Create an array with all numbers from min to max without a loop

Question

I have two numbers, min and max, and I want to create an array that contains all number between them (including min and max).

The most obvious approach is to use a for loop for this, and push the single values onto an array. Nevertheless, this seems to be a quite naive approach, i.e. it's imperative programming.

Now I was thinking of how to create such an array in a more functional style. Basically, something such as the reverse of a reduce function: Instead of reducing an array to a number, building up an array from two numbers.

How could I do this? What is a functional approach to solve this problem?

Basically, I'm thinking of something such as 10..20 in some other languages. What's the most elegant equivalent for this in JavaScript?

Both ;-). It should be functional (from a technical point of view), but well readable as well. — Golo Roden, Mar 18 '14 at 12:00

score 10 · Answer 1 · edited May 23 '17 at 11:48

10

Inspired by this

var min = 3, max = 10;
var x = Array.apply(null, {length: max + 1}).map(Number.call, Number).slice(min);
console.log(x);
// [ 3, 4, 5, 6, 7, 8, 9, 10 ]

The optimum version

var min = 3, max = 10;
var x = Array.apply(null, {length: max + 1 - min}).map(function(_, idx) {
    return idx + min;
});
console.log(x);
// [ 3, 4, 5, 6, 7, 8, 9, 10 ]

edited May 23 '17 at 11:48

Community

1
1

answered Mar 18 '14 at 10:23

thefourtheye

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This works. It's only drawback IMHO is that you first create an array with all number from `1` to `max` (which is not so very nice if you need only ten numbers which start at 1 billion ;-)). But apart from that: Nice solution :-) – Golo Roden Mar 18 '14 at 10:27
2

It seems overly complicated. A for loop does the job as well, minus the useless complexity. – Virus721 Mar 18 '14 at 10:40
You should use `slice` instead of `splice` – Bergi Mar 18 '14 at 11:50
@Bergi Got it. Updated. :) – thefourtheye Mar 18 '14 at 12:24
@Virus721 I was asking for a version without loops ;-) – Golo Roden Mar 18 '14 at 13:38
1

@GoloRoden Did you check the optimum version, that creates an array of required size only. So, it doesn't waste memory. – thefourtheye Mar 18 '14 at 13:53
@Virus721 Its a style of programming called functional programming. Almost everything is done with functions. – thefourtheye Mar 18 '14 at 14:00

score 5 · Answer 2 · answered Feb 12 '21 at 13:32

A one line way. Inspired by How to create an array containing 1...N.

Array.from({length: max-min+1}, (_, i) => i + min);

Initiate an array from a computed length based on max and min. Then map each index value by a function which is "index + min". The reference is here.

Example:

const min = 12;
const max = 20;
const arr = Array.from({length: max-min+1}, (_, i) => i + min);

score 4 · Accepted Answer · edited Mar 18 '14 at 12:22

4

You can think of a "functional" definition of range:

range(low, hi) = [], if low > hi
range(low, hi) = [low] (+) range(low+1,hi), otherwise,

which leads to the JS definition:

function range(low,hi){
  function rangeRec(low, hi, vals) {
     if(low > hi) return vals;
     vals.push(low);
     return rangeRec(low+1,hi,vals);
  }
  return rangeRec(low,hi,[]);
}

edited Mar 18 '14 at 12:22

Random Dev

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answered Mar 18 '14 at 11:34

Mau

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1

I think, you can remove one more line and do `return rangeRec(low+1,hi, vals.concat(low));` – thefourtheye Mar 18 '14 at 12:39
2

@thefourtheye: Yes, that would be much more functional (and you should move `rangeRec` outside of `range` then), but creating a new array on each iteration slows it down dramatically (`O(n²)` runtime) – Bergi Mar 18 '14 at 12:50

user568109 · Answer 4 · 2014-03-18T14:21:22.070

1

If you have harmony generators you can use this:

function* range(lorange,hirange){
  var n = lorange;
  while (n <= hirange){
    yield n++;
  }
}

rval= range(3,6);

Now you can :

Use the for-of comprehension for iterators as substitute of array
```
for (i of rval)
console.log(i);

3
4
5
6
```
Or you can use it to create an array like you want
```
rarray = [];
for (i of rval)
rarray.push(i);
```

edited Mar 18 '14 at 14:21

answered Mar 18 '14 at 10:40

user568109

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The idea is great :-)). Unfortunately, I need such an ranged array to explain what generators are about ;-). – Golo Roden Mar 18 '14 at 12:01
@GoloRoden This is a good substitute for the 10..20 range interpolation. It is like the xrange in python 2.7 which returns elements only when needed. – user568109 Mar 18 '14 at 12:43

score 1 · Answer 5 · answered Nov 09 '21 at 16:45

1

Might be late to the party, but, based on this (es6)

const rangexy = (start, end) => Array.from({length: (end+1 - start)}, (v, k) => k + start);

answered Nov 09 '21 at 16:45

aazev

86
5

And then, just after posting I realized @juminet posted a very similar one. Hehe – aazev Nov 09 '21 at 16:46

score 0 · Answer 6 · answered Dec 07 '22 at 14:51

0

Here is good exmaples.

const range = (min, max) => [...Array(max - min + 1).keys()].map((i) => i + min);

answered Dec 07 '22 at 14:51

TiiGRUS

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Create an array with all numbers from min to max without a loop

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