gosieve.go

package main

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

func main() {
	n, _ := strconv.Atoi(os.Args[1])
	primes := make([]bool, n+1)

	primes[0], primes[1] = true, true // 0 and 1 are not primes

	max := int(math.Ceil(math.Sqrt(float64(n))))

	for i := 2; i <= max; i++ { // the sieve
		if primes[i] == false {
			for j := i * i; j <= n; j += i {
				primes[j] = true
			}
		}
	}

	count := 0
	for _, v := range primes {
		if v == false {
			count += 1
		}
	}
	fmt.Println(count)
}