CODEWITHSUNDEEP
ruby
ab217
ab217

Reputation: 17160

What Does Ruby's Array#shift do?

I am having a hard time understanding what the shift and unshift methods of the Array class do in Ruby. Can somebody help me understand what they do?

Upvotes: 95

Views: 59634

Answers (5)

Alberto Santini
Alberto Santini

Reputation: 6562

shift and unshift acts in similar way as pop and push: they are meant to use arrays as stacks to which you can append and remove elements (usually one per time). The difference is just that shift and unshift add/remove elements at the beginning of an Array, actually shifting all other elements, while pop and push add/remove elements at the end of the Array, so preserving other elements' indices.

Examples:

                      # Spacing for clarity:
a = [2, 4, 8]    # a =>       [2, 4, 8]
a.push(16, 32)   # a =>       [2, 4, 8, 16, 32]
a.unshift(0, 1)  # a => [0, 1, 2, 4, 8, 16, 32]
a.shift          # a =>    [1, 2, 4, 8, 16, 32]
a.pop            # a =>    [1, 2, 4, 8, 16]

Upvotes: 60

Rob Grant
Rob Grant

Reputation: 7348

It returns the first element of the array, and removes it from the array, shifting the elements back one place.

So shifting [1,2,3,4,5]

returns 1, and sets the array to be [2,3,4,5].

More here.

Upvotes: 1

mipadi
mipadi

Reputation: 410672

It grabs the first element, removes it from the array, and returns the removed element. It's basically a way to treat an array like a stack: shift is pop, unshift is push.

Upvotes: 9

Tony Delroy
Tony Delroy

Reputation: 106106

If you can think of the array as being like a queue of values to be processed, then you can take the next (front) value and "shift" the other valuess over to occupy the space made available. unshift puts values back in - maybe you're not ready to process some of them, or will let some later code handle them.

Upvotes: 2

Steve Weet
Steve Weet

Reputation: 28392

Looking at the Ruby Documentation

Array.shift removes the first element from the array and returns it

a = [1,2,3] 
puts a.shift
 => 1 
puts a
 => [2, 3]

Unshift prepends the provided value to the front of the array, moving all other elements up one

a=%w[b c d]
 => ["b", "c", "d"] 
a.unshift("a")
 => ["a", "b", "c", "d"]

Upvotes: 106

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