Kotlin Program to Check Whether a Number is Prime or Not

In this article, you'll learn to check whether a number is prime or not. This is done using a for-in loop in Kotlin.

Example 1: Program to Check Prime Number using a for-in loop

fun main(args: Array<String>) {
    val num = 29
    var flag = false
    for (i in 2..num / 2) {
        // condition for nonprime number
        if (num % i == 0) {
            flag = true
            break
        }
    }

    if (!flag)
        println("$num is a prime number.")
    else
        println("$num is not a prime number.")
}

When you run the program, the output will be:

29 is a prime number.

Like Java, in the above program, for loop is used to determine if the given number num is prime or not. We only have to loop through 2 to half of num, because no number is divisible by more than its half.

Inside the for loop, we check if the number is divisible by any number in the given range (2..num/2). If it is, flag is set to true and we break out of the loop. This determines num is not a prime number.

If num isn't divisible by any number, flag is false and num is a prime number.

Here's the equivalent Java code: Java Program to Check Prime Number


Example 2: Program to Check Prime Number using a while loop

fun main(args: Array<String>) {
    val num = 33
    var i = 2
    var flag = false
    while (i <= num / 2) {
        // condition for nonprime number
        if (num % i == 0) {
            flag = true
            break
        }
        ++i
    }

    if (!flag)
        println("$num is a prime number.")
    else
        println("$num is not a prime number.")
}

When you run the program, the output will be:

33 is not a prime number.

In the above program, while loop is used instead of a for loop. The loop runs until i <= num/2. On each iteration, whether num is divisble by i is checked and the value of i is incremented by 1.

Visit this page to learn, how you can display all prime numbers between two intervals.