Comparison of programming languages (list comprehension)

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.

Examples of list comprehension

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"]

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);

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])

CoffeeScript

largeNumbers = (number for number in list when number > 100)

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))

Cobra

List the names of customers:

names = for cust in customers get cust.name

List the customers with balances:

names = for cust in customers where cust.balance > 0

List the names of customers with balances:

names = for cust in customers where cust.balance > 0 get cust.name

The general forms:

for VAR in ENUMERABLE [where CONDITION] get EXPR
for VAR in ENUMERABLE where CONDITION

Note that by putting the condition and expression after the variable name and enumerable object, editors and IDEs can provide autocompletion on the members of the variable.

Elixir

for x <- 0..100, x * x > 3, do: x * 2

Erlang

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

F#

Lazily-evaluated sequences:

seq { for x in 0 .. 100 do if x*x > 3 then yield 2*x }

Or, for floating point values

seq { for x in 0. .. 100. do if x**2. > 3. then yield 2.*x }

Lists and arrays:

[ for x in 0. .. 100. do if x**2. > 3. then yield 2.*x ]
[| for x in 0. .. 100. do if x**2. > 3. then yield 2.*x |]

List comprehensions are the part of a greater family of language constructs called computation expressions.

Groovy

(0..100).findAll{ x -> x * x > 3 }.collect { x -> 2 * x }

Haskell

[x * 2 | x <- [0 .. 99], x * x > 3]

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]

Io

By using Range object, Io language can create list as easy as in other languages:

Range 0 to(100) asList select(x, x*x>3) map(*2)

ISLISP

List comprehensions can be expressed with the for special form. Conditionals are expressed with if, as follows:

(for ((x 0 (+ x 1))
      (collect ()))
     ((>= x 100) (reverse collect))
     (if (> (* x x) 3)
         (setq collect (cons (* x 2) collect))))

JavaScript

Borrowing from Python, JavaScript has array comprehensions.^[1] Although this feature has been proposed for inclusion in the sixth 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
  using a Python-inspired generator function */
function* range(n) {
  for (var i = 0; i < n; i++)
    yield i;
}

[2 * x for (x of range(100)) if (x * x > 3)]

Julia

Julia supports comprehensions using the syntax:

 y = [x^2+1 for x in 1:10]

and multidimensional comprehensions like:

 z = [(x-5)^2+(y-5)^2 for x = 0:10, y = 0:10]

Mythryl

 s = [ 2*i for i in 1..100 where i*i > 3 ];

Multiple generators:

 pyth = [ (x,y,z) for x in 1..20 for y in x..20 for z in y..20 where x*x + y*y == z*z ];

Nemerle

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

OCaml

OCaml supports List comprehension through OCaml Batteries. ^[2]

Perl 6

my @s = ($_ * 2 if $_ ** 2 > 3 for 0 .. 99);

Python

Python uses the following syntax to express 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).

R

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

Racket

(for/list ([x 100] #:when (> (* x x) 3)) (* x 2))

An example with multiple generators:

(for*/list ([x (in-range 1 21)] [y (in-range 1 21)] [z (in-range 1 21)]
            #:when (= (+ (* x x) (* y y)) (* z z)))
  (list x y z))

Ruby

(0..100).select { |x| x**2 > 3 }.map { |x| 2*x }

Scala

Using the for-comprehension:

val s = for (x <- 0 to 100; if x*x > 3) yield 2*x

Scheme

List comprehensions are supported in Scheme through the use of the SRFI-42 library.^[3]

(list-ec (: x 100) (if (> (* x x) 3)) (* x 2))

An example of a list comprehension using multiple generators:

(list-ec (: x 1 21) (: y x 21) (: z y 21) (if (= (+ (* x x) (* y y)) (* z z))) (list x y z))

SETL

s := {2*x : x in {0..100} | x**2 > 3 };

Smalltalk

((1 to: 100) select: [ x | x squared > 3 ]) collect: [ x | x * 2 ]

Visual Prolog

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

Windows PowerShell

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

which is short-hand notation of:

$s = 0..100 | where {$_*$_ -gt 3} | foreach {2*$_}

References

External links

• Comparison of list comprehensions on rosettacode.org