素数程序给出错误答案

Question

I recently created a program which will tell you if a number is prime or not. It gets all answers right but the prime number of 9.

package main

import "fmt"

func isPrime(num int) bool {
    for x := 2; x > 1; x++ {
        if num%2 == 0 {
            return false
        } else {
            return true
        }
    }
    return true
}

func main() {
    fmt.Printf("is it a prime number: %v 
", isPrime(9))
}

What is the problem?
Thanks in advance!

Answer 1

1- Here is the working version of your code: try it on The Go Playground.

package main

import "fmt"

func isPrime(num int) bool {
    if num < 2 {
        return false
    }
    for x := 2; x < num; x++ {
        if num%x == 0 {
            return false
        }
    }
    return true
}

func main() {
    fmt.Printf("is it a prime number: %v 
", isPrime(9))
}

---

2- The only even prime is 2, so it is better to check it first:

if n == 2 {
    return true
}

Then there is no other even prime number: remove them all, and there is no prime number less than 2:

if n < 2 || n&1 == 0 {
    return false
}

and you don't need to check more than square root of n:

sqrt := int(math.Sqrt(float64(n)))

And now you may start from 3, with just odd numbers (i += 2):

for i := 3; i <= sqrt; i += 2 {
    if n%i == 0 {
        return false
    }
}

Try it on The Go Playground:

package main

import (
    "fmt"
    "math"
)

func isPrime(n int) bool {
    if n == 2 {
        return true
    }
    if n < 2 || n&1 == 0 {
        return false
    }
    sqrt := int(math.Sqrt(float64(n)))
    for i := 3; i <= sqrt; i += 2 {
        if n%i == 0 {
            return false
        }
    }
    return true
}

func main() {
    fmt.Printf("is it a prime number: %v 
", isPrime(9))
    for i := 0; i < 120; i++ {
        if isPrime(i) {
            fmt.Print(i, " ")
        }
    }
    fmt.Println()
}

output:

is it a prime number: false 
2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113

---

See:
Prime number
Generating primes