CODEWITHSUNDEEP
scalarecursionprimes
Asjal Ahmad
Asjal Ahmad

Reputation: 73

Is there a more effective way to check if a number is prime using recursive function?

I am writing a recursive function to check if a non-negative Integer is Prime in SCALA. My function takes two inputs to do so. Here is my code:

import io.StdIn._
def isPrime (x:Int, i:Int): Boolean = {
  if (i < 3) {
    true
  }else if (x % i == 0) {
    false
  }else{
    isPrime(x,i-1)
  }
}

print("Enter a number to check if it is a prime number: " )
val num1 = readInt()
println(isPrime(num1,num1-1))

My question is that is there a way for me to take one input as the parameter for the function? The code has to use a recursive function. I know that there are more effective ways to check if a number is prime (using iteration perhaps), but I'm just doing this as a practice problem.

Upvotes: 1

Views: 572

Answers (2)

Martijn
Martijn

Reputation: 12102

One option is to make an inner method

def isPrime(x:Int): Boolean = {
  def loop(i: Int): Boolean = 
    if (i<3 ) {
      true
    } else if (x % i==0) {
      false
    } else {
      isPrime(x,i-1)
    }
  loop(x, x - 1)
}

another option is to make a default parameter. Those can't refer back to the previous parameter, so you'll have to use a workaround. Here I pass some j, and make i = x - j, so I can just increment j starting from 1

def isPrime(x: Int, j: Int = 1): Boolean = {
  val i = x - j
  if (i < 3) {
    true
  } else if (x % i==0) {
    false
  } else {
    isPrime(x, j + 1)
  }
}

Unrelatedly, there is a bug in your code: 4 should not be prime.

Upvotes: 1

Keith Pinson
Keith Pinson

Reputation: 1725

It is perfectly fine, I'd say even standard practice, to wrap recursive functions with a calling function. For example:

def isPrime( x:Int ) = { 
  def recIsP( n:Int, i:Int ) : Boolean = i == n || n % i != 0 && recIsP(n, i + 1) 
  recIsP(x,2) 
}

isPrime(3301)

Upvotes: 1

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