Question

Simple question, but this is giving me trouble.

I have a list with lists inside, can be a function with two variables, say x,y and the result, z, so the list is built like

a = [ [[x1,y1,z11], [x1,y2,z12], ...],
      [[x2,y1,z21], [x2,y2,z22], ...],
      ... ]

Now, I want to build another of this objects, with a different function, let say w:

b = [ [[x1,y1,w11], [x1,y2,w12], ...],
      [[x2,y1,w21], [x2,y2,w22], ...],
      ... ]

Then, to do it, I do this:

b = [[0.0]*len(a[0])]*len(a)
for i in range(len(a)):
    for j in range(len(a[0])):
        c = a[i][j][:2]
        c.append(w(i,j))
        b[i][j] = c

ok. Now, when I ask for a[i][j], I get b[i][j]. This is puzzling me because when I try this in the terminal I get the correct a[i][j]. Any help?

Answer 1

The problem is here:

 b = [[0.0]*len(a[0])]*len(a)
 # or in two steps
 x = [0.0] * len(a[0]) # one list
 b = [x] * len(a)      # lots of references to that same list 

What you need to make are new lists:

# makes a new inner list for every m
b = [[0.0]*len(a[0]) for m in xrange(len(a))]