The Strangeness of Arrays

ruby-talk
1

Hi

The behaviour below has me completely confuzzled. Why on earth does my
array suddenly have all rows equal to the last row?

Can anyone tell me if this is expected; or if I have to start wearing my
Stupid Hat again.

Thanks
R

irb(main):014:0> @t = Array.new(5, Array.new(4))
=> [[nil, nil, nil, nil], [nil, nil, nil, nil], [nil, nil, nil, nil],
[nil, nil, nil, nil], [nil, nil, nil, nil]]
irb(main):025:0> row = 0
=> 0
irb(main):026:0> while (row <= 4)
irb(main):027:1> column = 0
irb(main):028:1> while (column <= 3)
irb(main):029:2> printf(".%d,%d: %d. “, column, row, @t[row][column] =
row+column)
irb(main):030:2> column += 1
irb(main):031:2> end
irb(main):032:1> printf(”\n")
irb(main):033:1> row += 1
irb(main):034:1> end
.0,0: 0. .1,0: 1. .2,0: 2. .3,0: 3.
.0,1: 1. .1,1: 2. .2,1: 3. .3,1: 4.
.0,2: 2. .1,2: 3. .2,2: 4. .3,2: 5.
.0,3: 3. .1,3: 4. .2,3: 5. .3,3: 6.
.0,4: 4. .1,4: 5. .2,4: 6. .3,4: 7.
=> nil
irb(main):035:0> @t
=> [[4, 5, 6, 7], [4, 5, 6, 7], [4, 5, 6, 7], [4, 5, 6, 7], [4, 5, 6, 7]]

2

Roger Bannister wrote:

Hi

The behaviour below has me completely confuzzled. Why on earth does my
array suddenly have all rows equal to the last row?

Can anyone tell me if this is expected; or if I have to start wearing my
Stupid Hat again.

It’s the strangeness of objects, not arrays. See below…

Thanks
R

irb(main):014:0> @t = Array.new(5, Array.new(4))

This fills @t with five references to the same four-item array.

If you change one, you change them all.

I think in recent Ruby you can do this:

@t = Array.new(5) { Array.new(4) }

and you’ll get different sub-arrays. Try it…

Hal

3

Hal Fulton wrote:

Roger Bannister wrote:

Hi

The behaviour below has me completely confuzzled. Why on earth does
my array suddenly have all rows equal to the last row?

Can anyone tell me if this is expected; or if I have to start wearing
my Stupid Hat again.

It’s the strangeness of objects, not arrays. See below…

Thanks
R

irb(main):014:0> @t = Array.new(5, Array.new(4))

This fills @t with five references to the same four-item array.

If you change one, you change them all.

I think in recent Ruby you can do this:

@t = Array.new(5) { Array.new(4) }

and you’ll get different sub-arrays. Try it…

Hal

Right, it’s the hat for me! I’d completely forgotten the basic example
of a = 1; b = a.

Thanks guys.