odd-not-prime.md

Odd Not Prime ^{7 Kyu}

^{LINK TO THE KATA - ALGORITHMS}

Description

For "x", determine how many positive integers less than or equal to "x" are odd but not prime. Assume "x" is an integer between 1 and 10000.

Example: 5 has three odd numbers (1,3,5) and only the number 1 is not prime, so the answer is 1

Example: 10 has five odd numbers (1,3,5,7,9) and only 1 and 9 are not prime, so the answer is 2

Solution

const isPrime = n => {
  if (n <= 1) return false

  if (n <= 3) return true

  if (n % 2 === 0 || n % 3 === 0) return false

  let i = 5

  while (i * i <= n) {
    if (n % i === 0 || n % (i + 2) === 0) return false
    i += 6
  }

  return true
}

const oddNotPrime = n => {
  let count = 0

  for (let i = 1; i <= n; i += 2) {
    if (!isPrime(i)) count++
  }

  return count
}