How to sum in a For or a While Loop in Python

# Table of Contents

1. Sum in a for loop in Python
2. Sum the numbers in a certain range using a for loop
3. Sum numbers taken from user input in a for loop
4. Sum of N numbers using a while loop in Python

# Sum in a for loop in Python

To sum in a for loop in Python:

1. Declare a new variable and set it to 0.
2. Use a for loop to iterate over a sequence of numbers.
3. Reassign the variable to its value plus the current number.

Copied!
my_list = [2, 4, 6, 8]

total = 0

for num in my_list:
    total += num

print(total)  # 👉️ 20

We used a for loop to sum the numbers in a list.

The first step is to declare a new variable and initialize it to 0.

The following 2 lines of code achieve the same result:

• total += num
• total = total + num

Here is an example that uses the longer reassignment syntax.

Copied!
my_list = [2, 4, 6, 8]

total = 0

for num in my_list:
    total = total + num

print(total)  # 👉️ 20

# Sum the numbers in a certain range using a for loop

If you need to add the numbers in a certain range using a for loop, create the range with the range() class.

Copied!
total = 0

for num in range(1, 5):
    total += num

print(total)  # 👉️ 10

print(list(range(1, 5)))  # 👉️ [1, 2, 3, 4]

The range class is commonly used for looping a specific number of times in for loops and takes the following parameters:

# Sum numbers taken from user input in a for loop

If you need to sum numbers taken from user input in a for loop, use the input() function.

Copied!
# 👇️ user enters 1 2 3 4
user_input = input('Enter space-separated numbers: ')

my_list = list(map(int, user_input.split()))

print(my_list)  # 👉️ [1, 2, 3, 4]

total = 0

for num in my_list:
    total += num

print(total)  # 👉️ 10

The input function takes an optional prompt argument and writes it to standard output without a trailing newline.

We used the str.split() function to split the string on each space.

The str.split() method splits the string into a list of substrings using a delimiter.

The method takes the following 2 parameters:

If the separator is not found in the string, a list containing only 1 element is returned.

Here is an example that splits the user-provided string on each comma.

Copied!
# 👇️ user enters 1,2,3,4
user_input = input('Enter comma-separated numbers: ')

my_list = list(map(int, user_input.split(',')))

print(my_list)  # 👉️ [1, 2, 3, 4]

total = 0

for num in my_list:
    total += num

print(total)  # 👉️ 10

After splitting the string, we get a list of strings, so we used the map() function to convert each string in the list to an integer.

Copied!
# 👇️ user enters 1,2,3,4
user_input = input('Enter comma-separated numbers:')

# 👇️ ['1', '2', '3', '4']
print(user_input.split(','))

The map() function takes a function and an iterable as arguments and calls the function with each item of the iterable.

The map() function passes each string to the int() class and converts it to an integer.

# 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.

Copied!
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.

To move toward 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

# Sum of N numbers using a while True loop

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

Copied!
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 the loop
    if num <= 0:
        break

print(sum_of_numbers)  # 👉️ 15  (5 + 4 + 3 + 2 + 1)

print(num)  # 👉️ 0

The while True loop iterates until it is interrupted by a break statement.

I've also written an article on how to sum all values in a dictionary or a list of dictionaries.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Sum the elements of a Tuple or List of Tuples in Python
• How to Sum all Numbers in a Range in Python
• Subtract a Value from every Number in a List in Python
• How to sum a List of Strings in Python
• RuntimeError: no running event loop in Python [Solved]
• How to keep a Python script output Window open