functional loop given a number instead of an array

Question

Say I have a number 18, instead of an array, in hand.

What is the best way to create a functional loop in JS given a number X instead of array of X elements?

I can do this:

[1,2,3].forEach(function(){

));

but if I have the number 3

I can do

for(var i = 0; i < 3; i++){

}

but I want that loop to be functional instead

We have moved on past functional-style and are now using generators. Back to the `for` statement! — , May 18 '16 at 04:27
@torazaburo , i am not sure if he asked the same question as you have marked it duplicate for i think he just wants to iterate x number times without creating an array or using for loop. not sure maybe. — AJS, May 18 '16 at 12:59
something like this [JSBIN](http://jsbin.com/jaxoyiweva/edit?js,console,output) — AJS, May 18 '16 at 13:08
Iterating *n* times in a "functional way" is the same, for all practical purposes, as creating an array of n numbers and then using some iterating-type function like `map` or `forEach` on the result, so yes, I would say it is a duplicate. — , May 18 '16 at 13:58

score 7 · Answer 1 · edited May 23 '17 at 12:31

7

If you have a number and you want to create a loop then you can use the number in limiter condition in the for loop.

for(var i = 0; i < number; i++)

Edit 1: you can use foreach on arrays only, in that case since you have a number already you can create a array of that length and then use the foreach on it.

var foo = new Array(number).fill(0);

foo.foreach()

Also another option is

var N = 18;

Array.apply(null, {length: N}).map(Number.call, Number)

result [0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17]

Many more options available in this thread Create a JavaScript array containing 1...N

edited May 23 '17 at 12:31

Community

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answered May 18 '16 at 04:01

Rajshekar Reddy

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yes, but I was looking for a *functional* for loop, etc :) – Alexander Mills May 18 '16 at 04:01
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@AlexMills, meaning a `forEach` style call? – Matt May 18 '16 at 04:03
@AlexMills what do you mean by functional for loop?? I never heard that before.. May be some links to it will help – Rajshekar Reddy May 18 '16 at 04:06
yes, forEach, but with a number – Alexander Mills May 18 '16 at 04:06
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`Array.apply(null, Array(number)).map(function () {}).forEach(...)` is about the closest you can get. – Matt May 18 '16 at 04:07
ok I made some edits, thanks Matt, do you think you could add that as an answer? – Alexander Mills May 18 '16 at 04:08
@Reddy. Just to note, an Array with a constructor length is not iterable, hence the `.Apply` approach. – Matt May 18 '16 at 04:21
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Actually, `new Array(5).forEach` will **not** do what you want. – May 18 '16 at 04:23

score 5 · Accepted Answer · edited Mar 05 '22 at 00:59

5

I don't understand why you want to do this. An equivalent to:

[1,2,3].forEach(function(){ ... ));

Is

var limit = n;
while (--limit) {(  // Note: 0 is falsy
    function(){ ... }
)(limit);}

Or if you really want to use an array structure, the following will do:

new Array(limit).fill(0).forEach(function(){...});

You might be interested in Myth of the Day: Functional Programmers Don't Use Loops.

edited Mar 05 '22 at 00:59

luiscla27

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answered May 18 '16 at 04:24

RobG

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Note, `.fill` is new to ES6 so might not be supported on all modern browsers just yet. – Matt May 18 '16 at 04:38
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because I need to pass the value of the index to the function – Alexander Mills May 18 '16 at 21:32
I think it's limit-- not --limit – Alexander Mills May 18 '16 at 21:38
@AlexanderMills—yes, `limit--`. The OP makes no use of the index, but if that's required, then *limit* counts down and `n - limit` counts up. – RobG Mar 05 '22 at 05:57

score 1 · Answer 3 · edited May 23 '17 at 12:23

1

Per this question, you can "functionally" iterate over a linear sequence relatively easily using:

Array.apply(null, Array(number)).map(function () {}).forEach(...)

Not sure what advantage this gives you versus a regular for-loop with an index, though it is a neat trick.

edited May 23 '17 at 12:23

Community

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answered May 18 '16 at 04:11

Matt

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I think the advantage is that it's way more code than `while(i--)...` ;-) – RobG May 18 '16 at 04:15
To be fair, the `while(i--)` is a reversed linear sequence :) – Matt May 18 '16 at 04:23
If the index matters (and it doesn't in the OP) then `while (++i < limit) ...` or `do ... while(i++ < limit)` and so on. – RobG May 18 '16 at 04:29
That's fair. I think we're a little OT at this point. – Matt May 18 '16 at 04:33
index does matter in OP :) – Alexander Mills May 18 '16 at 21:33

functional loop given a number instead of an array

3 Answers3