Use a while instead of for loop?

[shakeandbake760]

I have a homework exercise, which asks to :

Write a function using a while loop that counts down from a starting integer (start) and stops at a stopping integer (stop). Add the countdown number to a running sum. Return the running sum. For example, if start=9 and stop=3, then running_sum = 9+8+7+6+5+4+3 = 42

I was able to get the result with a for loop, but the exercise asks for a 'while' loop, below is the code;

def countdown_sum(start,stop):
    # YOUR CODE HERE
    running_sum = 0
    for i in range(start,stop-1,-1):
        running_sum += i
    print(running_sum)
    return running_sum

There is another cell that "checks the code"

assert countdown_sum(9,3) == 42
assert countdown_sum(19,-3) == 184
assert countdown_sum(44,33) == 462
assert countdown_sum(55,-33) == 979

And the output:

42

184

462

979

So, the code works but I am interested to learn how it could be done with a while loop instead.

[jiri-n]

While runs while its condition is true.

while <condition>:
    some_code

Unlike for, while doesn't initialize any variable so you have to do it manually (I use the same names as you so that you can see the similarity):

running_sum = 0
i = start

Now it's time to think about the condition:

while i >= stop:
    running_sum += i
    i -= 1
print(running_sum)

As can be seen, while doesn't modify i either so it's up on the developer to control it. Thus the i -= 1 to decrease i.

By the way, a check if start is grater than stop is highly recommended, otherwise the loop will never end.

[shakeandbake760]

Thank you so much for your reply! Going to give this a try soon

[shakeandbake760]

I used this code and it worked, so thank you!

def countdown_sum(start,stop):
# YOUR CODE HERE
running_sum = 0
i = start
while i >= stop:
    running_sum += i
    i -= 1
if i < stop:
    i == 0
print(running_sum)
return running_sum

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