Comparison of programming languages (list comprehension)

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[edit] List Comprehensions

List comprehension is a syntactic construct available in some programming languages for creating a list based on existing lists. It follows the form of the mathematical set-builder notation (set comprehension.) as distinct from the use of map and filter functions.

[edit] Boo

List with all the doubles from 0 to 10 (exclusive)

doubles = [i*2 for i in range(10)]

List with the names of the customers based on Rio de Janeiro

rjCustomers = [customer.Name for customer in customers if customer.State == "RJ"]

[edit] C#

var ns = from x in Enumerable.Range(0,100)
         where x*x > 3
         select x*2;

The previous code is syntactic sugar for the following code written using lambda expressions:

var ns = Enumerable.Range(0, 100)
        .Where(x => x*x > 3)
        .Select(x => x*2);

[edit] Clojure

An infinite lazy sequence:

(for [x (iterate inc 0) :when (> (* x x) 3)] (* 2 x))

A list comprehension using multiple generators:

(for [x (range 20), y (range 20), z (range 20) :when (== (+ (* x x) (* y y)) (* z z))] [x y z])

[edit] Common Lisp

List comprehensions can be expressed with the loop macro's collect keyword. Conditionals are expressed with if, as follows:

(loop for x from 0 to 100 if (> (* x x) 3) collect (* 2 x))

[edit] Erlang

L = lists:seq(0,100).
S = [2*X || X <- L, X*X > 3].

[edit] F#

{ for x in 0 .. 100 when x*x > 3 -> 2*x }

Or, more correctly for floating point values

{ for x in 0.0 .. 100.0 when x**2.0 > 3.0 -> 2.0*x }

[edit] Haskell

An example of a list comprehension using multiple generators:

pyth = [(x,y,z) | x <- [1..20], y <- [x..20], z <- [y..20], x^2 + y^2 == z^2]

[edit] JavaScript

Borrowing from Python, JavaScript 1.7 and later have array comprehensions.^[1] Although this feature has been proposed for inclusion in the fourth edition ECMAScript standard, Mozilla is the only implementation that currently supports it.

/* There is no "range" function in JavaScript's standard
   library, so the application must provide it. */
function range(n) {
  for (var i = 0; i < n; i++)
    yield i;
}
 
[2 * x for (x in range(100)) if (x * x > 3)]

JavaScript 1.8 adds Python-like generator expressions.

[edit] Nemerle

$[x*2 | x in [0 .. 100], x*x > 3]

[edit] Perl

List comprehensions are supported in Perl through the use of the List::Comprehensions CPAN module.^[2]

[edit] Python

The Python programming language uses the following syntax for expressing list comprehensions over finite lists:

S = [2*x for x in range(100) if x**2 > 3]

A generator expression may be used in Python versions >= 2.4 which gives lazy evaluation over its input, and can be used with generators to iterate over 'infinite' input such as the count generator function which returns successive integers:

from itertools import count
S = (2*x for x in count() if x**2 > 3)

(Subsequent use of the generator expression will determine when to stop generating values).

[edit] R

x = (0:100)
S = 2 * x[x ^ 2 > 3]

[edit] Scala

Using the for-comprehension:

val s = for (x <- Stream.from(0); if x*x > 3) yield 2*x

[edit] Visual Prolog

S = [ 2*X || X = list::getMember_nd(L), X*X > 3 ]

[edit] Windows PowerShell

$s = ( 0..100 | ? {$_*$_ -gt 3} | % {2*$_} )

[edit] References