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1. Print "Hello, World!"2. Add Two Numbers3. Swap Two Variables4. Find ASCII Value of a Character5. Check Even or Odd6. Find Largest of Two Numbers7. Find Largest of Three Numbers8. Check Positive, Negative, or Zero9. Convert Kilometers to Miles10. Convert Celsius to Fahrenheit11. Calculate Area of a Triangle12. Calculate Simple Interest13. Calculate Compound Interest14. Calculate Square Root15. Convert Seconds to Hours, Minutes, and Seconds16. Calculate Body Mass Index (BMI)17. Check Leap Year18. Find Absolute Value19. Calculate Age from Birth Year20. Sum of First N Natural Numbers21. Calculate Power Without Using pow()22. Print Multiplication Table23. Convert Decimal to Binary24. Convert Binary to Decimal25. Calculate Electricity Bill

Sum of First N Natural Numbers in Python

N(N+1)/2. A loop also works. The formula is O(1) and easier to get wrong with / vs //.

BeginnerBasic Python ProgramsExample 20 of 25
sum-of-first-n-natural-numbers.py
1# Program to find the sum of first N natural numbers
2

3n = int(input("Enter a positive integer N: "))
4

5if n <= 0:
6    print("Please enter a positive integer.")
7else:
8    total = n * (n + 1) // 2
9    print("Sum of first", n, "natural numbers is:", total)

Output

Enter a positive integer N: 10
Sum of first 10 natural numbers is: 55

In this one

  • N*(N+1)//2, check N=10 → 55
  • // keeps it an int

What's going on

1+2+...+N = N(N+1)/2. For N=10 that's 55. Gauss, school-story, whatever.

n * (n + 1) // 2 uses integer division so you get 55 not 55.0. Regular / is fine too if you don't mind the float.

A for loop that adds 1 through N is clearer and slower. For N=10 you will not notice. For N=10 million you will.

n <= 0 is rejected. The formula still "works" for 0 (sum is 0) but "first 0 natural numbers" is a bad prompt.

After this program

If the idea is still fuzzy, read Day 1: Introduction to Python in the Python course. For a short check, try the Python quiz (300 questions). Stay with all 25 basic python programs, or see every topic on Python exercises.

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