Trying to convert nested loop to recursive function

Question

I'm trying to create a recursive version of the following nested loops and to get the same results as the reference code. The example is below.

This is a version on Codepen http://codepen.io/anon/pen/XbQMLv

(The intent of the code is to output only unique combinations of integers from the indexes.)

Original code and output:

var len = 4;

for (var a = 0; a < len; a++) {
  for (var b = a + 1; b < len; b++) {
    for (var c = b + 1; c < len; c++) {
      console.log(a, b, c);
    }
  }
}
// Outputs:
// 0 1 2
// 0 1 3
// 0 2 3
// 1 2 3

Recursive code and output:

var len = 4;
var end = 3;
var data = [];

var loop = function (index) {
  if (index === end) {
    console.log(data);
    return;
  }
  for (var i = index; i < len; i++) {
    data[index] = i;
    loop(i + 1);
  }
}

loop(0);
// Outputs:
// [ 0, 1, 2 ]
// [ 0, 2, 3 ]
// [ 1, 3, 2 ]
// [ 2, 3, 3 ]

Not sure what I'm missing here.

Answer 1

You have a single little error in your code:

You call a recursive function from your i + 1, but not your index + 1.
It causes index to be equal not current array index but it's value.

For example, when you passed [0, 1, 2], your data now is [0, 1] and you are about to insert 3, you call loop(3 + 1), index 4 goes out of an array range. if (index === end) condition fails and it doesn't output. for (var i = index; i < len; i++) loop fails as well, and everything is going wrong.

It should be:

var len = 4;
var end = 3;
var data = [];

var loop = function (index) {
  if (index === end) {
    console.log(data);
    return;
  }
  for (var i = index; i < len; i++) {
    data[index] = i;
    loop(index + 1); // <--- HERE
  }
}

loop(0);

Here is the working JSFiddle demo.

Update:

Oh, now I see. You need a[i] > a[i-1] condition to be true for all combinations. Just add a start variable which will save the last inserted value in order to comply with this rule.

var len = 4;
var end = 3;
var data = [];

var loop = function (start, index) {
  if (index === end) {
    document.body.innerHTML += "<br/>" + data;
    return;
  }
  for (var i = start; i < len; i++) { // We start from 'start' (the last value + 1)
    data[index] = i;
    loop(i + 1, index + 1); // Here, we pass the last inserted value + 1
  }
}

loop(0, 0); // At beginning, we start from 0

Updated JSFiddle demo with passing argument.

You can check the previous value instead of passing a value as an argument if it looks wrong for you. Condition will be like "if it is a first number, start from 0; else - start from the next number after the previous one".

var start = (index === 0 ? 0 : data[index-1] + 1);  

Updated JSFiddle demo with calculating start.

Answer 2

As you are having three for loops one inside another, you have to pass 2 arguments for recursion function.

var len = 4;
var end = 3;
var data = [];

var loop = function(start, index) {
  if (index === end) {
    console.log(data);
    return;
  }
  for (var i = start; i < len; i++) {
    data[index] = i;
    loop(i + 1, index + 1); //Pass as like a+1 & b+1
  }
}

loop(0, 0);