Primes

primes.go

package main

import (
    "os"
    "fmt"
    "math"
    "strconv"
)

/*
 * Determines if a number is prime. The second
 * argument must be a list of all primes lower 
 * than n
 */
func isprime(n int, primes []int) bool {
    max := int(math.Sqrt(float64(n)))

    for j := 0; j < len(primes) && max >= primes[j]; j++ {
        if n % primes[j] == 0 {
            return false
        }
    }

    return true
}

/*
 * Returns a list of the first n prime numbers
 */
func nprimeslist(n int) []int {
    primes := []int{}

    for i := 2; len(primes) < n; i++ {
        if isprime(i, primes) {
            primes = append(primes, i)
        }
    }

    return primes
}

/*
 * Returns a list of the prime numbers less
 * than max
 */
func maxprimeslist(max int) []int {
    primes := []int{}

    for i := 2; i < max; i++ {
        if isprime(i, primes) {
            primes = append(primes, i)
        }
    }

    return primes
}

/*
 * Finds the nth prime number
 */
func nthprime(n int) int {
    prime := 0

    if n >= 1 {
        primes := nprimeslist(n)
        prime = primes[n - 1]
    }

    return prime
}

func main() {
    if len(os.Args) != 2 {
        fmt.Printf("Usage: %s number\n", os.Args[0])
        os.Exit(1)
    }

    n, _ := strconv.Atoi(os.Args[1])
    prime := nthprime(n)
    fmt.Printf("The %dth prime is %d\n", n, prime)
}