Python Program to find the factorial of a number without recursion

When it is required to find the factorial of a number without using recursion, the while loop can be used. The factorial of a number n is the product of all positive integers from 1 to n, denoted as n!.

What is Factorial?

Factorial of a number n (written as n!) is defined as:

• n! = n × (n-1) × (n-2) × ... × 2 × 1
• 0! = 1 (by definition)
• Examples: 5! = 5 × 4 × 3 × 2 × 1 = 120

Using While Loop

We can calculate factorial by multiplying numbers from n down to 1 ?

# Method 1: Multiplying from n down to 1
num = 7
factorial = 1

while num > 0:
    factorial = factorial * num
    num = num - 1

print(f"The factorial of 7 is: {factorial}")

The factorial of 7 is: 5040

Using For Loop (Alternative Approach)

We can also use a for loop with range() for a more concise solution ?

# Method 2: Using for loop
num = 5
factorial = 1

for i in range(1, num + 1):
    factorial *= i

print(f"The factorial of {num} is: {factorial}")

The factorial of 5 is: 120

Complete Program with Input

Here's a complete program that takes user input and handles edge cases ?

def factorial_iterative(n):
    if n < 0:
        return "Factorial is not defined for negative numbers"
    
    factorial = 1
    while n > 0:
        factorial = factorial * n
        n = n - 1
    
    return factorial

# Test with different numbers
numbers = [0, 1, 5, 7]

for num in numbers:
    result = factorial_iterative(num)
    print(f"Factorial of {num} is: {result}")

Factorial of 0 is: 1
Factorial of 1 is: 1
Factorial of 5 is: 120
Factorial of 7 is: 5040

How It Works

The algorithm follows these steps:

1. Initialize: Set factorial = 1 and get the input number
2. Loop: While the number is greater than 0
3. Multiply: factorial = factorial × current number
4. Decrement: Reduce the number by 1
5. Repeat: Continue until number becomes 0
6. Return: The final factorial value

Comparison of Methods

Conclusion

Using iterative approaches (while or for loop) to calculate factorial is memory-efficient with O(1) space complexity. The while loop method is straightforward and avoids the overhead of recursive function calls.