C Program to find factorial using iterative method

In this tutorial, we will learn to write the program to find the factorial of numbers using the C programming language. Here we are going to follow an iterative approach.

Please follow this article for factorial program in C using recursion

Before moving forward let’s understand

What is Factorial of a number?

Factorial of a whole number ‘n’ is the product of that number ‘n’ with its every whole number in descending order till 1.

For example

Factorial of 5 is 5*4*3*2*1 = 120

And factorial of 5 can be written as 5!.

Note: To store the largest output we are using long long data type. Long long takes double memory as compared to single long.

There are multiple ways to write the program in C to calculate the factorial of the whole number.

In this tutorial, we will learn to write using

1). Using For Loop

2). Using Function

1). Factorial Program in C of a given number using for Loop

#include<stdio.h>
int main() {
  int i, num, factorial = 1;
  printf("=== Factorial Program in C === \n");
  printf("Enter a whole number to find Factorial: ");
  scanf("%d", &num);
  for (i = 1; i<=num; i++) {
    factorial = factorial * i;
  }
  printf("Factorial of %d is: %d", num, factorial);
  return 0;
}

Output

=== Factorial Program in C === 
Enter a whole number to find Factorial: 5
Factorial of 5 is: 120

Program Explanations

1. Header Inclusion and Main Function Declaration:
   • #include <stdio.h> includes the Standard Input Output library necessary for input and output operations through printf and scanf.
   • int main() is the starting point for the program.
2. Variable Declaration:
   • int i, num, factorial = 1; declares three integer variables:
     • i is used as a loop counter.
     • num will store the user input number for which the factorial is to be calculated.
     • factorial is initialized to 1 and is used to compute the factorial of num.
3. Input from User:
   • The program prompts the user to enter a whole number (num). The input is read using scanf.
4. Calculation of Factorial:
   • A for loop runs from 1 to num. Within the loop, factorial is updated by multiplying it with i during each iteration.
   • This multiplication continues until i is equal to num, effectively calculating num! (num factorial).
5. Output the Result:
   • The factorial of the number is displayed using printf, which outputs the factorial stored in factorial.
6. Program Termination:
   • The program returns 0, indicating successful execution.

2). Factorial Program in C using Function and While Loop

#include<stdio.h>
int fact(int num) {
  int i = 1, fact = 1;
  while (i <= num) {
    fact *= i;
    i++;
  }
  return fact;
}
int main() {
  int i, num, factorial = 1;
  printf("=== Factorial Program in C === \n");
  printf("Enter a whole number to find Factorial: ");
  scanf("%d", & num);
  printf("Factorial of %d is: %d", num, fact(num));
  return 0;
}

Output

=== Factorial Program in C === 
Enter a whole number to find Factorial: 5
Factorial of 5 is: 120

Program Explanations

1. Header Inclusion:
   • #include <stdio.h>: Includes the Standard Input Output library necessary for using the printf and scanf functions for input and output operations.
2. Function Definition for fact:
   • int fact(int num): This function takes an integer num as an argument and returns its factorial.
   • Within the function, two integer variables i and fact are initialized to 1. The variable fact will store the computed factorial.
   • A while loop iterates from 1 to num. In each iteration, fact is multiplied by i, and then i is incremented by 1.
   • Once the loop completes (when i exceeds num), the function returns the value of fact, which by then represents the factorial of num.
3. Main Function:
   • int main(): The main entry point of the program.
   • Variables:
     • int i, num, factorial = 1: Declares variables for loop counters and storage (factorial is initialized but not actually used for computation in main).
   • Input:
     • The program prompts the user to enter a whole number using printf, and the input is read into the variable num using scanf.
   • Calculation and Output:
     • The factorial of num is computed by calling the fact function with num as an argument.
     • The result is then printed to the console using printf, displaying the factorial of the entered number.
   • Program Termination:
     • return 0;: The main function ends and returns 0, indicating successful execution.