September 24, 2009

Filtering lists

Recently, my friend Gav wrote about using STL to filter a vector of values in C++ in which he explained a surprising gotcha. I’m sure he knows what he’s talking about, but it struck me how ugly this (presumably idomatic) code was. So I figured I’d see what it would look like in a few more “modern” languages:

Ruby

>> numbers = 1..9
=> 1..9
>> numbers.reject { |n| n.even? }
=> [1, 3, 5, 7, 9]

Or, if you skip the separate assignment of the input data:

>> (1..9).reject { |n| n.even? }
=> [1, 3, 5, 7, 9]

Python

>>> numbers = range(1,10)
>>> [n for n in numbers if n % 2]
[1, 3, 5, 7, 9]

>>> [n for n in range(1, 10) if n % 2]
[1, 3, 5, 7, 9]

Clojure

user=> (def numbers (range 1 10))
#'user/numbers
user=> (filter odd? numbers)
(1 3 5 7 9)

user=> (filter odd? (range 1 10))
(1 3 5 7 9)

Yeah, I get that this wasn’t the point of the original post–sometimes you’re just stuck with C++. But if you do have the choice, other languages can be far more expressive for this common kind of list processing.

If you have examples in other languages (or improvements to my efforts) send them in and I’ll post them here.

Update: From Julian Doherty:

Erlang

1> Numbers = lists:seq(1,9).
[1,2,3,4,5,6,7,8,9]
2> [X || X <- Numbers, X rem 2 =/= 0].
[1,3,5,7,9]

Update: From Ben MacLeod:

C#

using System;
using System.Linq;

// ...

    var numbers = Enumerable.Range(1, 10).Where(n => n % 2 != 0);
    // or, equivalently:
    //var numbers = (from n in Enumerable.Range(1, 10) where n % 2 != 0 select n);
    foreach(var number in numbers) {
        Console.WriteLine(number);
    }

// ...

Update: From John Carney:

PHP

5.2

function not_even($x) {
    return $x & 1 ;
}

$numbers = array(1, 2, 3, 4, 5, 6, 7, 8, 9) ;
$numbers = array_filter($numbers, "not_even") ;

5.3

$numbers = array(1, 2, 3, 4, 5, 6, 7, 8, 9) ;
$numbers = array_filter($numbers, function($x) { return $x & 1 ; }) ;

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