Prime Number Sample Code

Here are some simple functions are finding prime numbers. These are meant as examples of basic Go programming. An example of table-driven testing is included, a popular testing technique in the Go community.

// primes.go

package main

import (
  "fmt"
)

func isPrime(n int) bool {
  if n < 2 {
    return false
  } else if n == 2 {
    return true
  } else if n%2 == 0 {
    return false
  } else { // n > 2
    d := 3
    for d*d <= n {
      if n%d == 0 {
        return false
      }
      d += 2
    } // for
    return true
  } // if
}

func isPrime2(n int) bool {
  switch {
  case n < 2:
    return false
  case n == 2:
    return true
  case n%2 == 0:
    return false
  default: // n > 2
    d := 3
    for d*d <= n {
      if n%d == 0 {
        return false
      }
      d += 2
    } // for
    return true
  } // switch
}

//
// table-driven testing
//
var prime = []int{2, 3, 5, 7, 11, 13, 17, 31, 37, 101}
var notPrime = []int{-2, -1, 0, 1, 4, 6, 8, 9, 10, 12, 91, 100}

func testIsPrime(isPrime func(int) bool) {
  for _, n := range prime {
    if !isPrime(n) {
      fmt.Printf("testIsPrime: %v is prime!\n", n)
      panic("error!")
    }
  }
  for _, n := range notPrime {
    if isPrime(n) {
      fmt.Printf("testIsPrime: %v is not prime!\n", n)
      panic("error!")
    }
  }
  fmt.Println("all testIsPrime tests passed")
}

func countPrimesLessThan(n int) (count int) {
  if n < 2 {
    return 0
  } else {
    for i := 2; i < n; i++ {
      if isPrime(i) {
        count++
      }
    }
    return count
  }
}

func main() {
  testIsPrime(isPrime)
  testIsPrime(isPrime2)

  // nvals := []int{10, 100, 1000, 10000, 100000}
  // for _, n := range nvals {
  //  fmt.Printf("countPrimesLessThan(%v)=%v\n", n, countPrimesLessThan(n))
  // }
  // for i := 0; i < len(nvals); i++ {
  //  fmt.Printf("countPrimesLessThan(%v)=%v\n", nvals[i], countPrimesLessThan(nvals[i]))
  // }
  // for i := 0; i <= 25; i++ {
  //  fmt.Printf("isPrime2(%v) = %v\n", i, isPrime2(i))
  // }
} // main