3.8. while Loops

for Loops either iterate on the contents of a container, such as a list or a dictionary, or until a given counter exceeds a prespecified threshold. In contrast, a while loop runs until a specified condition is true, as shown below.

>>> counter = 0
>>> while counter < 5:
...     print counter,
...     counter+=1
...
0 1 2 3 4

Warning

The above loop would run infinitely if the value of counter was not incremented in each iteration. This is a very common pitfall which the application developer should watch out for!

There are various scenarios where the flexibility of while loops proves advantageous, such as when operating with inputs given by the user, or when checking the status of a boolean flag. Similar to Java or C++, Python provides a break command to exit out of a loop early. This allows us to write loops where the termination criterion may not be known in advance, or is pretty large in terms of computational cost. For example:

>>> active_list = [5,9,14,23,31]
>>> counter = 0
>>> while counter < 10000:
...     if(counter in active_list): break
...     else: print counter,
...     counter+=1
...
0 1 2 3 4

In the example above, the termination criterion can make the loop run for quite some time. Instead, we check if the value of counter is present in active_list and if so, we break out early. This stops the loop in just \(5\) iterations. Python also provides a continue command to skip all statements coming after it in a loop, as shown below.

>>> counter = 0
>>> while counter<100:
...     counter+=1
...     if counter in inactive_list: continue
...     print counter,
...
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

In the example above, note that counter is incremented before the statement with continue. If instead, we incremented counter afterwards, then we would have encountered an infinite loop! Thus, when using continue or while loops in general, the application developer should be extremely mindful of such logical errors. Also note that the commands break and continue are not specific to while loops, and can be used with for loops as well.