The factorial of a positive number `n` is given by:

factorial of n (n!) = 1 * 2 * 3 * 4 * ... * n

## Example 1: Find Factorial of a number using for loop

```
fun main(args: Array<String>) {
val num = 10
var factorial: Long = 1
for (i in 1..num) {
// factorial = factorial * i;
factorial *= i.toLong()
}
println("Factorial of $num = $factorial")
}
```

When you run the program, the output will be:

Factorial of 10 = 3628800

In this program, we've used for loop to loop through all numbers between 1 and the given number `num` (10), and the product of each number till `num` is stored in a variable `factorial`.

Unlike Java, in Kotlin, you can use *ranges* (`1..num`

) and *in* operator to loop through numbers between 1 to `num`.

Also, we've used `long` instead of `int` to store large results of factorial.

However, it's still not big enough to store the value of bigger numbers (say 100). For results that cannot be stored in a long variable, we use `BigInteger`

variable declared in `java.math`

library.

Here's the equivalent Java code: Java Program to Find Factorial of a Number.

## Example 2: Find Factorial of a number using BigInteger

```
import java.math.BigInteger
fun main(args: Array<String>) {
val num = 30
var factorial = BigInteger.ONE
for (i in 1..num) {
// factorial = factorial * i;
factorial = factorial.multiply(BigInteger.valueOf(num.toLong()))
}
println("Factorial of $num = $factorial")
}
```

When you run the program, the output will be:

Factorial of 30 = 205891132094649000000000000000000000000000000

Here, instead of `long`

, we use `BigInteger`

variable factorial.

Since, `*`

cannot be used with `BigInteger`

, we instead use `multiply()`

for the product. Also, `num` should be casted to `BigInteger`

for multiplication.

Likewise, we can also use a while loop to solve this problem.

## Example 3: Find Factorial of a number using while loop

```
fun main(args: Array<String>) {
val num = 5
var i = 1
var factorial: Long = 1
while (i <= num) {
factorial *= i.toLong()
i++
}
println("Factorial of $num = $factorial")
}
```

When you run the program, the output will be:

Factorial of 5 = 120

In the above program, unlike a for loop, we have to increment the value of `i` inside the body of the loop.

Though both programs are technically correct, it is better to use for loop in this case. It's because the number of iteration (upto `num`) is known.

Visit this page to learn to *find factorial of a number using recursion*.