Why does Array#push append to all arrays in nested arrays and not only the specified index?

Question

Recently I attempted to append a number to one of the nested arrays in my script. I expected only the nested array at the specified index to have the number appended to it, but what actually happened was that all of the nested arrays had the number appended to them.

This behaviour seems very odd, why do ruby arrays function this way?

irb(main):001:0> [1,2,3].push 3 
=> [1, 2, 3, 3]
irb(main):002:0> layered = [[]] * 5
=> [[], [], [], [], []]
irb(main):003:0> layered[0] << 2
=> [2]
irb(main):004:0> layered
=> [[2], [2], [2], [2], [2]]
irb(main):005:0>

Hauleth · Accepted Answer · 2015-04-12 18:07:33Z

4

Because all of them are the same array. You can check it by calling Object#object_id on each element:

[3] pry(main)> layered = [[]] * 5
=> [[], [], [], [], []]
[4] pry(main)> layered[0].object_id
=> 70207042910540
[5] pry(main)> layered[1].object_id
=> 70207042910540
[6] pry(main)> layered[2].object_id
=> 70207042910540

To create new array for each element then use Array.new(5) { [] }.

answered Apr 12, 2015 at 18:07

Hauleth

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2 Comments

Nirma Over a year ago

Thank you so much!! This seems a little confusing as to why [ [] ] * 5 would do this. Do you know of any practical examples of when using [ [] ] * 5 would be appropriate?

Hauleth Over a year ago

When you will replace whole array at assign (as you should do). So if you replace layered[0] << 2 with layered[0] += [2] then it should work.

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