Difference of results between while loop and for loop when working with list

Question

I was trying to answer an exercise in a beginner's guide to python book. I tried using the while loop first and got the right output, but I experimented by using the for loop, and got an unexpected output. Why the difference in results?

sandwich_orders = ['mushrooom sandwich','cheese sandwich','jalapeno sandwich','bacon sandwich','pastrami','pastrami','pastrami']
finished_sandwiches = []

for sandwich in sandwich_orders:
    finished_sandwich = sandwich_orders.pop()
    if finished_sandwich != 'pastrami':
        print("I made your " + finished_sandwich + ".")
        finished_sandwiches.append(finished_sandwich)
    else:
        print("The deli has run out of pastrami.")


print("\nFinished sandwiches: ")
for sandwich in finished_sandwiches:
    print("\t" + sandwich)
print("\n")
while 'pastrami' in sandwich_orders:
    sandwich_orders.remove('pastrami')

Output (incorrect):

The deli has run out of pastrami.
The deli has run out of pastrami.
The deli has run out of pastrami.
I made your bacon sandwich.

Finished sandwiches: 
    bacon sandwich

Correct output:

I made your mushroom sandwich.
I made your jalapeno sandwich.
The deli has run out of pastrami.
The deli has run out of pastrami.
I made your cheese sandwich.
The deli has run out of pastrami.
I made your bacon sandwich.

Finished sandwiches: 
    mushroom sandwich
    jalapeno sandwich
    cheese sandwich
    bacon sandwich

Please [edit] your question and post a [mre]. We can't run this code as-is. — MattDMo, Dec 25 '21 at 23:33
Does this answer your question? [How to remove items from a list while iterating?](https://stackoverflow.com/questions/1207406/how-to-remove-items-from-a-list-while-iterating) — Silvio Mayolo, Dec 25 '21 at 23:34

score 0 · Answer 1 · answered Dec 25 '21 at 23:54

Don't modify a list while iterating over it.

Here is a fixed version:

sandwich_orders = ['mushrooom sandwich','cheese sandwich','jalapeno sandwich','bacon sandwich','pastrami','pastrami','pastrami']
finished_sandwiches = []

for sandwich in sandwich_orders:
    if sandwich != 'pastrami':
        print("I made your " + sandwich + ".")
        finished_sandwiches.append(sandwich)
    else:
        print("The deli has run out of pastrami.")

Output:

I made your mushrooom sandwich.
I made your cheese sandwich.
I made your jalapeno sandwich.
I made your bacon sandwich.
The deli has run out of pastrami.
The deli has run out of pastrami.
The deli has run out of pastrami.

Difference of results between while loop and for loop when working with list

1 Answers1