The HCF or GCD of two integers is the largest integer that can exactly divide both numbers (without a remainder).

## Example 1: Find GCD of two numbers using while loop

```
fun main(args: Array<String>) {
val n1 = 81
val n2 = 153
var gcd = 1
var i = 1
while (i <= n1 && i <= n2) {
// Checks if i is factor of both integers
if (n1 % i == 0 && n2 % i == 0)
gcd = i
++i
}
println("G.C.D of $n1 and $n2 is $gcd")
}
```

When you run the program, the output will be:

G.C.D of 81 and 153 is 9

Here, two numbers whose GCD are to be found are stored in `n1` and `n2` respectively.

Then, a while loop is executed until `i` is less than both `n1` and `n2`. This way, all numbers between 1 and smallest of the two numbers are iterated to find the GCD.

If both `n1` and `n2` are divisble by `i`, `gcd` is set to the number. This goes on until it finds the largest number (GCD) which divides both `n1` and `n2` without remainder.

Unlike Java, you cannot use a for loop with conditions for this problem. Here's the equivalent Java code: Java Program to Find GCD of two Numbers.

There is a better alternative for finding GCD in Kotlin as follows:

## Example 2: Find GCD of two numbers (Better Alternative)

```
fun main(args: Array<String>) {
var n1 = 81
var n2 = 153
while (n1 != n2) {
if (n1 > n2)
n1 -= n2
else
n2 -= n1
}
println("G.C.D = $n1")
}
```

When you run the program, the output will be:

G.C.D = 9

This is a better way to find the GCD. In this method, smaller integer is subtracted from the larger integer, and the result is assigned to the variable holding larger integer. This process is continued until `n1` and `n2` are equal.

The above two programs works as intended only if the user enters positive integers. Here's a little modification of the second example to find the GCD for both positive and negative integers.

## Example 3: GCD for both positive and negative numbers

```
fun main(args: Array<String>) {
var n1 = 81
var n2 = -153
// Always set to positive
n1 = if (n1 > 0) n1 else -n1
n2 = if (n2 > 0) n2 else -n2
while (n1 != n2) {
if (n1 > n2)
n1 -= n2
else
n2 -= n1
}
println("G.C.D = $n1")
}
```

When you run the program, the output will be:

G.C.D = 9