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# tylerkerr/nthprime.go

Created June 18, 2017 18:55
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 package main import "fmt" import "os" import "strconv" import "math" func sieve(n int) []int { isprime := make([]bool, n+1) for i := range isprime { // help. how do i initialize the array as all true isprime[i] = true } isprime[0] = false if len(isprime) > 1 { isprime[1] = false } max := int(math.Ceil(math.Sqrt(float64(n)))) for i := 2; i <= max; i++ { if isprime[i] == true { for j := i * i; j <= n; j += i { isprime[j] = false } } } var primes []int for i, v := range isprime { if v == true { primes = append(primes, i) } } return primes } func main() { nthprime, err := strconv.Atoi(os.Args[1] - 1) if err != nil { fmt.Println(err) } var searchbound int var searchmultiplier float64 if nthprime == 1 { searchbound = 2 } else { if nthprime < 11 { searchmultiplier = 3 } else { searchmultiplier = 1.2408246135107 // found this via trial and error } searchbound = int(math.Log(float64(nthprime)) * float64(nthprime) * searchmultiplier) } primes := sieve(searchbound) // efficiency := float64(nthprime) / float64(len(primes)) // fmt.Printf("searched up to %d and found %d primes (efficiency of %.4f). prime number %d is:\n", searchbound, len(primes), efficiency, nthprime) fmt.Println(primes[nthprime]) }
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