Python: Efficiently iterating over a multidimensional list

Question

I'm using for-loops to iterate over two-dimensional list:

def itr(lpic, lH, lW, x, y):
    '''lpic=2D-Array; lH=Row_count; lW=Column_count;'''
    stack = []
    range_x = range(x-1, x+2)
    range_y = range(y-1, y+2)
    append = stack.append
    for i in range_x:
                if 0<=i<lH:#i is a valid index *Updated
                    for j in range_y:
                        if (0<=j<lW) and (lpic[i][j]=="0"):
                            lpic[i][j] = "1"
                            append([i, j])
    return stack

I'd like to know if there is a better way to do the same with Python2.5.

Answer 1

There are two simple optimizations for your code:

1. Use xrange instead for range. This will prevent from generation two temporary lists.

2. Use min and max in parameters for xrange to omit 'if' in outer loop. So you code will look like that:

def itr(lpic, lH, lW, x, y):
    '''lpic=2D-Array; lH=Row_count; lW=Column_count;'''
    stack = []
    range_x = xrange(max(0,x-1), min(lH,x+2))
    range_y = xrange(max(0,y-1), min(lW,y+2))
    append = stack.append
    for i in range_x:
      for j in range_y:
          if lpic[i][j]=="0":
              lpic[i][j] = "1"
              append([i, j])
    return stack

This will slightly increase performance.