How to map with index in Ruby?

If you're using ruby 1.8.7 or 1.9, you can use the fact that iterator methods like each_with_index, when called without a block, return an Enumerator object, which you can call Enumerable methods like map on. So you can do:

arr.each_with_index.map { |x,i| [x, i+2] }

In 1.8.6 you can do:

require 'enumerator'
arr.enum_for(:each_with_index).map { |x,i| [x, i+2] }

Ruby has Enumerator#with_index(offset = 0), so first convert the array to an enumerator using Object#to_enum or Array#map:

[:a, :b, :c].map.with_index(2).to_a
#=> [[:a, 2], [:b, 3], [:c, 4]]

In ruby 1.9.3 there is a chainable method called with_index which can be chained to map.

For example:

array.map.with_index { |item, index| ... }