Python Nested For and While Unexpected Behaviour

Asked Jan 06 '17 at 16:22

Active Jan 06 '17 at 21:14

Viewed 44 times

While writing some code I notice something strange, if you take this code:

data = ['a','b','c','d','e']
print("start", len(data))
while(len(data) > 0):
  print("while", len(data))
  for i in data:
    print("for", i)
    del data[data.index(i)]
print("end", len(data))

If I execute it, it returns:

('start', 5)
('while', 5)
('for', 'a')
('for', 'c')
('for', 'e')
('while', 2)
('for', 'b')
('while', 1)
('for', 'd')
('end', 0)

This is not what I expected to happen, this is what i thought would happen:

('start', 5)
('while', 5)
('for', 'a')
('for', 'b')
('for', 'c')
('for', 'd')
('for', 'e')
('end', 0)

It's both running the list out of order (which is annoying but that wholly unexpected) and running more than one while loop. As I said in the brackets then list returning out of order is not really the problem its the multiple while loops cycles that confused me. Can someone explain why this is happening? A solution would be pleasant too.

Here is a link to the demo code on trinket.io: https://trinket.io/python/46f433793d

edited Jan 06 '17 at 21:14

Jongware

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asked Jan 06 '17 at 16:22

Duncan

2

Do not modify a list while iterating it with a for loop. – tobias_k Jan 06 '17 at 16:23
What exactly do you not understand? The `for` loop deletes every second element from the list (see tons of other questions on that topic), and the `while` loop runs until the list is empty, which takes three runs here. – tobias_k Jan 06 '17 at 16:25
what exactly are you trying to accomplish ? – amphibient Jan 06 '17 at 16:27
@amphibient delete each item in the for loop in one iteration of the while loop (added example to question) – Duncan Jan 06 '17 at 16:30
@tobias_k yes I know you shouldn't – Duncan Jan 06 '17 at 16:34

Python Nested For and While Unexpected Behaviour

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