each
One uses the #each method when in need to yield a reference to each element in the collection.
arr = [1, 2, 3, 4]
arr.each do |num|
puts num
end
On line 3-5 we are calling the each method on the array assigned to local variable arr, passing in the do..end block as an argument. The block is executed once for each element in the collection it is called on, passing that element as a parameter. Afterwards, the each method returns the collection itself.
Sometimes, one needs to use the element as well as the current element numerical index. For this, there is the #each_with_index method.
arr = [1, 2, 3, 4]
arr.each_with_index do |num, index|
puts "#{index}: #{num}"
end
Note that #each returns the collection it’s called upon, as-is.
map
Like #each, the #map method iterates over a collection and apply a block to each element of it. The difference is it also returns a new array with the results of each block.
Refer to the irb session below to see the difference in the return value of bo th methods.
irb :001 > arr = [1, 2, 3, 4]
irb :002 > arr.each { |num| num + 1 }
=> [1, 2, 3, 4]
irb :003 > arr.map { |num| num + 1 }
=> [2, 3, 4, 5]
On line 3, we are calling the map method on an array assigned to local variable arr, passing in the {} block as an argument. The block is executed once for each element in the collection, passing that element as a parameter. Afterwards, the map method returns a new array containing the return value of each block.
#each returns the collection it’s called upon, as we saw earlier, while #map returns a new array with the results of each block.
#map as an alias called #collect.
select
Like #each, the #select method iterates over a collection and returns a new collection, however it doesn’t populate this new collection with the results of each block. Rather it select results of each block if they return true to the provided expression.
irb :001 > arr = [1, 2, 3, 4]
irb :002 > arr.select { |num| num < 3 }
=> [1, 2]
On line 2 we are calling the select method on the array assigned to local variable arr, passing in the {} block as an argument. The block is executed once for every element of the collection, passing in that element as a parameter. Afterwards, the select method returns a new collection of element for which the return value of the {} block evaluates to true.
On this example, the expression returns true for the first two elements (1 and 2); #select returns them in a new array.