Python Program to Find Factorial of a Number

mediumFunctionsPython

Factorial (n!) is n × (n − 1) × … × 1. Writing it as a function is one of the most common PR-3 questions.

The question

Write a function factorial(n) that returns n! using a loop. Read n and print its factorial by calling your function.

The code

program.py

def factorial(n):
    result = 1
    for i in range(2, n + 1):
        result *= i
    return result

n = int(input())
print("Factorial of", n, "is", factorial(n))

Try it Yourself »

Input

Output

Factorial of 10 is 3628800

How it works

1def factorial(n): creates a reusable function.
2result starts at 1 (starting at 0 would make every product 0).
3The loop multiplies result by every number from 2 to n.
4return sends the answer back to the line that called the function.

Common mistakes

Starting result at 0 — the answer is always 0.
Printing inside the function instead of returning — the question asks for a function that returns n!.

Now solve a similar question yourself

A new Python question every time, checked instantly.

Practice questions