Javascript generate logical array based on condition

Question

Coming from a matlab background, I am trying to replicate the following scenario in Javascript

A = [1, 2, 3, 4, 5];
B = 4;
C = A == B;
answer => C = [0, 0, 0, 1, 0]

In other words, it generates a logical array where only the value compared is set to 1. I can do this using a loop but I was wondering if there a 1 liner solution to this in javascript?

No there is not. JavaScript is not that kind of programming language. There's the .filter() function on the Array prototype however. (Well .map(); not enough coffee yet :) — Pointy
– Pointy, Commented Dec 17, 2013 at 14:40

Qantas 94 Heavy · Accepted Answer · 2013-12-17 14:48:16Z

4

You can use the map() function to do something similar to what you were looking for:

var A = [1, 2, 3, 4, 5];
var B = 4;
var C = function (x) { return +(x === B); };
var answer = A.map(C);

_{var C = x => +(x === B); would look cleaner, but that's ES6 code (experimental).}

edited Dec 17, 2013 at 14:48

answered Dec 17, 2013 at 14:42

Qantas 94 Heavy

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Pointy · Accepted Answer · 2013-12-17 14:41:26Z

2

About the fanciest you could get would be

var C = A.map(function(v) { return v == B ? 1 : 0; });

That's supported in newer JavaScript runtime systems.

In JavaScript it'd probably be more idiomatic to prefer a result array containing boolean values:

var C = A.map(function(v) { return v == B; });

answered Dec 17, 2013 at 14:41

Pointy

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Matt Burland · Accepted Answer · 2013-12-17 14:48:34Z

2

There's not a one-liner, but using Array.map can get you pretty close to what you want:

 var a = [1, 2, 3, 4, 5];
 var b = 4;
 var c = a.map(function(item) { return item === b? 1: 0; });

 console.log(c);

Fiddle

Note map isn't supported by older browsers, the MDN link above has polyfil code or you can include any number of libraries that provide something equivalent (e.g. jQuery has a .map() function).

edited Dec 17, 2013 at 14:48

answered Dec 17, 2013 at 14:43

Matt Burland

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1 Comment

dopplesoldner Over a year ago

Thanks for pointing out the it is only supported in newer browsers

score 1 · Accepted Answer · 2015-04-11 19:07:03Z

1

You could write your own function :

function equals(a, b) {
    var result = [];
    while (result.length < a.length) {
        result.push(+(a[result.length] == b));
    }
    return result;
}

var A = [1, 2, 3, 4, 5];
var B = 4;
var C = equals(A, B); // [0, 0, 0, 1, 0]

edited Apr 11, 2015 at 19:07

answered Dec 17, 2013 at 15:29

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