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How to Sum Numbers in For and While Loops in Python

Summing numbers in a loop is a fundamental programming task.

This guide explores various techniques for calculating sums using both for and while loops in Python, covering different scenarios like summing items from a list, numbers in a range, and dynamically collecting values from user input.

How to Sum Numbers in a List with a for Loop​

To sum numbers stored in a list, use a for loop to iterate through the list and accumulate the sum:

my_list = [2, 4, 6, 8]
total = 0
for num in my_list:
    total += num
print(total)  # Output: 20
  • Initialize a variable to store the sum (e.g., total = 0).
  • Iterate over each number in the list and add it to the total.
  • After the loop finishes you get the sum of all items in the list.

How to Sum Numbers in a Range with a for Loop​

To sum numbers in a specific range, use the range() function with a for loop:

total = 0
for num in range(1, 5):
    total += num
print(total)                # Output: 10

print(list(range(1, 5)))    # Output: [1, 2, 3, 4]
  • range(1, 5) generates numbers from 1 up to (but not including) 5.
  • Inside the loop we add the current number generated by range to the total sum, so in this case, we are summing all of the numbers starting from 1 until 4.

How to Sum Numbers from User Input with a for Loop​

To sum numbers entered by the user, combine input(), split(), and map() functions:

user_input = input('Enter space-separated numbers: ')
my_list = list(map(int, user_input.split()))

total = 0
for num in my_list:
    total += num

print(total)
# Example
# Input: 1 2 3 4
# Output: 10
  • The code prompts the user to input space-separated numbers using input().
  • Then the split() method splits the string into multiple string numbers separated by spaces.
  • The map() function and int() method are then used to transform each string to a number, and construct a new list of integers with these numbers.
  • Finally, a for loop is used to sum the numbers as we did in the first example.
  • Use try/except statements for more robust code that also handles invalid input, like what would happen if the user types a instead of 1.

How to Sum N Numbers with a while Loop​

You can also use a while loop to sum a fixed number of integers.

sum_of_numbers = 0
num = 5

while num > 0:
    sum_of_numbers += num
    num -= 1
print(sum_of_numbers) # Output: 15 (5 + 4 + 3 + 2 + 1)
  • The while loop will continue as long as num > 0 to add numbers to the sum.
  • num -= 1 is then used to decrement the number by one to reach the base case, where the while condition will be False.

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