Decrementing for loop in coffeescript

Question

I know how to do a incrementing for loop in coffeescript such as:

Coffeescript:

for some in something

Generated Javascript:

for (_i = 0, _len = something.length; _i < _len; _i++)

How do I create a decrementing for loop similar to this in Coffeescript?

for (var i = something.length-1; i >= 0; i--)

Answer 1

There doesn't seem to be an elegant way to loop in reverse.

I checked the GitHub ticket regarding this, but it has been closed: https://github.com/jashkenas/coffee-script/issues/1187

The syntax used to be:

for some in something by -1

But it has been removed in recent versions. EDIT: it works now (for 1.6.2 @time of edit)

Answer 2

Idiomatic way (from docs) along the lines of:

lst = ['a', 'b', 'c']
for n in (num for num in [lst.length-1..0])
  alert lst[n]

(Edited after @Trevor's note)

Edit:

Although, if performance is critical, this equivalent but maybe less aesthetic snippet will generate less resulting javascript:

lst = [1,2,3]
i = lst.length
alert lst[i] while i--

Answer 3

A different take just for the record:

i = something.length
while item = something[--i]
  #use item

(will break on falsy values)

Answer 4

EDIT: As of CoffeeScript 1.5 by -1 syntax is supported.

First, you should familiarize yourself with the by keyword, which lets you specify a step. Second, you have to understand that the CoffeeScript compiler takes a very naïve approach to loop endpoints (see issue 1187, which Blender linked to), which means that

for some in something by -1 # don't do this!!!

will result in an infinite loop—it starts at index 0, increments the index by -1, and then waits until the index hits something.length. Sigh.

So you need to use the range loop syntax instead, which lets you specify those endpoints yourself—but also means you have to grab the loop items yourself:

for i in [something.length - 1..0] by -1
  some = something[i]

Obviously that's pretty messy. So you should strongly consider iterating over something.reverse() instead. Just remember that reverse() modifies the array that you call it on! If you want to preserve an array but iterate over it backwards, you should copy it:

for some in something.slice(0).reverse()

Answer 5

For a decrementing, index-based for loops, you can use:

for i in [0...something.length].reverse()

This removes the messiness @TrevorBurnham mentioned is an issue with specifying endpoints yourself.

When something.length is 0, this is equivalent to

for i in []

Answer 6

As of coffee-script 1.5.0 this is supported:

for item in list by -1
  console.log item

This will translate into

var item, _i;
for (_i = list.length - 1; _i >= 0; _i += -1) {
  item = list[_i];
  console.log(item);
}