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

1. It generates unnecessary and expensive slice calls
2. 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?

Answer 1

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

Answer 2

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++]