Prime numbers. Go newbie

[xorornot]

Doing a great Go class at Udemy and to practice im trying to migrate some Python scripts to Go. There is a very simple script im trying to make, that basically find the prime / non-prime numbers on a range.

Appreciate if someone could point out what im doing wrong…

Im able to detect the non-primes fine (code below):

package main

import "fmt"

func main() {
	var numbers int
	fmt.Print("please enter the number you want analyzed? ")
	fmt.Scan(&numbers)
	for i := 1; i < numbers; i++ {
		check(i)
	}
}

func check(numbers int) {
	if numbers > 1 {
		for i := 2; i < numbers; i++ {
			if numbers%i == 0 {
				fmt.Println(numbers, "isnt a prime number //", i, "times", numbers/i, "is", numbers)
				break
			}
		}
	} else {
		fmt.Println(numbers, "isnt a prime number")
	}
}

Once I put an else statement to show the prime numbers, the program blows up. (below).

func check(numbers int) {
	if numbers > 1 {
		for i := 2; i < numbers; i++ {
			if numbers%i == 0 {
				fmt.Println(numbers, "isnt a prime number //", i, "times", numbers/i, "is", numbers)
				break
			} else {
				fmt.Println(numbers, "its prime!") // ADDED THIS!
			}
		}
	} else {
		fmt.Println(numbers, "isnt a prime number")
	}
}

[joncalhoun]

Your logic is off a bit. Think about the if statement you added - tey walking through it in your head or on paper if numbers is 7. That else statement will execute like 6 times.

If you want more info on how to fix it I can help but given that you are doing this to practice I won’t post anymore unless asked. I want you to have a chance to debug

[xorornot]

Hi @joncalhoun
I already cracked my head on this. Not having the “while” loop is freezing my brain. Can you please tell me how an experienced Go programmer would do it?
Thx

[NobbZ (Norbert Melzer)]

There is a while in go, just that it is written out for.

for i <= 10 {
  i++
}

[joncalhoun]

Try running this: https://play.golang.org/p/sugffCQzHH

See how the print statement prints out 5 different times? That is because a number is NOT prime if it is divisible by any number other than 1 and itself, but you have to check ALL numbers from 2 to sqrt(number) to see if a number is prime (technically you don’t need to check that many numbers but that is a minor detail here). Your code goes from 2 to numbers - 1 which is fine, just slightly slower. Your real issue stems from the fact that you are declaring a number as prime before that ever happens.

How can you change your code so that it only prints “this is a prime” if all numbers from 2 to sqrt(numbers) are checked?

PS - It honestly looks like your problem isn’t with Go, but is with your logic being used. I’m unclear how a while loop would change this.