Tjyylli

I am trying to write a code using while loop to sum all the negative numbers in a list. I am getting -10 instead of -17. Any idea why? Thanks!

# sum all the negative numbers using a while loop

given_list3 = [7, 5, 4, 4, 3, 1, -2, -3, -5 ,-7]

total6 = 0

i = 0

while -1 < 0:

        total6 += given_list3[i]

        i += -1

        if given_list3[i] > 0:

            break

print(total6)

Various thing wrong

• the -1 < 0 does not make sense, it wont terminate


• you break when you find the first negative element

You should do something like

index = 0



while index < len(lst):

    value = lst[index]



    if value > 0:

        continue



    total += value



    index += 1

Note that, in python, it's more common to iterate over the values directly

for value in lst:

    if value >= 0:

        total += value

or use a list comprehension

total = sum([x for x in lst if x >= 0])

To be honest this isn't the best way to write this, however if you have a sorted list where all the negative numbers are on one side than this will work.

This issue is that you set i equal to 0, which would be 7 in that list. So what your while loop is doing is, 7 + -7 + -5 + -3 + -2... You will likely want to start i = -1 so that the first object it adds is -7 which will give you your desired results.

# sum all the negative numbers using a while loop

given_list3 = [7, 5, 4, 4, 3, 1, -2, -3, -5 ,-7]

total6 = 0

i = -1

while -1 < 0:

    total6 += given_list3[i]

    i += -1

    if given_list3[i] > 0:

        break

print(total6)

To explain why this works you need to understand the positioning in a list or array. Given your list:

List items:  [7, 5, 4, 4, 3, 1, -2, -3, -5 ,-7]

Positions:   [0][1][2][3][4][5] [6] [7] [8] [9]



List items:  [  7,  5,  4,  4,  3,  1, -2, -3, -5 ,-7]

From Reverse:[-10][-9][-8][-7][-6][-5][-4][-3][-2][-1]

If you want to look at it you can see it this way as a continuous spectrum:

List:     [   7,  5,  4,  4,  3,  1, -2, -3, -5 ,-7, 7, 5, 4, 4, 3, 1, -2, -3, -5 ,-7]

Positions:[ -10][-9][-8][-7][-6][-5][-4][-3][-2][-1][0][1][2][3][4][5] [6] [7] [8] [9]

While your list only contains 7,..-7, the call to "given_list3" operates on the above spectrum. Allowing negative numbers to work from the right while 0 and higher work from the left.

If you'll have a List wherein the negative values are scattered it might be best to loop through every single object, compare if it's < 0 and then add it to your total.

given_list = [-2,-3,5,7,4,-5,4,3,1,-7]

total = 0

for num in given_list:

    if num < 0:

        total = total + num

print(total)

In this case, you won't have to worry about starting on the other side of the list.

You should put a debugger and step through each line that is executed. You will see that the first value added is the numbers[0], which is the positive number 7. From there on, it just works like you expect it to; it loops back and adds the numbers till it finds a positive number and then exits.

You can use higher order functions like filter and sum to make your code more elegant and less error prone.

numbers = [7, 5, 4, 4, 3, 1, -2, -3, -5 ,-7]



sum(filter(lambda x: x<0, numbers))