Prime numbers problem

[zakkor]

I have no idea why this program of mine doesn't work.

#include <iostream>
using namespace std;

int a, nr, n;

int main()
{
    cin>>n;
    for(int i=1; i<=n; i++)
    {
        cin>>a;
        if(a%2!=0 && a%3!=0 && a%5!=0 && a%7!=0)
        {
            nr++;
        }
    }
    if(nr==n)
    {
        cout<<"all the numbers are prime.";
    }
    if(nr!=n)
    {
        cout<<"all the numbers are not prime.";
    }
    return 0;
}

It's supposed to input a set amount of numbers, then tell you if all of them are prime or not.

Also excuse me for my bad english I'm from Sweden.

[Naureg]

Hej, har du sett min häst?

Han är grön och liten ;_;

[Stewbond]

you didn't initialize nr.

Set:
int a, nr=0, n;

[zakkor]

I did what you said, but it still doesn`t work... It tells me that all the numbers are not prime even if i input only prime numbers..

[Chervil]

Well, your test at line 12 is not the proper way to test for prime numbers.

[Stewbond]

Let's say you input a = 3.

a%3 is the remainder of 3/3. The remainder is 0. 0!=0 so that will evaluate to false. false && anything is false, therefore you will not enter the if statement and nr will not be incremented.

do something like:

for (int i = 2; i < sqrt(a); ++i)
{
    if (a%i)
    {
        a is not prime;
        break
    }
    a will be prime if not broken in the if in any iteration;
}

[ResidentBiscuit]

For a number to be prime, it can't be divisible by any number less than itself, besides 1. You're only testing divisibility by 2,3,5, and 7.