Trying to figure out what I am doing wrong.

[kapua51]

Let me start off by saying that I am new to programming in C++. I know this may be basic but can someone help me in telling me what I am doing wrong with this code?

#include "conio.h"
#include <iostream>
#include <cmath>
#include <stdio.h>
using namespace std;

int main()
{
double n;
double d;
double y;
double fmod;

for (n = 3; n <=100; n++)
{for (d = (n - 1); d>=2; d--);
fmod(n,d);}
cout<<n<<"is not prime";
if (fmod=0)
cout<<n<<"is not prime";
else
cout<<n<<"is prime"<<endl;

return 0;
}

[f1engineer]

#include "conio.h"
#include <iostream>
#include <cmath>
#include <stdio.h>
using namespace std;

int main()
{
double n;
double d;
double y;
double fmod;

for (n = 3; n <=100; n++)
{
for (d = (n - 1); d >= 2; d--) //Here You havn't semicolon
fmod(n,d);
}
cout<<n<<"is not prime";
if (fmod=0)
cout<<n<<"is not prime";
else
cout<<n<<"is prime"<<endl;

return 0;
}

[David Debonnaire]

#include "conio.h"
#include <iostream>
#include <cmath>
#include <stdio.h>
using namespace std;

int main()
{
double n;
double d;
double y;
double fmod;

for (n = 3; n <=100; n++)
{
for (d = (n - 1); d >= 2; d--) //Here You havn't semicolon
fmod(n,d);
}
cout<<n<<"is not prime";
if (fmod=0) //**** Testing for equality : 'if (fmod == 0) ....'
cout<<n<<"is not prime";
else
cout<<n<<"is prime"<<endl;

return 0;
}

//**** Testing for equality : 'if (fmod == 0) ....'

[Banfa]

Generally speaking you should only use floating point variables (float and double) if you need to and you should not test the for equality to anything as floating point arithmatic is not acurate due to rounding errors.

You are trying to calculate the primeness (prime or not prime) of integers (only integers can be prime or not prime), therefore you have used the wrong basic types to implement this program. The calculations should be intinsically integer based so so should your variables.

This will also make the program faster and less prone to rounding errors.