Unexpected array behavior

Question

The following modify method somehow modifies the whole @x array instead of just simply generating another element to be pushed later. How come?

def modify i, s
  t = @x[-1]
  t[i] = t[i] + s
  t
end

@x = [ [10, 12] ]
@x << modify(0, 1)

puts @x

Edited The following code have done the trick. Still I wonder if its possible to get rid of the p-argument

def modify p, i, s
  a = p.to_a
  a[i] += s*@delta[i]
  Vector.elements( a )
end

Answer 1

@x[-1] is the inner array [10, 12]. You set t to reference this array. You then modify this array with the t[i] = part.

Answer 2

You should probably redo from start, but in the spirit of small changes ... try changing your method to reduce side effects.

If you want to return a single Fixnum element:

def modify i, s
  t = @x[-1]
  r = t[i] + s
  r
end

or, if you want to return an array to inject into the greater array of tuples

def modify i, s
  t = @x[-1].dup
  t[i] = t[i] + s
  t
end

In the spirit of understanding, you should read up on Ruby objects and how they are references and how Ruby methods have has side-effects.

Study this:

a=[1,2]
b=a
b[0]=4
puts a
> [4,2]

You might want to do a dupe

a=[1,2]
b=a.dup
b[0]=4
puts a
> [1,2]