Maths Programming Help

Hey, I new to C++ and I was trying to make a program that solved a maths problem for me (I wanted to find out how many times the anwer would be a positive integer, given that n is a positive integer):
n+17
------ // That's a dividing sign
n-7

I tried a few methods, but couldn't get what I want.
Here's my latest attempt:

Code:

---

#include <stdafx.h>
#include <iostream>

using namespace std;


float main()
{
        float x, y, z;
        for ( float n = 0; n < 1000; n = n + 14){
                x = n - 7;
                y = n + 17;
                z = n / x;
                if (z > 0){
                        cout<<y <<endl;
                        n = n - 17;
                        x = x - 7;
                }
                else {
                        cout<<y <<" is not a positive integer" <<endl;
                }
        }
        cin.get();
        return 0;
}

---

I'm using the Visual C++ Express 2005 program if tht helps. I thought I might make the variables floats to make sure the decimals aren't truncated in an int thinking the program will see them as positive integers anyway. Thanks!

If you want to know whether (n+17)/(n-7) is an integer, just check whether (n+17)%(n-7) == 0 using integer arithmetic. The % is the modulus (remainder) operator.

Edit: BTW, it's "int main()", not "float main()".

Thanks, I tried it once and it didn't work:

Code:

---

int main()
{
        for (int n = 0; n < 1000; n = n + 14){       
                if ((n + 17)%(n - 7) == 0){
                        cout<<n <<endl;
                }
        }
        cin.get();
        return 0;
}

---

so then I assigned x the value of (n + 17)%(n - 7) here:

Code:

---

int main()
{
        for (int n = 0; n < 1000; n = n + 14){
                int x = (n + 17)%(n - 7);
                if ( x == 0){
                        cout<<n <<endl;
                }
        }
        cin.get();
        return 0;
}

---

It still didn't work.
Note: There are very few answers to (n+17)/(n-7) which is why the computer might not be printing out anything.

On the one hand, you know that if n > 7, then (n+17)/(n-7) is greater than 1. On the other hand, as n increases, it decreases, and as n goes to infinity, it approaches 1, so for large enough n it will be less than 2 and so will never be an integer again. Keeping this in mind, you can probably work out by hand what all the possible values of n are.

> It still didn't work.
Have you tried

int n = someValueWhichIsKnownToWork;
int x = (n + 17)%(n - 7);

To check the code does actually produce the right answer given correct input?

Robatino, yeh I wanted to try and make a program that would find those for me, as I haven't learnt how to do it yet (at school). And in one of the different attempts the computer printed -10, and as the condition was set to < 10,000,000 in the for loop (I think) (I set it that high because I wanted to see if the numbers would ever come down to -1, before it was just 1000), it kept printing -10 then started printing -9 and so forth until -2, each time taking more loops to change the number, then started printing -1.9 and so forth until -1.1 taking even more time per number, and finally reached -1 (whole) (probably because the computer couldn't calculate any lower decimals).

Salem, as I said above, I wanted to find the answer with the help of a computer as I didn't know how to do it by hand.

Anyway thanks for your help, I'll just leave it unless someone knows maybe a differen way of doing this. Thanks again!

Code:

---

#include <iostream>
using namespace std;

int main() {
        int n = 8;
        while ((n+17)/(n-7) >= 2) {
                if ((n+17)&#37;(n-7) == 0) cout << n << "\n";
                n++;
        }
        return 0;
}

---

Just a pretty simple loop if you do some basic math to determine where it will converge and where to start from. Obviously you don't want to start from 7 because you will get divide by 0, anything less will result in a negative number which isn't wanted. Keep looping until it's less than 2 because the fraction goes to 1 for inf, and anything between 1 and 2 can't be a positive integer. (Basically what everyone said above ;))

Btw, why did you increment n by 14 each time. The first "n" is actually 8 (25/1 = 25). Also when I ran it none were multiples of 14 - they were 8,9,10,11,13,15,19,31.

It's easier to do some of this by hand - for example, if (x+17)/(x-7) == 2, it's trivial to solve for x to get x == 31. So one can just check by hand (or in one's head) values of n between 8 and 30 (of course n == 31 is also a solution).

Stay away from floats and doubles if you need to work with real mathematical fractions.

You had the correct solution before, except you were doing n = n + 14

Code:

---

#include <iostream>

int main()
{
        for (int n = 0; n < 1000; ++n){       
                // Careful to not divide by 0
                if (n != 7 && (n + 17)&#37;(n - 7) == 0){
        std::cout<< n <<std::endl;
                }
        }
  std::cin.get();
        return 0;
}

---