Mersenne Prime Oracle

Question

Please give me any feedback regarding good coding standards and readability.

fun main(arg: Array<String>) {

    //do while user guess != Mersenne Prime.
    do {
        //initializing variable
        var number: Int = 3
        try {
            number = readLine()!!.toInt()

            if (isMersennePrime(number)) 
            println("${number} is a Mersenne Prime. Congratulations.")

            else 
            println("${number} is not a Mersenne Prime.")
        }
        catch (e: NumberFormatException) {
            println(" is not a valid number. Exiting.")
        }
    } while (!isMersennePrime(number))
}

    // is Number both prime and 2^n - 1? 
    fun isMersennePrime(Number: Int): Boolean =    
    (twonminus1(Number.toDouble()) && isPrime(Number))

    // is x Prime?
    fun isPrime(x: Int): Boolean = (2..x-1).all{x % it != 0 && x != 2}

    // is x = 2^n - 1?
    fun twonminus1(x: Double): Boolean = Math.log(x + 1.0)/Math.log(2.0) % 1.0 == 0.0

Answer 1

Others already made some good points. Calling isMersennePrime twice is really bad because that can be very computationally intensive.

I'm not sure why you are not allowing 2 to be a prime number. Where I'm from 2 is prime.

When scanning for primes, you can stop at sqrt(x) instead of x - 1. If y * z = x, one of y or z is smaller than sqrt(x).

@Roland gave you a trick on checking if an integer is a power of two, but I just wanted to add a reference to wikipedia with some explanation.

Also var number: Int = 3 is quite odd. It's better to use immutable val. Instead:

while (true) {
    print("Enter a number: ")
    val text = readLine()
    val number = try {
        text!!.toInt()
    } catch (e: NumberFormatException) {
        println("$text is not a valid number. Exiting.")
        break
    }
    if (isMersennePrime(number)) {
        println("${number} is a Mersenne Prime. Congratulations.  Exiting.")
        break
    } else {
        println("${number} is not a Mersenne Prime.  Try again.")
    }
}

Answer 2

Do not use double arithmetics when integer arithmetics can solve the problem as well. The function isPowerOfTwoMinus1 can be written simply as ((x + 1) & x) == 0.

Don't call isMersennePrime twice, just break from the loop.

Answer 3

1. (2..x-1).all{x % it != 0 && x != 2} can be replaced with (3..x-1).all{x % it != 0}
2. Your variables should follow camelCase. You have a Number variable in isMersennePrime
3. Your try-catch could cover only the number = readLine... line