Prime numbers

[acr2176]

Hey, this is my first post around here!
I've been playing with c++ programming for the last couple weeks and thought joining a forum would be helpful in learning more about c++

Anyway, i'm here bacause i've been working on a program that would display only prime numbers within a interval given by the user.

I was kind of proud of myself when I finally got one that worked! lol

Problem is, i can't figure out a way to include 2,3,5,7 in it...
I tried doing a if statement such as
if((a%x)!=0) cout<< a<<endl;
and adding a for (x=2; x=7; x++){

it kind of makes sense that it would work, but i don't know why it doesn't!

I know is probably a silly mistake somewhere, but what is it?? haha, it's driving me crazy

this is my code pasted:

#include <iostream>

using namespace std;

void function ();
int m, a;
int main(){

while (true){
system ("CLS");
cin.clear();

cout<<"FINDING PRIME NUMBERS WITHIN A RANGE"<<endl<<endl;
cout<<"Enter range"<<endl<<endl;
cout<<"From one to:";
cin>> m;
cout<<endl;
function ();

system ("PAUSE");
}
return 0;

}

void function (){

cout<<"2"<<endl<<"3"<<endl<<"5"<<endl<<"7"<<endl; //This is the part I cheated.
for ( a=1; a<=m; a++ ){
int d=2;
int t=3;
int f=5;
int s=7;
if ((a%d)!=0 && (a%t)!=0 && (a%f)!=0 && (a%s)!=0 && a!=1) cout<< a<< endl;
}
}

[eker676]

You need two for loops to make this work properly. Look at this
http://en.allexperts.com/q/C-1040/finding-prime-numbers-1-1.htm

That person takes a number and moduluses it by all of the numbers greater than 1 and less than itself. If the two numbers tested come out to be an even divide (the modulus of the two numbers equals zero) then the number is not prime.

[fatllama]

First you should think about what makes a number prime: it can only be divided evenly by 1 and itself.

Remember you can divide it by all the numbers from 2 to the square root of the number: any number greater than that can not be a factor of the number chosen.
The way I wrote my prime number program was by making an "is_prime" boolean. This is set to true as a default.
I then loop a variable from 2 until it is < the square root of the number.
Each cycle, the given number is divided by the variable: if it has no remainder, the loop is broken, the boolean is set to false, and it goes to the results:
It then displays "Your number is False" or "Your number is True" depending on what the boolean is set to :-).
I hope this helps.