I need a hand with functions.

[smerfguy2]

Hello I am trying to write a simple program that basically tells a user whether the number they entered is prime or not. The code looks like this and I can't figure out why it isn't working.

#include <iostream>
#include <cmath>
using namespace std;
bool prime(int n);


int main()
{
    int u;

    while(true){
        cout << "Enter an number\n";
        cin >> u;
        if(u == 0){
        break;
        }
        if(prime(u)){
            cout << u << " Is prime\n";
        }else{
        cout << u << " Is not prime\n";
        }

    }

    return 0;
}

bool prime(int n){
    int i;
    for(i = 2; i <= sqrt(n); i){
        if(n % i != 0){
            return true;
        }
            return false;

                                }

}

Also I could make this program in a much simpler version but the chapter in the book I am working on is all about functions. Could someone explain this to me?

[liquidfuzz]

Hi!

You need to start checking if parts of the code does what you intend. Nerds call it 'debugging' and I'm pretty sure that most of the time coding is spend debugging in the world. Insert some cout and see. If you debug you'll be able to narrow down the gnarly.

[smerfguy2]

Awesome thanks. Im a beginner so any of the tips I can get help me!

[Pravesh Koirala]

@OP
in your Prime() function

bool prime(int n){
    int i;
    for(i = 2; i <= sqrt(n); i){  //you missed i++
        if(n % i != 0){     //this part is wrong
            return true;
        }
            return false;

                                }

}

Replace it with

bool prime(int n){
    int i;
    for(i = 2; i <= sqrt(n); i++){
        if(n % i == 0){
            return false;
        }
    return true;
}

I want you to figure out what is wrong with your code. Try and post it. I'll explain it to you if you can't figure it out.

[smerfguy2]

My problem here is that I can't seem to figure out the right equation, and functions barely make sense to me. I can't just skip them because they are 100% needed all the time. and I'm not sure why I need i++ or i <= sqrt(n)

If you could explain that it would be awesome. The program runs fine until I enter 9, then it tells me 9 is prime which it isn't. 3 * 3 = 9....

[Pravesh Koirala]

Yup! that's the problem.
When you enter 9, and you call the function prime(9). Then the for loop will iterate from
i = 2 to i = 3.
For i=2
your statement (9%2 != 0) becomes true
so you end up returning true. But you never check 9 for i = 3.
You got the problem?

In my fix. I first check 9 for all the values (2 and 3 in this case). If any of the values divides 9 perfectly (3 in this case), then I simply return false.
And if none of them divide the number prefectly, I return false.
.
You got my point?

[Nisheeth]

I'm not sure why I need i++ or i <= sqrt(n)

i++, will increase i after iteration of the loop.
This is to check if it is divisible by all numbers beginning from 2 to sqrt(n).
The upper limit is determined by i <= sqrt(n).

And as for why we are using sqrt(n), as per Wikipedia:
if n is composite then it can be factored into two values, at least one of which must be less than or equal to