Write a C program to input a number and calculate its factorial using for loop. How to find factorial of a number in C program using loop. Logic to find factorial of a number in C programming.

**Example**

**Input**

Input number: 5

**Output**

Factorial: 120

## Required knowledge

### What is factorial?

Factorial of a number `n` is product of all positive integers less than or equal to `n`. It is denoted as `n!`

.

For example factorial of 5 = 1 * 2 * 3 * 4 * 5 = 120

## Logic to find factorial of a number

Step by step descriptive logic to find factorial of a number.

- Input a number from user. Store it in some variable say
`num`. - Initialize another variable that will store factorial say
`fact = 1`

.Why initialize

`fact`with 1 not with 0? This is because you need to perform multiplication operation not summation. Multiplying 1 by any number results same, same as summation of 0 and any other number results same. - Run a loop from 1 to
`num`, increment 1 in each iteration. The loop structure should look like`for(i=1; i<=num; i++)`

. - Multiply the current loop counter value i.e.
`i`with`fact`. Which is`fact = fact * i`

.

## Program to find factorial of a number

```
/**
* C program to calculate factorial of a number
*/
#include <stdio.h>
int main()
{
int i, num;
unsigned long long fact=1LL;
/* Input number from user */
printf("Enter any number to calculate factorial: ");
scanf("%d", &num);
/* Run loop from 1 to num */
for(i=1; i<=num; i++)
{
fact = fact * i;
}
printf("Factorial of %d = %llu", num, fact);
return 0;
}
```

Since factorial cannot be negative and can grow rapidly, therefore I have used `unsigned long long`

data type to store factorial. If your compiler doesn't supports `long long`

then, you can use `unsigned long`

which is guaranteed to run on all compilers. If you are using `unsigned long`

then you also need to replace the format specifier from `%llu`

to `%lu`

.

Advance your skills by learning this program using recursive approach.

Read more - Program to find factorial using recursion.

Output

Enter any number to calculate factorial: 5 Factorial of 5 = 120

Happy coding 😉

<pre><code> ----Your Source Code---- </code></pre>