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

ruby arrays iteration
brad picture brad · Nov 21, 2009 · Viewed 63.4k times · Source

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

Mark Byers picture Mark Byers · Nov 21, 2009

Use map to create a new array from the old one:

arr2 = arr.map {|item| item * 3}

Use map! to modify the array in place:

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

See it working online: ideone

Related questions

How can I use Array#delete while iterating over the array?

I have an array that I want to iterate over and delete some of the elements. This doesn't work: a = [1, 2, 3, 4, 5] a.each do |x| next if x < 3 a.delete x # do something with x end a #=> [1, 2, 4] I want …

Read More
Get every n-elements of array

How to select every n-elements (e.g. six) of an array using Ruby 1.9+? Example: a = [0, 1, 2, 3, 4, ... , 33] # solution # => [[0, 1, 2, 3, 4, 5], [6, 7, 8, 9, 10, 11], ... ]

Read More
How to check if a value exists in an array in Ruby

I have a value 'Dog' and an array ['Cat', 'Dog', 'Bird']. How do I check if it exists in the array without looping through it? Is there a simple way of checking if the value exists, nothing more?

Read More