How do I modify an array while I am iterating over it in Ruby?

Question

I'm just learning Ruby so apologies if this is too newbie for around here, but I can't work this out from the pickaxe book (probably just not reading carefully enough). Anyway, if I have an array like so

arr = [1,2,3,4,5]

and I want to, say, multiply each value in the array by 3, I have worked out that doing the following

arr.each {|item| item *= 3}

will not get me what I want (and I understand why, I'm not modifying the array itself).

What I don't get is how to modify the original array from inside the code block after the iterator. I'm sure this is very easy.

Answer 1

You should probably use map instead of directly modifying the array:

arr = arr.map {|item| item * 3}

If you really want to modify the array directly, do this:

arr.each_with_index {|item, index| arr[index] = item * 3}

Answer 2

To directly modify the array, use arr.map! {|item| item*3}. To create a new array based on the original (which is often preferable), use arr.map {|item| item*3}. In fact, I always think twice before using each, because usually there's a higher-order function like map, select or inject that does what I want.