Ruby - Difference between Array#<< and Array#push

Question

From examining the documentation for Ruby 1.9.3, both Array#<< and Array#push were designed to implement appending an element to the end of the current array. However, there seem to be subtle differences between the two.

The one I have encountered is that the * operator can be used to append the contents of an entire other array to the current one, but only with #push.

a = [1,2,3]
b = [4,5,6]

a.push *b
=> [1,2,3,4,5,6]

Attempting to use #<< instead gives various errors, depending on whether it's used with the dot operator and/or parentheses.

Why does #<< not work the same way #push does? Is one not actually an alias for the other?

compare this to append vs extend in python http://stackoverflow.com/questions/252703 — dreftymac, Mar 15 '13 at 00:55

score 100 · Accepted Answer · answered May 13 '12 at 05:29

100

They are very similar, but not identical.

<< accepts a single argument, and pushes it onto the end of the array.

push, on the other hand, accepts one or more arguments, pushing them all onto the end.

The fact that << only accepts a single object is why you're seeing the error.

answered May 13 '12 at 05:29

x1a4

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To extend on this you can shorthand 'push' 2 arrays with +=. Use << to add a single value to an array. – TomDunning May 13 '12 at 18:54
@Isotope - That creates extra objects, so it should be considered an anti-pattern unless you really don't want to modify the first array. – x1a4 Jun 06 '13 at 19:42
1

I do believe the current version of Ruby does allow for the "< – Alfonso Vergara Sep 02 '16 at 18:34

score 13 · Answer 2 · edited Jun 14 '21 at 22:32

13

Another important point to note here is that << is also an operator, and it has lower or higher precedence than other operators. This may lead to unexpected results.

For example, << has higher precedence than the ternary operator, illustrated below:

arr1, arr2 = [], []

arr1.push true ? 1 : 0
arr1
# => [1] 

arr2 << true ? 1 : 0
arr2
# => [true]

edited Jun 14 '21 at 22:32

tdy

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answered Apr 08 '15 at 15:09

Santhosh

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score 10 · Answer 3 · answered May 13 '12 at 05:30

10

The reason why << does not work and push does is that:

push can accept many arguments (which is what happens when you do *b).
<< only accepts a single argument.

answered May 13 '12 at 05:30

David Weiser

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score 9 · Answer 4 · edited Jun 07 '18 at 22:35

9

The main difference between Array#<< and Array#push is

Array#<< # can be used to insert only single element in the Array

Array#push # can be used to insert more than single element in the Array

Another significant difference is, In case of inserting single element,

Array#<< is faster than Array#push

Benchmarking can help in finding out the performance of these two ways, find more here.

edited Jun 07 '18 at 22:35

aardvarkk

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answered Aug 15 '13 at 06:17

Akshay Mohite

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score 0 · Answer 5 · answered May 13 '12 at 06:07

0

The push method appends an item to the end of the array.It can have more than one argument. << is used to initialize array and can have only one argument , adds an element at the end of array if already initialized.

answered May 13 '12 at 06:07

Abhishek Singh

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Ruby - Difference between Array#<< and Array#push

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