Python – Sum of First N Natural Numbers

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Find the sum of first N natural numbers with Python Program

In this tutorial, we will find the sum of first N natural numbers. We shall explain the problem statement, and explore different ways to solve this problem.

Problem Statement

Find the sum of first N natural numbers.

1 + 2 + 3 + 4 . . + N

Example: If N = 6, sum of first 6 Natural Numbers is calculated as below.

1 + 2 + 3 + 4 + 5 + 6
= 21

User will provide the N through console. So, you have to read the N from console. And you should print the sum to the console.

Solution 1

Take an integer in n.
answer = 0
You can use a for loop to iterate from 1 to n.
1. In the for loop, add the number to answer.
After you come out of the loop, you have the sum of first n natural numbers in your answer.

Python Program using For loop

n = 10
answer = 0

for i in range(0, n+1):
    answer += i;

print(answer)

Run Code Copy

Python Program using While loop

n = 10
answer = 0

i = 1
while i <= N:
    answer += i
    i += 1

print(answer)

Run Code Copy

Solution 2

The formula to find the sum of first N natural numbers is given below:

Sum of first N natural numbers = (N*(N+1))/2

We will use this formula and write a python program to compute the answer.

Python Program using formula

n = 20
answer = (n * (n + 1)) / 2
answer = int(answer)
print(answer)

Run Code Copy

Output

Validating input from User

In all the above programs, we have not considered the case where user could provide an incorrect input.

Let us validate the input to be a natural number. For that, we should check if the given input is a number and then if it is greater than zero.

Python Program with input validation

# Read the input
N = input("Enter a natural number: ")

# Assume everything is fine
validation = True

# If N is not numeric, validation fails
if not(N.isnumeric()):
    validation = False
else:
    N = int(N)
    #if n is less than 1, it is not a natural number
    if (N<1):
        validation = False

if validation:
    answer = (N*(N+1))/2
    answer = int(answer)
    print(answer)
else:
    print('Input is not a natural number. Try again.')

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These validation cases are only for demonstration purpose. You may think of some more validations and incorporate in the Python program.