23), minute (0-59), and second (0-59).

result of date subtraction

add time duration

local timezone

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.

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;

microseconds

sleep

How to sleep for half a second.

perl:

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

use Time::HiRes qw(sleep);

sleep 0.5;

timeout

Arrays

The names given by the languages for their basic container types:

perl:

Arrays are a data type. Lists are a context.

lua:

Note that Lua uses the same data structure for arrays and dictionaries.

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.

size

How to get the number of elements in an array.

lua:

The # operator returns the index of a non-nil value in the array that is followed by a nil value. Unfortunately different Lua implementations behave differently when there are multiple such indices. In particular there is no guarantee that the index will be the index of the last non-nil value in the array.

# a returns the number of times that a for loop used with ipairs will execute.

lookup

How to access a value in an array by index.

perl:

A negative index refers to the length - index element.

lua:

Lua arrays are indexed from one.

update

How to change the value stored in an array at given index.

out-of-bounds behavior

What happens when a lookup is performed on an out-of-bounds index.

index of element

How to get the first or last index containing a value.

perl:

Some techniques for getting the index of an array element.

slice

How to slice a subarray from an array.

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)];

slice to end

How to slice an array to the end

manipulate back

How to push or pop elements from the back an array. The return value of the pop command is the value of the item which is removed.

With these commands an array can be used as a stack.

manipulate front

How to shift or unshift elements to the front of an array. The return value of the shift command is the value of the item which is removed.

With these commands an array can be used as a stack. When combined with the commands from the previous section the array can be used as a queue.

concatenate

How to concatenate two arrays.

replicate

How to create an array consisting of n copies of a value.

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 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;

arrays as function arguments

How arrays are passed as arguments.

iterate over elements

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.

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.

reverse

How to reverse the order of the elements in an array.

sort

How to sort an array.

dedupe

How to remove extra occurrences of elements from an array.

membership

How to test whether a value is an element of an array; how to test whether a value is not an element of an array.

intersection

How to compute an intersection.

union

How to compute the union of two arrays.

relative complement

How to find the elements of an array that are not in another array.

map

How to apply a function to the elements of an array.

None of the examples modify the source array.

lua:

map implementation

filter

How to select the elements of an array for which a predicate is true.

None of the examples modify the source array.

lua:

filter implementation

reduce

How to reduce an array using a binary operation and an initial value.

None of the examples modify the source array.

lua:

reduce implementation

universal and existential tests

How to test whether all elements in an array satisfy a predicate; how to test whether at least one element in an array satisfies a predicate.

shuffle

How to shuffle an array.

zip

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

perl:

zip expects arrays as arguments, which makes it difficult to define the arrays to be zipped on the same line as the invocation. It can be done like this:

@a = zip @{[1,2,3]}, @{['a','b','c']};

Dictionaries

literal

The syntax for a dictionary literal.

perl:

Curly brackets create a hash and return a reference to it:

$h = { 'hello' => 5, 'goodbye' => 7 }

size

How to get the number of entries in a dictionary.

lookup

How to look up the value associated with a key.

perl:

Use the ampersand prefix @ to slice a Perl hash. The index is a list of keys.

%nums = ('b'=>1, 't'=>2, 'a'=>3);
@nums{('b','t')}

update

How to set or update the value associated with a key.

out of bounds behavior

What happens when a lookup is performed for a key the dictionary does not have.

is key present

How to find out whether a value is associated with a key.

delete

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

from array of pairs, from even length array

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

merge

How to merge the values of two dictionaries.

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

invert

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

iteration

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

keys and values as arrays

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

sort by values

How to sort the data in a dictionary by its values.

default value, computed value

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

perl:

How to use a tied hash. If the CPAN module Tie::ExtraHash is installed there is a shorter way.

Functions

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

define function

How to define a function.

perl:

One can also use shift to put the arguments into local variables:

sub add {
  my $a = shift;
  my $b = shift;

  $a + $b;
}

invoke function

How to invoke a function.

nested function

Can function definitions be nested; are nested functions visible outside of the function in which they are defined.

missing argument behavior

How incorrect number of arguments upon invocation are handled.

perl:

Perl collects all arguments into the @_ array, and subroutines normally don't declare the number of arguments they expect. However, this can be done with prototypes. Prototypes also provide a method for taking an array from the caller and giving a reference to the array to the callee.

extra argument behavior

If a function is invoked with more arguments than are declared, how the function can access them.

default argument

How to declare a default value for an argument.

variable number of arguments

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

lua:

In version 5.1 and earlier the global variable arg was set automatically. In version 5.2 one must explicitly assign to a local array.

It is possible to get the number of arguments that were provided without assigning to a local array:

function foo(...)
  print('passed ' .. select('#', ...) .. ' arguments')
end

return value

How the return value of a function is determined.

multiple return values

lua:

If a function returns multiple values, the first value can be selected by placing the invocation inside parens:

function roots(x)
  return math.sqrt(x), -math.sqrt(x)
end
(roots(4))

anonymous function literal

How to define a lambda function.

invoke anonymous function

How to invoke a lambda function.

function as value

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

function with private state

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

default scope

What scope do variables declared inside a function have by default.

Execution Control

if

The if statement.

perl:

When an if block is the last statement executed in a subroutine, the return value is the value of the branch that executed.

while

How to create a while loop.

perl:

Perl provides until, do-while, and do-until loops.

An until or a do-until loop can be replaced by a while or a do-while loop by negating the condition.

break and continue

break exits a for or while loop immediately. continue goes to the next iteration of the loop.

for

How to create a C-style for loop.

lua:

Provide a third argument to set the step size to something other than the default of one:

for i = 0, 90, 10 do
  print(i)
end

raise exception

How to raise an exception.

catch exception

How to handle an exception.

finally/ensure

Clauses that are guaranteed to be executed even if an exception is thrown or caught.

uncaught exception behavior

System behavior if an exception goes uncaught. Most interpreters print the exception message to stderr and exit with a nonzero status.

closure

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

generator

How to create and use a generator.

Streams

print to standard output

read from standard input

How to read a line from standard input.

standard file handles

Constands for the standard file handles.

open file

How to create a file handle for reading.

open file for writing

How to create a file handle for writing.

close file

How to close a file handle.

read line

iterate over a file by line

chomp

Remove a newline, carriage return, or carriage return newline pair from the end of a line if there is one.

read file

write to file

flush file handle

Files

file exists test, file regular test

copy file, remove file, rename file

set file permissions

temporary file

Directories

Processes and Environment

command line arguments

How to access the command line arguments.

environment variable

How to access an environment variable.

exit

How to terminate the process and set the status code.

set signal handler

How to register a signal handler.

external command

How to execute an external command.

backticks

How to execute an external command and read the standard output into a variable.

Libraries and Namespaces

library

What a library looks like.

import library

How to import a library.

library path

How to access the library path.

library path environment variable

The environment variable which governs the library path.

namespace declaration

How to declare a namespace.

namespace separator

The separator used in namespace names.

list installed packages, install a package

How to list the installed 3rd party packages; how to install a 3rd party package.

Objects

define class

How to define a class.

create object

How to instantiate a class.

create blank object

How to create a blank object without any attributes or methods, or at least without any but the default attributes and methods.

instance variable visibility

object literal

The syntax for an object literal.

set attribute

How to set an object attribute.

get attribute

How to get an object attribute.

define method

How to define a method for an object.

invoke method

How to invoke an object method.

handle undefined method invocation

define class method

invoke class method

define class variable

get and set class variable

Inheritance and Polymorphism

subclass

Reflection

inspect type

basic types

inspect class

has method?

message passing

eval

list object methods

list object attributes

list loaded libraries

list loaded namespaces

inspect namespace

pretty print

source line number and file name

command line documentation

Net and Web

get local hostname, dns lookup, reverse dns lookup

How to get the hostname and the ip address of the local machine without connecting to a socket.

The operating system should provide a method for determining the hostname. Linux provides the uname system call.

A DNS lookup can be performed to determine the IP address for the local machine. This may fail if the DNS server is unaware of the local machine or if the DNS server has incorrect information about the local host.

A reverse DNS lookup can be performed to find the hostname associated with an IP address. This may fail for the same reasons a forward DNS lookup might fail.

http get

How to make an HTTP GET request and read the response into a string.

http post

absolute url

How to construct an absolute URL from a base URL and a relative URL as documented in RFC 1808.

When constructing the absolute URL, the rightmost path component of the base URL is removed unless it ends with a slash /. The query string and fragment of the base URL are always removed.

If the relative URL starts with a slash / then the entire path of the base URL is removed.

If the relative URL starts with one or more occurrences of ../ then one or more path components are removed from the base URL.

The base URL and the relative URL will be joined by a single slash / in the absolute URL.

parse url

How to extract the protocol, host, port, path, query string, and fragment from a URL. How to extract the parameters from the query string.

url encode/decode

How to URL encode and URL unencode a string.

URL encoding, also called percent encoding, is described in RFC 3986. It replaces all characters except for the letters, digits, and a few punctuation marks with a percent sign followed by their two digit hex encoding. The characters which are not escaped are:

A-Z a-z 0-9 - _ . ~

URL encoding can be used to encode UTF-8, in which case each byte of a UTF-8 character is encoded separately.

When form data is sent from a browser to a server via an HTTP GET or an HTTP POST, the data is percent encoded but spaces are replaced by plus signs + instead of %20. The MIME type for form data is application/x-www-form-urlencoded.

base64 encode/decode

How to encode binary data in ASCII using the Base64 encoding scheme.

A popular Base64 encoding is the one defined by RFC 2045 for MIME. Every 3 bytes of input is mapped to 4 of these characters: [A-Za-z0-9/+].
If the input does not consist of a multiple of three characters, then the output is padded with one or two hyphens: =.

Whitespace can inserted freely into Base64 output; this is necessary to support transmission by email. When converting Base64 back to binary whitespace is ignored.

Unit Tests

test class

run tests, run test method

equality assertion

approximate assertion

regex assertion

exception assertion

setup

teardown

Debugging and Profiling

check syntax

flags for stronger and strongest warnings

lint

debugger

debugger commands

benchmark code

profile code

Perl

perldoc
core modules

The first character of a Perl variable $ @ % determines the type of value that can be stored in the variable: scalar, array, hash. Using an array variable @foo in a scalar context yields the size of the array, and assigning a scalar to an array will modify the array to contain a single element. $foo[0] accesses the first element of the array @foo, and $bar{'hello'} accesses the value stored under 'hello' in the hash %bar. $#foo is the index of the last element in the array @foo.

Scalars can store a string, integer, or float. If an operator is invoked on a scalar which contains an incorrect data type, perl will perform an implicit conversion to the correct data type: non-numeric strings evaluate to zero.

Scalars can also contain a reference to a variable, which can be created with a backslash: $baz = \@foo; The original value can be dereferenced with the correct prefix: @$baz. References are how perl creates complex data structures, such as arrays of hashes and arrays of arrays. If $baz contains a reference to an array, then $baz->[0] is the first element of the array. if $baz contains a reference to a hash, $baz->{'hello'} is the value indexed by 'hello'.

The literals for arrays and hashes are parens with comma separated elements. Hash literals must contain an even number of elements, and the => operator can be used in placed of a comma between a key and its value. Square brackets, e.g. [1, 2, 3], create an array and return a reference to it, and curly brackets, e.g. {'hello' => 5, 'bye' => 3} , create a hash and return a reference to it.

By default Perl variables are global. They can be made local to the containing block with the my keyword or the local keyword. my gives lexical scope, and local gives dynamic scope. Also by default, the perl interpreter creates a variable whenever it encounters a new variable name in the code. The use strict; pragma requires that all variables be declared with my, local, or our. The last is used to declare global variables.

Perl functions do not declare their arguments. Any arguments passed to the function are available in the @_ array, and the shift command will operate on this array if no argument is specified. An array passed as an argument is expanded: if the array contains 10 elements, the callee will have 10 arguments in its @_ array. A reference (passing \@foo instead of @foo) can be used to prevent this.

Some of Perl’s special variables:

• $$: pid of the perl process
• $0: name of the file containing the perl script (may be a full pathname)
• $@: error message from last eval or require command
• $& $` $’: what last regex matched, part of the string before and after the match
• $1 … $9: what subpatterns in last regex matched

Lua

Lua 5.1 Reference Manual
Lua Tutorial Directory

The Lua REPL is actually a REL; that is, it doesn't print the value of statement. Furthermore expressions cannot in general be used in the position of a statement. An expression can be converted to a statement and printed by preceding it with an equals sign:

> =1 + 1
2
> ="lorem ipsum"
lorem ipsum

There is no built-in pretty printer for tables. All numbers are stored as double precision floats. The table type can be used as both an array and a dictionary.

Groovy

User Guide

An interpreted Groovy script can be nothing more than a few top level statements which are executed in order. The Groovy interpreter handles providing an entry point class and main method, compiling the source to Java byte code, and launching the JVM.

$ cat > hello.groovy
s = "Hello, World!"
println(s)

$ groovy hello.groovy
Hello, World!

Groovy classes can be placed in their own files and compiled to Java byte code explicitly. The resulting .class files depend on the groovy.jar and the asm.jar but otherwise can be used like .class files compiled from Java source files.

$ cat > Greeting.groovy
class Greeting {
  void say(name) {
    println("Hello, ${name}!")
  }
}

$ cat > UseGreeting.java
public class UseGreeting {
  public static void main(String[] args) {
    Greeting g = new Greeting();
    g.say(args[0]);
  }
}

$ groovyc Greeting.groovy

$ javac UseGreeting.java

$ java -cp.:/PATH/TO/groovy-2.2.1.jar:/PATH/TO/asm-4.1.jar UseGreeting Fred
Hello, Fred!

All Groovy values have a Java type which can be inspected at runtime with the getClass() method.

A Groovy variable can optionally be declared to have a type. This places a constraint on the values which can be stored in the variable which is enforced at run time. The following code generates a org.codehaus.groovy.runtime.typehandling.GroovyCastException:

Integer i = 'foo'

Groovy does not use any of the Java primitive types directly: byte, short, int, long, float, double, boolean, char. Instead it used it uses the wrapper classes: Byte, Short, Integer, Long, Float, Double, Boolean, Char. Nevertheless it is still possible to use the primitive type in a variable declaration.

Groovy performs the following imports:

import java.lang.*
import java.util.*
import java.io.*
import java.net.*
import groovy.lang.*
import groovy.util.*
import java.math.BigInteger
import java.math.BigDecimal