Python Program to Calculate the Sum of Natural Numbers

In this python tutorial, you will learn how to Calculate the Sum of Natural Numbers using the if and else statements along with the  assignment operators of the python programming language.

How to Calculate the Sum of Natural Numbers?

Let’s take a look at the source code , here the values are given as input by the user in the code, the if and else statements and its conditions along with the while loop carry out the function.

num = int(input("Enter a number: "))  
if num < 0:  
   print("Enter a positive number")  
   sum = 0  
   # use while loop to iterate un till zero  
   while(num > 0):  
       sum += num  
       num -= 1  
   print("\nThe sum is",sum)




Enter a number: 
The sum is 136
  1. Here we give the user the option to enter the values and the input values are scanned using the input function and are stored in the variable num with the statements/strings ("Enter a positive number") , we use the int function and declare the  input value as an integer value.
  2. In the STDIN section of the code editor the input values are entered.
  3. We declare the if statement with the start iteration  sum = 0 , then a while loop with the condition (num > 0) followed by a :  with the assignment operators sum += num and num -= 1.
  4. In the if statement, if the condition is satisfied, the print function  will display the statement ("Enter a positive number"). If not, it moves to the next step which is the else statement.
  5. At the start, the sum is initialized to 0. And, the number is stored in variable num . Then, we used the while loop to iterate until num becomes zero. In each iteration of the loop, we have added the num to sum and the value of num is decreased by 1.
  6.  Once the num value becomes zero the iteration comes to an end the print function will display the statement ("\nThe sum is",sum).


  • The input() function allows a user to insert a value into a program, it returns a string value.
  • The if and else statements evaluates whether an expression is true or false. If a condition is true, the “if” statement is executed otherwise, the “else” statement is executed.
  • The colon : at the end of the if and else statement tells Python that the next line of code should only be run if the condition is true.
  • The While Loop is used to execute a block of statements repeatedly until a given condition is satisfied. And when the condition becomes false, the line of code immediately after the loop in the program is executed.
  • The Python += operator lets you add two values together and assign the resultant value to a variable, the  -= operator function is similar which lets you subtract two values together and assign the resultant value to a variable.
  • The statement for the input function are enclosed in single quotes and parenthesis.
  • The \n in the code indicates a new line or the end of a statement line or a string.
  • The print statement/string to be displayed in enclosed in double quotes.