Detecting prime numbers

[b52]

My solution seems quite different from the one in the 'Solution Guide'.
Will mine detect all primes or will it break? It appears to work but
I don't know enough to be sure. Thanks for any insight.

b52

#include <iostream>
using namespace std;

int main()
{
  int i,j;
  for (i = 2; i < 1000; i++)
  {
    for (j = 2; j < i; j++)
      if (i % j * j == 0) break;
    if (j < i)
      continue;
    else
      cout << i << endl;
  }
  return 0;
}

[helios]

What's the point of the "* J" on line 10? i%j*j can only ==0 if j==0 or i%j==0, and j can never be zero.

[b52]

I see what you mean. When I googled prime formula I must have misunderstood the information that I read. Thanks, I would never have noticed that - even when I debugged the code.

b52

[jankidudel]

I variant :

#include <iostream>
using namespace std;

int main()
{
    int  count=0;
    for(int i=2; i<1000; i++) 
    {
        for(int j=2; j<i; j++) 
        {
            if(i%j==0)
                count++;
        }
        if(count==0)
        {
            cout<<"Prime: "<<i<<endl;
            count =0;
        }
        else
            count =0;    
    }  
    
    return 0;
}

II variant :

#include <iostream>
using namespace std;

int main()
{
    for(int i =2; i<1000; i++)
    {
        if((i%2!=0 && i%3!=0 && i%5!=0)|| ( i==2 || i==5 || i==3))
            cout<<"Prime : "<<i<<endl;
    }
    return 0;
}

[helios]

The second variant doesn't work. The first one is inefficient.

[jankidudel]

Yea, first is inefficient, but why you're saying that second variant doesn't work? Do you had tried it?

[Skillless]

No but its obvious it doesnt work because it only tries divisions by 2, 3, and 5, not all other prime numbers that come after those

(It would say, 21 is a prime number, even though 21%7 == 0)

[spaggy]

helios is right the second one won't work. 169 is not divisible by 2,3 or 5 but is not a prime number

[helios]

PROTIP: It's not necessary to test something to demonstrate that it's incorrect, and just because something passes a test, it doesn't make it correct. Testing can only prove the presence of bugs, not their absence.

[D80DSL3A]

Agreed that 2nd method won't work. Skillless needed a higher # for his counter example as 21%3==0 is tested for. Method fails if lowest prime factor is > 5 so lowest "false positive" comes for 7*7 = 49.

[Skillless]

Oh lol yeah ofcourse, sorry >.> my brain doesnt work when im in an office