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?

Answer 1

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 ]

Answer 2

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,[]);
}

Answer 3

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);

Answer 4

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 :

1. Use the for-of comprehension for iterators as substitute of array

   for (i of rval)
   console.log(i);
   
   3
   4
   5
   6
   

2. Or you can use it to create an array like you want

   rarray = [];
   for (i of rval)
   rarray.push(i);
   

Answer 5

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);