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.
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 in Rio de Janeiro
rjCustomers = [customer.Name for customer in customers if customer.State == "RJ"]
Further information: C# 3.0 Language Integrated Query |
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);
Further information: Ceylon, Ceylon comprehensions |
Filtering numbers divisible by 3:
value divisibleBy3 = { for (i in 0..100) if (i%3==0) i };
// type of divisibleBy3 is Iterable<Integer>
Multiple "generators":
value triples = { for (x in 0..20) for (y in x..20) for (z in y..20) if (x*x + y*y == z*z) [x,y,z] };
// type of triples is Iterable<Integer[3]>
Further information: List comprehension § 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])
Further information: CoffeeScript |
largeNumbers = (number for number in list when number > 100)
Further information: 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))
Further information: 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.
Further information: Dart, Dart lists |
[for (var i in range(0, 100)) if (i * i > 3) i * 2]
var pyth = [
for (var x in range(1, 20))
for (var y in range(x, 20))
for (var z in range(y, 20)) if (x * x + y * y == z * z) [x, y, z]
];
Iterable<int> range(int start, int end) =>
List.generate(end - start, (i) => start + i);
Further information: Elixir |
for x <- 0..100, x * x > 3, do: x * 2
Further information: Erlang |
L = lists:seq(0,100).
S = [2*X || X <- L, X*X > 3].
Further information: 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.
Further information: 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]
Further information: 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)
Further information: 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))))
Further information: 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]
It is also possible to add a condition:
v = [3x^2 + 2y^2 for x in 1:7 for y in 1:7 if x % y == 0]
And just changing square brackets to the round one, we get a generator:
g = (3x^2 + 2y^2 for x in 1:7 for y in 1:7 if x % y == 0)
Further information: 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 ];
Further information: Nemerle |
$[x*2 | x in [0 .. 100], x*x > 3]
Nim has built-in seq, set, table and object comprehensions on the sugar standard library module:[1]
import sugar
let variable = collect(newSeq):
for item in @[-9, 1, 42, 0, -1, 9]: item + 1
assert variable == @[-8, 2, 43, 1, 0, 10]
The comprehension is implemented as a macro that is expanded at compile time, you can see the expanded code using the expandMacro compiler option:
var collectResult = newSeq(Natural(0))
for item in items(@[-9, 1, 42, 0, -1, 9]):
add(collectResult, item + 1)
collectResult
The comprehensions can be nested and multi-line:
import sugar
let values = collect(newSeq):
for val in [1, 2]:
collect(newSeq):
for val2 in [3, 4]:
if (val, val2) != (1, 2):
(val, val2)
assert values == @[@[(1, 3), (1, 4)], @[(2, 3), (2, 4)]]
Further information: OCaml |
OCaml supports List comprehension through OCaml Batteries.[2]
Further information: Perl |
my @s = map {2 * $_} grep {$_ ** 2 > 3} 0..99;
Array with all the doubles from 1 to 9 inclusive:
my @doubles = map {$_ * 2} 1..9;
Array with the names of the customers based in Rio de Janeiro (from array of hashes):
my @rjCustomers = map {$_->{state} eq "RJ" ? $_->{name} : ()} @customers;
Filtering numbers divisible by 3:
my @divisibleBy3 = grep {$_ % 3 == 0} 0..100;
Further information: PowerShell |
$s = ( 0..100 | ? {$_*$_ -gt 3} | % {2*$_} )
which is short-hand notation of:
$s = 0..100 | where-object {$_*$_ -gt 3} | foreach-object {2*$_}
Further information: Python syntax and semantics § Comprehensions |
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).
Further information: R |
x <- 0:100
S <- 2 * x[x ^ 2 > 3]
Further information: 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))
Further information: Raku (programming language) |
my @s = ($_ * 2 if $_ ** 2 > 3 for 0 .. 99);
Further information: Scala |
Using the for-comprehension:
val s = for (x <- 0 to 100; if x*x > 3) yield 2*x
Further information: 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))
Further information: SETL |
s := {2*x : x in {0..100} | x**2 > 3 };
Further information: Smalltalk |
((1 to: 100) select: [ :x | x squared > 3 ]) collect: [ :x | x * 2 ]
Further information: Visual Prolog |
S = [ 2*X || X = list::getMember_nd(L), X*X > 3 ]