Sqrt Problem Here

[bananae]

I've posted where the problem in my code is; really don't know what the problem is with it. Sorry if my code is a little confusing, I made it like this first so I can see if there are any problems with it before I compress it.

//Quadratic Formula
//First build 1/31/09

#include <iostream>
#include <string>
#include <math.h>

using namespace std;

int main()
{
    //making variables up in here
    string sAnswer = "null";
    long double a = 0;
    long double b = 0;
    long double c = 0;
    long double num1 = 0;
    long double num2 = 0;
    long double num3 = 0;
    long double num4 = 0;
    
    labelA: //label for my goto because im too lazy for loops
    
    cout << "I can do the quadratic formula!" << endl;
    cout << "I just need you to input the variables." << endl;
    cout << "But first I want you to tell me if you want output" << endl;
    cout << "at each calculation so you can write it down and call it yours." << endl;
    cout << "Please type in a 'y' or a 'n' to indicate if I should tell you or not." << endl;
    
    cin >> sAnswer;
    
    cout << sAnswer << endl; //this line is for testing purposes
    cout << "I'm not going to give you the output just yet. This is an alpha build!" << endl; //delete this later
    //input if statement here
    
    cout << "Please tell me the value of a." << endl;
    cin >> a;
    cout << "Please tell me the value of b." << endl;
    cin >> b;
    cout << "Please tell me the value of c." << endl;
    cin >> c;
    cout << "Alrighty I'm working on it now, buddy." << endl;
    
    //the output for each calculation is to see where the problem lays in the program
    //the error is occuring at the square root calculation
    
    num1 = b * b;
    cout << "B squared is: " << num1 << endl;
    
    num2 = a * c * -4;
    cout << "-4ac is: " << num2 << endl;
    
    num3 = num1 + num2;
    cout << "b squared plus -4ac is: " << num3 <<  endl;
    
    num1 = sqrt(num3);
    cout << "Square root of b squared plus -4ac" << num1 << endl;
    
    num2 = b * -1;
    num4 = num1;
    num3 = num2 + num1;
    num1 = a * 2;
    num3 = num3/num1;
    cout << "First result (addition): " << num3 << endl;
    num3 = num2 - num4;
    num4 = a * 2;
    num3 = num3/num4;
    cout << "Second result (subtraction): " << num3 << endl;
    cout << "Go again? y/n?" << endl;
    cin >> sAnswer;
    cout << "Haha just kidding alpha build" << endl;
    cout << "End of test" << endl;
    system("PAUSE");
    return 0;
}    

As you can see I'm going to add a goto and the checks for it but thats after I get the math working right. Thanks for the help!

[helios]

If num3 is <0, you can't do sqrt().
I don't understand what's with all those steps:

if (!a)
    //function is linear
num3=b*b-4*a*c;
if (num3<0)
    //function has no roots, IIRC
num3=sqrt(num3);
minus=(-b-num3)/2*a;
plus=(-b+num3)/2*a;

[bananae]

I had the steps expanded so I can output the result of each calculation, I was going to add an If statement at the beginning that would get rid of that option and then I would have something similar to your equation in the place of my expanded steps if the user didn't want to see the result at each calculation.

[seymore15074]

Line 12 kills me. Nice.

[bananae]

Would it make much of a difference performance wise if I didn't assign my variables values at conception?

[firedraco]

It's generally a good idea to assign them at creation anyway, just to make sure they have a value. Also, what helios said is basically right, if num3 < 0 then you are no real roots and that will cause some problems.

[bananae]

Ok I'll include that in the next iteration of it.
Anything else wrong besides that?