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Asked: 2026-05-23T08:48:53+00:00

I misunderstand Array behaviour When i created this matrix matrix, cell = [], [];

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I misunderstand Array behaviour

When i created this matrix

matrix, cell = [], []; 5.times { cell << [] } # columns
3.times { matrix << cell } # lines
matrix
sample_data = (0..5).to_a
matrix[1][2] = sample_data.clone
matrix.each { |line| puts "line : #{line}" }

I have this result 

line : [[], [], [0, 1, 2, 3, 4, 5], [], []]
line : [[], [], [0, 1, 2, 3, 4, 5], [], []]
line : [[], [], [0, 1, 2, 3, 4, 5], [], []]

Instead the expected result

line : [[], [], [], [], []]
line : [[], [], [0, 1, 2, 3, 4, 5], [], []]
line : [[], [], [], [], []]

What wrong ?

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1 Answer

  1. Editorial Team

    The problem is with your line:

    3.times { matrix << cell }

    You are using the same object cell as the three rows of matrix.

    The key is that Array is a mutable object. Even if you modify it, its identity does not change. The three occurences of cell are pointing to the same instance (object). If you access and modify it through one occurence of it, the other occurences will reflect that change.

    If you change this line to:

    3.times { matrix << cell.dup }

    then you will get the desired result.

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