How to map/collect with index in Ruby?

Question

What is the easiest way to convert

[x1, x2, x3, ... , xN]

to

[[x1, 2], [x2, 3], [x3, 4], ... , [xN, N+1]]

Answer 1

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] }

Answer 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]]

Answer 3

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| ... }

Answer 4

Over the top obfuscation:

arr = ('a'..'g').to_a
indexes = arr.each_index.map(&2.method(:+))
arr.zip(indexes)

Answer 5

I have always enjoyed the syntax of this style:

a = [1, 2, 3, 4]
a.each_with_index.map { |el, index| el + index }
# => [1, 3, 5, 7]

Invoking each_with_index gets you an enumerator you can easily map over with your index available.

Answer 6

Here are two more options for 1.8.6 (or 1.9) without using enumerator:

# Fun with functional
arr = ('a'..'g').to_a
arr.zip( (2..(arr.length+2)).to_a )
#=> [["a", 2], ["b", 3], ["c", 4], ["d", 5], ["e", 6], ["f", 7], ["g", 8]]

# The simplest
n = 1
arr.map{ |c| [c, n+=1 ] }
#=> [["a", 2], ["b", 3], ["c", 4], ["d", 5], ["e", 6], ["f", 7], ["g", 8]]

Answer 7

A fun, but useless way to do this:

az  = ('a'..'z').to_a
azz = az.map{|e| [e, az.index(e)+2]}

Answer 8

a = [1, 2, 3]
p [a, (2...a.size+2).to_a].transpose

Answer 9

module Enumerable
  def map_with_index(&block)
    i = 0
    self.map { |val|
      val = block.call(val, i)
      i += 1
      val
    }
  end
end

["foo", "bar"].map_with_index {|item, index| [item, index] } => [["foo", 0], ["bar", 1]]

Answer 10

I often do this:

arr = ["a", "b", "c"]

(0...arr.length).map do |int|
  [arr[int], int + 2]
end

#=> [["a", 2], ["b", 3], ["c", 4]]

Instead of directly iterating over the elements of the array, you're iterating over a range of integers and using them as the indices to retrieve the elements of the array.