Why is Python for loop not looping with a while loop inside

Question

I am really curious about this.

I have a for loop that is looping through a list. Inside the for loop I have a while loop that I want to loop through till a condition is met. When the condition of the while loop is met, stop the while loop and move to the next item of the list and start the while loop again.

Here is the example code:

course_ids = [1,2,3,4,5]

loop_control = 0
counter = 0

for ids in course_ids:
    while loop_control == 0:
        counter = counter + 1
        if counter == 2:
            loop_control = 1

The problem is that when the while loop condition is met, it breaks out of the for loop altogether.

How do I get the for loop to work as intended with a while loop inside of it?

What makes you think that it breaks out of the for loop too? It is true that when the condition of the while loop is not met (loop_control == 1), it remains the case for each subsequent for loop iteration. Technically the for loop continued, the iterations just didn't do much. — busybear
– busybear, Commented Mar 21, 2020 at 2:40
As it is, your while structure will loop twice. On the first iteration counter will be set to 1, on the second it will be set to 2. The if counter==2 statement will then become valid. At which point loop_control will be set to 1. On the subsequent attempt to loop, the expression loop_control==0 will evaluate to False and the while loop will fail. The second iteration of the for loop will proceed. Any subsequent iteration of the while loop will fail since loop_control will still equal 1. — Michael Ekoka
– Michael Ekoka, Commented Mar 21, 2020 at 2:49
If you want to repeat the while after each for loop iteration, you need to reset loop_control (and probably counter as well) to 0 right before the while, not before the for. — Michael Ekoka
– Michael Ekoka, Commented Mar 21, 2020 at 2:57
Nice. Thanks that did it. I add this python if counter != len(course_ids): loop_control = 0 — Justin Besteman
– Justin Besteman, Commented Mar 21, 2020 at 20:46

Rashid 'Lee' Ibrahim · Accepted Answer · 2020-03-21 02:42:05Z

3

You set loop control to 1 in the if statement. The while loop is set to only run if loop control is equal to 1. So basically the first time it runs, you set the condition to never run again.

If you were to reset loop control to zero on each iteration of the for loop, the while will run each time.

course_ids = [1,2,3,4,5]

# loop_control = 0 <-- Remove this line
counter = 0

for ids in course_ids:
    loop_control = 0 # place this line and see it work more than once.
    while loop_control == 0:
        counter = counter + 1
        if counter == 2:
            loop_control = 1

answered Mar 21, 2020 at 2:42

Rashid 'Lee' Ibrahim

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AzyCrw4282 · Accepted Answer · 2020-03-21 02:42:54Z

0

Your assumption is wrong that it breaks out of the for loop altogether. It doesn't, it runs but the while condition is never True and hence it never runs inside it.

You can also check this by adding a print statement inside the for loop and see its output.

course_ids = [1,2,3,4,5]

loop_control = 0
counter = 0

for ids in course_ids:
    print("here")
    while loop_control == 0:
        counter = counter + 1
        if counter == 2:
            loop_control = 1

Output:

here
here
here
here
here

As you can see it ran 5 times as expected. You should also consider visualizing the code to get a better understanding, see here

answered Mar 21, 2020 at 2:42

AzyCrw4282

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Comments

Justin Besteman · Accepted Answer · 2020-03-21 20:48:49Z

0

you all are right.

I did not reset the while loop condition

I have added these lines after the for loop and it fixed the issue

if counter != len(course_ids):
    loop_control = 0

answered Mar 21, 2020 at 20:48

Justin Besteman

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