for loop with range in CoffeeScript

Question

Noob question. I am trying to write a for loop with a range. For example, this is what I want to produce in JavaScript:

var i, a, j, b, len = arr.length;
for (i = 0; i < len - 1; i++) {
    a = arr[i];
    for (j = i + 1; i < len; j++) {
        b = arr[j];
        doSomething(a, b);
    }
}

The closest I've come so far is the following, but

It generates unnecessary and expensive slice calls
accesses the array length inside the inner loop

CoffeeScript:

for a, i in a[0...a.length-1]
    for b, j in a[i+1...a.length]
        doSomething a, b

Generated code:

var a, b, i, j, _i, _j, _len, _len1, _ref, _ref1;

_ref = a.slice(0, a.length - 1);
for (i = _i = 0, _len = _ref.length; _i < _len; i = ++_i) {
  a = _ref[i];
  _ref1 = a.slice(i + 1, a.length);
  for (j = _j = 0, _len1 = _ref1.length; _j < _len1; j = ++_j) {
    b = _ref1[j];
    doSomething(a, b);
  }
}

(How) can this be expressed in CoffeeScript?

epidemian · Accepted Answer · 2013-02-15 02:43:18Z

10

Basically, transcribing your first JS code to CS:

len = arr.length
for i in [0...len - 1] by 1
  a = arr[i]
  for j in [i + 1...len] by 1
    b = arr[j]
    doSomething a, b

answered Feb 15, 2013 at 2:43

epidemian

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2 Comments

alekop Over a year ago

Thanks, that works. I don't understand why it still generates a few unnecessary loop counters, but I can live with that.

epidemian Over a year ago

@alekop I think the main rational for those extra counters is that if you happen to modify the loop variable (i or j) inside the loop, you don't change the loop's iterations.

Slai · Accepted Answer · 2017-10-02 18:40:07Z

1

Seems like the only way to avoid the extra variables is with a while loop http://js2.coffee

i = 0
len = arr.length 

while i < len - 1
  a = arr[i]
  j = i + 1
  while j < len
    b = arr[j]
    doSomething a, b
    j++
  i++

or a bit less readable:

i = 0; len = arr.length - 1
while i < len
  a = arr[i++]; j = i
  while j <= len
    doSomething a, arr[j++]

edited Oct 2, 2017 at 18:40

answered Sep 29, 2017 at 23:03

Slai

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2 Comments

alekop Over a year ago

That's not exactly what the original code does. The outer loop range is 0..n-1, while the inner range is i+1..n.

Slai Over a year ago

@alekop sorry about that .. I didn't read the question carefully.

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