Ruby array strange behaviour

Question

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 ?

sawa · Accepted Answer · 2011-06-24 13:20:02Z

6

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.

edited Jun 24, 2011 at 13:20

answered Jun 24, 2011 at 13:04

sawa

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1 Comment

DGM Over a year ago

Or since cell is empty just use a new [] instead of specifying cell... 3.times { matrix << [] }

Pavling · Accepted Answer · 2011-06-24 13:05:05Z

2

You're putting the same object (cell) into the matrix three times.

this will fix your bug:

3.times { matrix << cell.clone } # lines

...but you might want to explain what problem you're trying to solve with this code, as there may be better ways...

answered Jun 24, 2011 at 13:05

Pavling

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