Challenge 1: Extension Function on Int — Solution

fun Int.isPrime(): Boolean {
    if (this < 2) return false
    for (i in 2 until this) {
        if (this % i == 0) return false
    }
    return true
}

fun main() {
    println(2.isPrime())
    println(7.isPrime())
    println(9.isPrime())
    println(1.isPrime())
}

/*
Output:
true
true
false
false

Notes:
- fun Int.isPrime() adds isPrime() to every Int in the codebase, called
  exactly like a real member function: 7.isPrime().
- Inside the function, "this" refers to the specific Int the function was
  called on (7, in 7.isPrime()).
- Numbers less than 2 return false immediately; otherwise the loop checks
  for any divisor between 2 and (this - 1) — finding one means it isn't
  prime.
*/