Can you make sense of this Array issue

ruby-talk
1

I want to have an array, say 5x5 with all nils unless set to something else.
However, when I set m[0][0] to 0 .. all 0 indices are set to zero.. Hmm..
never asked for that to be the case.
There's some funky stuff going on here.

irb(main):001:0> n = 5
=> 5
irb(main):002:0> m = [[nil]*n]*n
=> [[nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil,
nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
irb(main):003:0> m[0][0] = 0
=> 0
irb(main):004:0> m
=> [[0, nil, nil, nil, nil], [0, nil, nil, nil, nil], [0, nil, nil, nil,
nil], [0, nil, nil, nil, nil], [0, nil, nil, nil, nil]]

Any ideas?

2

O I think I did figure this out. It seems the * operator just duplicates the
pointers in some optimized fashion. If the nxn array is constructed by
setting each m[i][j] element everything works fine.. I should have thought
of that before.

irb(main):001:0> n = 5
=> 5
irb(main):002:0> m =
=>
irb(main):003:0> for i in 0..(n-1)
irb(main):004:1> m[i] ||=
irb(main):005:1> for j in 0..(n-1)
irb(main):006:2> m[i][j] = nil
irb(main):007:2> end
irb(main):008:1> end
=> 0..4
irb(main):009:0> m
=> [[nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil,
nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
irb(main):010:0> m[0][0] = 0
=> 0
irb(main):011:0> m
=> [[0, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil,
nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]

···

On 5/26/07, Roland Mai <roland.mai@gmail.com> wrote:

I want to have an array, say 5x5 with all nils unless set to something
else.
However, when I set m[0][0] to 0 .. all 0 indices are set to zero.. Hmm..
never asked for that to be the case.
There's some funky stuff going on here.

irb(main):001:0> n = 5
=> 5
irb(main):002:0> m = [[nil]*n]*n
=> [[nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil,
nil, nil], [nil, nil, nil, nil, nil], [nil, nil, nil, nil, nil]]
irb(main):003:0> m[0][0] = 0
=> 0
irb(main):004:0> m
=> [[0, nil, nil, nil, nil], [0, nil, nil, nil, nil], [0, nil, nil, nil,
nil], [0, nil, nil, nil, nil], [0, nil, nil, nil, nil]]

Any ideas?

3

Nothing odd here, you're making an array containing the SAME n-ary array
n times.

-s

···

In message <b034db400705261216x40b8da17xf2396c9013ac6dd2@mail.gmail.com>, "Roland Mai" writes:

I want to have an array, say 5x5 with all nils unless set to something else.
However, when I set m[0][0] to 0 .. all 0 indices are set to zero.. Hmm..
never asked for that to be the case.
There's some funky stuff going on here.

irb(main):001:0> n = 5
=> 5
irb(main):002:0> m = [[nil]*n]*n

4

You might be interested in comparing:

Array.new(10, [])
and
Array.new(10) {[]}

5

Here's my interpretation so far.

Array.new(10, ) creates an array with being duplicated 10 times.
Basically, each is a mirror/pointer to the original object.

Array.new(10) {} creates the same array but the difference is that each
time creates a different object unlike above where the new object was
merely a pointer.

···

On 5/26/07, Paul Stickney <pstickne@gmail.com> wrote:

You might be interested in comparing:

Array.new(10, )
and
Array.new(10) {}