Map an Array With Index in Ruby

1. Combine map and each_with_index Methods in Ruby
2. Combine map and with_index Methods in Ruby
3. Use map Together With the Ruby Range

The map method is quite handy in Ruby, and it comes in handy when you need to transform an array into another one. For example, the code below uses map to convert an array of small letters into an array of capital letters.

Example code:

small_letters = ['a', 'b', 'c', 'd']
capital_letters = small_letters.map { |l| l.capitalize }
puts capital_letters

Output:

["A", "B", "C", "D"]

In the above example, say we want to map and get the respective index of each of the letters in the array. How do we do that? Listed below are different ways to achieve that.

Combine map and each_with_index Methods in Ruby

The each_with_index is a Ruby enumerable method.

It does what the name indicates. It iterates through an array or hash and extracts each element with its respective index. Calling map on the result of each_with_index gives us access to the indexes.

Here is the official documentation of each_with_index for more explanation.

Example code:

small_letters = ['a', 'b', 'c', 'd']
numbered_capital_letters = small_letters.each_with_index.map { |l, i| [i + 1, l.capitalize] }
puts numbered_capital_letters

Output:

[[1, "A"], [2, "B"], [3, "C"], [4, "D"]]

Combine map and with_index Methods in Ruby

The with_index is similar to each_with_index but with slight differences.

Unlike each_with_index, with_index cannot be called directly on an array. The array needs to be converted explicitly to an Enumerator first.

Since map automatically handles the conversion, the execution order matters here, invoke .map before calling .with_index.

Example code:

small_letters = ['a', 'b', 'c', 'd']
numbered_capital_letters = small_letters.map.with_index { |l, i| [i + 1, l.capitalize] }
puts numbered_capital_letters

Output:

[[1, "A"], [2, "B"], [3, "C"], [4, "D"]]

It’s also worth mentioning that with_index accepts an argument that is the index’s offset, where the index should start from. We can make use of that argument instead of having to do i + 1 inside the {} block.

Example code:

small_letters = ['a', 'b', 'c', 'd']
numbered_capital_letters = small_letters.map.with_index(1) { |l, i| [i, l.capitalize] }
puts numbered_capital_letters

Output:

[[1, "A"], [2, "B"], [3, "C"], [4, "D"]]

Here is the official documentation of with_index for more explanation.

Use map Together With the Ruby Range

The Ruby Range is a set of values with a beginning and an end.

For example, 1..10 or 1...11 is a Range that represents a set of values from 1 to 10. We can use this together with the map method to achieve the same result as what we have in the examples above.

Example code:

small_letters = ['a', 'b', 'c', 'd']
numbered_capital_letters = (0...small_letters.length).map { |i| [i + 1, small_letters[i].capitalize]  }
puts numbered_capital_letters

Output:

[[1, "A"], [2, "B"], [3, "C"], [4, "D"]]