for loop with range in CoffeeScript - 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?

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++]