Factorial with Recursion

CSC-115 · Semester I · C Programming

beginner25 minAvailable

Cin-browser interpreter · runs locally

#include <stdio.h>

int fact(int n) {
  if (n <= 1) return 1;
  return n * fact(n - 1);
}

int main() {
  int n = 6;
  printf("Factorial of %d = %d\n", n, fact(n));
  return 0;
}

13 lines

Output

Write some code, then press Run. Stdout and errors appear here.

Lab brief

Objective

Compute the factorial of a number with a recursive function and trace how the calls unwind.

Theory & concepts

The factorial of n (written n!) is defined recursively: n! = n * (n-1)!, with the base case 0! = 1! = 1. A recursive factorial function mirrors this definition directly, and the base case stops the chain of calls.

Rendering diagram…

Recursive factorial call chain

int factorial(int n) {
    if (n <= 1)
        return 1;                    /* base case */
    return n * factorial(n - 1);     /* recursive case */
}

Trace factorial(4)

The calls chain down 4 -> 3 -> 2 -> 1, the base case returns 1, then each call multiplies and returns up: 2, 6, 24. Watch for the missing base case - without it the function never stops.

Prerequisites

Lesson: Recursion
Lesson: Function definition and prototype

Instructions

1Write a recursive function factorial(int n) using the base case n <= 1.
2Read an integer n from the user in main.
3Guard against negative input - print an error for n < 0.
4Print the result of factorial(n).
5Trace the call chain for n = 4 by hand before running.

Tasks

Implement recursive factorial
Compute and print 5! which must equal 120.
Verify several inputs
Check 0! = 1, 1! = 1, and 6! = 720 against a calculator.
Trace the recursion
Write the sequence of calls for factorial(4) and the value each one returns.
Add input validation
Reject negative numbers and explain why factorial is undefined for them.

Self-check

factorial returns correct values for 0, 1, and positive inputs.
Negative input is handled without crashing.
The base case exists and is reachable.
The student can trace factorial(4) call by call.

Environment

editorstandard inputstdout consoletest cases