Help Please before i go crazy!!!!

[almul]

Hi there can you help me understand what is happening in this program or in other words try and explain the logic of it to me, i can understand the program but for one line.

This is the line: for(j=2; j <= i/2; j++)

Like is this for statement a loop or what is it. I just don't get the j<=i/2 bit can some please expalin this bit to me, before i go crazy.


The program basically is to find and output prime numbers between 1 - 100.


#include <iostream>
using namespace std;
int main() {
int i, j;
bool isprime;
for(i=1; i < 100; i++)
{
isprime = true;
// see if the number is evenly divisible
for(j=2; j <= i/2; j++)

// if it is, then it is not prime
if((i%j) == 0)
isprime = false;

if(isprime)
cout << i << " is prime.\n";
}
return 0;
}


Many Thanks
almul

[JVene]

Use code tags like this:

#include <iostream>
using namespace std;

int main() 
{
 int i, j; 
 bool isprime;
 for(i=1; i < 100; i++) 
   {
    isprime = true;
    // see if the number is evenly divisible 
    for(j=2; j <= i/2; j++)

    // if it is, then it is not prime
    if((i%j) == 0)
    isprime = false;

    if(isprime)
    cout << i << " is prime.\n";
   }

 return 0;
}

Would it make more sense to you like this

#include <iostream>
using namespace std;

int main() 
{
 int i, j; 
 bool isprime;
 for(i=1; i < 100; i++) 
   {
    isprime = true;
    // see if the number is evenly divisible 
    int limit = i / 2;

    for(j=2; j <= limit; j++)

    // if it is, then it is not prime
    if((i%j) == 0)
    isprime = false;

    if(isprime)
    cout << i << " is prime.\n";
   }

 return 0;
}

It's the same thing

[almul]

Thanks but what does the: "i/2" bit actually mean. Thats what im having trouble understanding.

Also is the second for statement an actually loop???

Can someone please explain.

Thanks

[Plasmator]

Thanks but what does the: "i/2" bit actually mean. Thats what im having trouble understanding. Can someone please explain.

Just like kindergarten arithmetic...

For integers:
1 / 2 = 0
2 / 2 = 1
3 / 2 = 1
4 / 2 = 2
5 / 2 = 2
6 / 2 = 3
7 / 2 = 3
8 / 2 = 4
etc.

[almul]

Ok got that bit sorted now for the second for statement. Is this 2nd for statement a loop if it is then where are its curley brackets?
Thanks

[jefranki]

As supplied, this code is equivilent to:

#include <iostream>
using namespace std;

int main() 
{
 int i, j; 
 bool isprime;
 for(i=1; i < 100; i++) 
   {
    isprime = true;
    // see if the number is evenly divisible 
    for(j=2; j <= i/2; j++) if((i%j) == 0) isprime = false;

    if(isprime) cout << i << " is prime.\n";
   }

 return 0;
}