Question

I am working on the exercises for python from Google and I can’t figure out why I am not getting the correct answer for a list problem. I saw the solution and they did it differently then me but I think the way I did it should work also.

# B. front_x
# Given a list of strings, return a list with the strings
# in sorted order, except group all the strings that begin with 'x' first.
# e.g. ['mix', 'xyz', 'apple', 'xanadu', 'aardvark'] yields
# ['xanadu', 'xyz', 'aardvark', 'apple', 'mix']
# Hint: this can be done by making 2 lists and sorting each of them
# before combining them.
def front_x(words):
  # +++your code here+++
  list = []
  xlist = []
  for word in words:
    list.append(word)
  list.sort()
  for s in list:
    if s.startswith('x'):
      xlist.append(s)
      list.remove(s)
  return xlist+list

The call is:

front_x(['bbb', 'ccc', 'axx', 'xzz', 'xaa'])

I get:
[‘xaa’, ‘axx’, ‘bbb’, ‘ccc’, ‘xzz’]
when the answer should be:
[‘xaa’, ‘xzz’, ‘axx’, ‘b
bb’, ‘ccc’]

I’ve don’t understand why my solution does not work

Thank you.

Answer 1

You shouldn’t modify a list while iterating over it. See the for statement documentation.

  for s in list:
    if s.startswith('x'):
      xlist.append(s)
      list.remove(s)    # this line causes the bug