In this tutorial, you shall learn how to write a program to check if given number is a prime number in Kotlin.

## Kotlin – Check Prime Number

To check if given number is a prime number in Kotlin, check if there is any factor other than 1 and itself. If there is one, then given number is not a prime number.

## Program

In the following program, we read a number from user and store it in `num`, then check if this number is a Prime number or not.

We write a function `isPrime()` that takes an integer as an argument, and returns a boolean value of true if the given number is Prime number, or false otherwise.

Main.kt

```fun isPrime(number: Int): Boolean {
if (number <= 1) {
return false
}

for (i in 2 until number) {
if (number % i == 0) {
return false
}
}

return true
}

fun main() {
print("Enter an integer : ")
if ( isPrime(num) ) {
println("\$num is a prime number")
} else {
println("\$num is not a prime number")
}
}```

Output #1

```Enter an integer : 17
17 is a prime number```

Output #2

```Enter an integer : 42
42 is not a prime number```

Related tutorials for the above program

## Program with Increased Performance

We can increase the performance of `isPrime()` function to take less iterations to check if the given number is Prime number or not. Instead of checking from i=2 to number, we can check from i=2 to number/i because if 2 is not a factor of the given number, then there is no factor between [number/2, number]. We take this fact into consideration and modify the For loop to a While loop.

Main.kt

```fun isPrime(number: Int): Boolean {
if (number <= 1) {
return false
}

var i = 2
while ( i <= number/i ) {
if (number % i == 0) {
return false
}
i++
}

return true
}

fun main() {
print("Enter an integer : ")
if ( isPrime(num) ) {
println("\$num is a prime number")
} else {
println("\$num is not a prime number")
}
}```

Output #1

```Enter an integer : 17
17 is a prime number```

Output #2

```Enter an integer : 42
42 is not a prime number```

Related tutorials for the above program

## Conclusion

In this Kotlin Tutorial, we learned how to check if given number is a Prime number or not.