不明白为什么循环没有停止

Question

I'm using a while loop in Python to do an action. However, the loop doesn't end when I expect it to (when teams becomes empty). This is the code:

while teams:
    if scores[0]<scores[1]:
        losers.append(teams[0])
        teams.remove(teams[0])
        teams.remove(teams[0])
        scores.remove(scores[0])
        scores.remove(scores[0])
    if scores[0]>scores[1]:
        losers.append(teams[1])
        teams.remove(teams[0])
        teams.remove(teams[0])
        scores.remove(scores[0])
        scores.remove(scores[0])
return losers

While testing the code, I put in print statements and found that the losers array has the expected result, however the loop continues after I expect it to stop, checking for scores[0] when I already deleted scores[0] on the last iteration of the loop. Why is my loop not ending?

Answer 1

Double check the conditions of the loop. Loops will become infinite if a condition never becomes equal to false .

See the "The Infinite Loop" section of http://www.tutorialspoint.com/python/python_while_loop.htm .

Answer 2

If the loop does not end is because teams never becomes empty. Your assumption "the loop doesn't end when [...] teams becomes empty" is incorrect and probably misguided you in your attempts to solve the problem.

Answer 3

The fix is to change the second if statement to elif :

while teams:
    if scores[0]<scores[1]:
        losers.append(teams[0])
        teams.remove(teams[0])
        teams.remove(teams[0])
        scores.remove(scores[0])
        scores.remove(scores[0])
    elif scores[0]>scores[1]:
        losers.append(teams[1])
        teams.remove(teams[0])
        teams.remove(teams[0])
        scores.remove(scores[0])
        scores.remove(scores[0])
return losers

The problem is that when the teams and scores lists get to the last pair, and the condition in the first if statement is True , that final pair is removed and both lists become empty.

Then the second if condition is evaluated, but the scores list is now empty (courtesy of the preceding if matching), so an IndexError is raised.

Changing it to an elif means that the second condition will only be evaluated if the first condition is False . Now the loop will only process one pair of teams/scores per iteration, and the test for an empty team will succeed.

You could make the while loop a bit clearer with this:

while teams:
    if scores[0] < scores[1]:
        losers.append(teams[0])
    elif scores[0] > scores[1]:
        losers.append(teams[1])
    else:
        # this should never happen
        print('Error: drawn match detected!')
        losers.append(None)    # or simply ignore if that makes sense

    teams[:] = teams[2:]    # remove the first 2 items
    scores[:] = scores[2:]