Select and Reject

The select method returns a new array containing only the elements for which the block returns a truthy value. It's how Ruby spells "filter".

numbers = [1, 2, 3, 4, 5, 6]
evens = numbers.select { |n| n.even? }
puts evens.inspect   # [2, 4, 6]

reject is the opposite, it keeps everything for which the block returns false:

odds = numbers.reject { |n| n.even? }
puts odds.inspect    # [1, 3, 5]

Both return a new array; the original is untouched.

The block is just a predicate, anything that evaluates to true or false works:

words = ["hi", "hello", "hey", "yo"]
long  = words.select { |w| w.length >= 3 }
puts long.inspect    # ["hello", "hey"]