How do you sum a number in a while loop in python?

num = 15
sum = 0

# use while loop to iterate until zero
while num > 0:
    sum += num
    num -= 1
print["The sum is", sum]

sum = 0

num = int[input["Enter a number: "]]
if num < 0:
    print["Please enter a positive number"]
else:
    sum = 0

# use while loop to iterate until zero
while num > 0:
    sum += num
    num -= 1
print["The sum is", sum]

Sum of N numbers using a while loop in Python #

To get the sum of N numbers using a while loop:

1. Iterate for as long as the number is greater than 0.
2. On each iteration, decrement the number by 1.
3. On each iteration, increment the total by the number.

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sum_of_numbers = 0

num = 5

while num > 0:
    # 👇️ reassign sum to sum + num
    sum_of_numbers += num

    # 👇️ reassign num to num - 1
    num -= 1

print[sum_of_numbers]  # 👉️ 15  [5 + 4 + 3 + 2 + 1]

print[num]  # 👉️ 0

The while loop in the example iterates for as long as the num variable stores a value greater than 0.

On each iteration, we use the += operator to reassign the sum_of_numbers variable to its current value plus num.

To move towards the base case, we also use the -= operator to reassign the num variable to its value minus 1.

The following 2 lines of code achieve the same result:

• sum_of_numbers += num
• sum_of_numbers = sum_of_numbers + num

Similarly, the -= operator is also a shorthand:

• num -= 1
• num = num - 1

Make sure to specify a base case that has to be met to exit the while loop, otherwise you might end up with an infinite loop.

A commonly used alternative is to use a while True loop with a break statement.

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sum_of_numbers = 0

num = 5

while True:
    # 👇️ reassign sum to sum + num
    sum_of_numbers += num

    # 👇️ reassign num to num - 1
    num -= 1

    # 👇️ if num is equal to or less than `0`, break out of loop
    if num