Better algorithm for finding diagnols

I have made a program I created a couple months ago(December). I just opened it up again. I noticed it takes a very long time to calculate higher sided polygons(of course). What algorithm would you use?

(Mine = s += n; n += 1, looped, s being sides, and n = added to it. 4 = 2, 5 = 5, 6 = 9.

s+= n; n+= 1;
2+=3;n+=1;
5+=4;n+=1;
9+=5;n+=1....)

Thanks(by the way, if you want to see it, just ask).

I'm confused, what exactly are you calculating?

Since s just seems to be the sum of the first n integers - 1 (ie 9 is the sum of 1+2+3+4-1)you can use the handy dandy equation where the sum of the first n integers is equal to (n*(n+1))/2.

Law of cosines, honey.

PJ, the number of diagnols in a polygon. My description wasn't clear enough. Here >>

Code:

---

#include <iostream>

int findNumber(int);

int main()
{
    int noOfSides;
    std::cout << "How many sides does the polygon have?\n";
    std::cin >> noOfSides;
    //int noOfDiagnols = findNumber(noOfSides);
    std::cout << "The polygon has " << findNumber(noOfSides) << " diagnols\n";
    system("PAUSE");
}

int findNumber(int nos)
{
    std::cout << "In findNumber"
              << "\n\n";
    int newNOS = nos - 3;
    int n = 2;
    std::cout << "\n";
    /*for( n = 2; newNOS <= 0; newNOS-- )
    {
        n += 1;
        std::cout << "in for()";
    }
    */
    int nOfDiag = 0;
    while(newNOS != 0)
    {
        std::cout << "In while()..\n"
                  << newNOS
                  << " steps to go;\n";
       
        nOfDiag += n;
        n += 1;
        newNOS--;
    }
    return nOfDiag;
}

---

(I realize I have the OS specific system("PAUSE"), comments, and other stuff I should have cleared out)

I don't think that what you said would work, PJ, as n has to get incremented(another thing I should have used) each time.

No, it doesn't have to increase every time, you wouldn't even need n.

The number of diagonals for a shape with s sides is the same as the sum of the first (s-2) integers minus 1.

So for example, number of diagonals for a square is 1+2-1=2.
Pentagon: 1+2+3-1=5.
Hexagon: 1+2+3+4-1=9.
etc etc

So to calculate for any value, use my above equation and subtract 1.
For example, number of diagonals for a shape with 200 sides is:

((198*199)/2)-1=19700