Map is a method that's available on Enumarable object, and one of them is array. It takes a block as its argument, and it'll produce a new object or modify the old one depending on how we call it. The resulting object is obtained from what's returned on the block that we send, because the block will be called for each element of Enumerable object that we have.
Using map from an array. Pay attention to the array variable. Even though we've performed map on it, the elements aren't changed.
irb> array = [1, 3, 5, 7, 9]
=> [1, 3, 5, 7, 9]
irb> array.map { |a| a * a }
=> [1, 9, 25, 49, 81]
irb> array
=> [1, 3, 5, 7, 9]Using map! from an array. This time, the elements of array variable will be changed.
irb> array = [1, 3, 5, 7, 9]
=> [1, 3, 5, 7, 9]
irb> array.map! { |a| a * a }
=> [1, 9, 25, 49, 81]
irb> array
=> [1, 9, 25, 49, 81]