Program to find the factorial of a number using Recursion

Algorithm (Step by Step):

1. Start
2. Define a recursive function factorial(n)
3. Check if n is 0 or 1 (base case)
   • If true, return 1
   • If false, proceed to recursive case
4. For recursive case, return n * factorial(n-1)
5. Input a number from user
6. Call factorial function with input number
7. Display the result
8. End

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Pseudocode:

FUNCTION factorial(n: INTEGER) -> INTEGER
    IF n = 0 OR n = 1 THEN
        RETURN 1
    ELSE
        RETURN n * factorial(n - 1)
    END IF
END FUNCTION

ALGORITHM FindFactorial
    BEGIN
        INPUT number
        result ← factorial(number)
        OUTPUT "The factorial of ", number, " is ", result
    END

Program:

def factorial(n):
    # Base case: factorial of 0 or 1 is 1
    if n == 0 or n == 1:
        return 1
    else:
        # Recursive case: n * factorial of (n - 1)
        return n * factorial(n - 1)

def main():
    try:
        # Input the number
        number = int(input("Enter a number: "))
        
        # Check for negative numbers
        if number < 0:
            print("Factorial cannot be calculated for negative numbers.")
            return
            
        # Calculate factorial
        result = factorial(number)
        
        # Print the result
        print(f"The factorial of {number} is {result}")
        
    except ValueError:
        print("Please enter a valid integer.")

# Call main function
if __name__ == "__main__":
    main() 

Flowchart:

flowchart TD A([Start]) --> B[/Enter a number/] B --> C{Is input valid?} C -->|No| D[/Display: Please enter a valid integer/] --> E([End]) C -->|Yes| F{Is number negative?} F -->|Yes| G[/Display: Factorial cannot be calculated for negative numbers/] --> E F -->|No| H[Call factorial n] H --> I{Is n = 0 or n = 1?} I -->|Yes| J[Return 1] I -->|No| K[Compute n * factorial n-1] K --> I J --> L[Calculate final result] L --> M[/Display: The factorial of n is result/] M --> E %% Styling style A fill:#f9f,stroke:#333,stroke-width:2px style E fill:#f9f,stroke:#333,stroke-width:2px style B fill:#90caf9,stroke:#333,stroke-width:2px style D fill:#90caf9,stroke:#333,stroke-width:2px style G fill:#90caf9,stroke:#333,stroke-width:2px style M fill:#90caf9,stroke:#333,stroke-width:2px style C fill:#ffd700,stroke:#333,stroke-width:2px style F fill:#ffd700,stroke:#333,stroke-width:2px style I fill:#ffd700,stroke:#333,stroke-width:2px style H fill:#a5d6a7,stroke:#333,stroke-width:2px style J fill:#a5d6a7,stroke:#333,stroke-width:2px style K fill:#a5d6a7,stroke:#333,stroke-width:2px style L fill:#a5d6a7,stroke:#333,stroke-width:2px

Important!
If you find any mistakes in my code or flowchart, please comment below this post. I will be happy to correct them and clear up any doubts you may have.

Flowchart Image: