Question

How do reject! and reject differ from delete_if for a Hash in Ruby? Can anyone explain the differences between them with simple code snippets?

Answer 1

Since the other answers are referring to Array#delete_if and not Hash#delete_if, which seems to be what you are asking, I thought I should clarify.

As others have pointed out, reject and reject! differ in that reject! version modifies the hash in-place, while reject creates a new hash. Meanwhile delete_if is almost the same as reject!.

In fact, for an Array, reject! and delete_if are exactly the same.

However, for a Hash, they are slightly different. reject! returns nil if no changes were made, or the hash if changes were made. delete_if always returns the hash.

hash = {a: 1, b: 2, c: 3}

return_value = hash.delete_if {|k, v| v > 100}
# hash is unchanged, return_value is {a: 1, b: 2, c: 3}

return_value = hash.reject! {|k, v| v > 100}
# hash is unchanged, return_value is nil

So if you wanted to check whether changes were made to the hash after deleting the elements, you could use reject! and check the return value.