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How to Sum Numbers in For and While Loops in Python

Learn how to calculate sums in Python using for and while loops, including summing lists, ranges, and user input with practical code examples.

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)
  • 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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