Quadratic solver - fail

[shooter]

Please help me. This is my code:

#include<iostream>
#include<math.h>
using namespace std;

double a;
double b;
double c;
double top;
double top2;
double bottom;

double x;
double x2;

void solvequadratic(){

top=-b+sqrt(pow(b,2)*-4*a*c);
top2=-b-sqrt(pow(b,2)*-4*a*c);

bottom=2*a;

x=top/bottom;
x2=top2/bottom;

}

int main(){

cout << "Enter a: " << endl;
cin >> a;

while(a==0){
cout << "Please, enter a again: " << endl;
cin >> a;
}

cout << "Enter b: " << endl;
cin >> b;

cout << "Enter c: " << endl;
cin >> c;

void solvequadratic();

cout << "x1 = " << x << endl;
cout << "x2 = " << x2 << endl;

getchar();
getchar();
}

When I compile it,run it and enter the values I always get answers of x1=0 and x2=0! Any ideas?

[hannes]

remove 'void' when you call solveQuadratic() in your main-function.

[shooter]

Thanks a lot for the quick reply, hannes! I use the solver on equations like ax^2+bx+c. But it gives me some strange answers like:

a=1, b=5, c=9

and I get x1 = -1.#IND and for x2 the same as x1!

What could the problem be ?

[Browni3141]

Look after the pow function on lines 17 and 18. You have an extra '*'. Also, beware of negative roots. Standard functions won't handle complex numbers.

[stravant]

Also, a small note. By convention when you need to square a number you should just use x*x rather than pow(x,2).

[shooter]

Thank a lot! Now it works!

Here`s the final code:

#include<iostream>
#include<math.h>
using namespace std;

double a;
double b;
double c;
double top;
double top2;
double bottom;

double x;
double x2;

void solvequadratic(){

top=-b+sqrt(pow(b,2)-4*a*c);
top2=-b-sqrt(pow(b,2)-4*a*c);

bottom=2*a;

x=top/bottom;
x2=top2/bottom;

}

int main(){

int again;

cout << "This program solves quadratic equations!" << endl;

cout << endl;

cout << "ax^2 + bx + c = 0" << endl;

cout << endl;

cout << "Enter a: " << endl;
cin >> a;

while(a==0){
cout << "Please, enter a again: " << endl;
cin >> a;
}

cout << "Enter b: " << endl;
cin >> b;

cout << "Enter c: " << endl;
cin >> c;

solvequadratic();

cout << "x1 = " << x << endl;
cout << "x2 = " << x2 << endl;

cout << endl;

cout << "Do you want to try again?(1-yes/2-no)" << endl;
cin >> again;

while(again==1){
main();
again=0;
}

if(again==2){
exit(1);
}

getchar();
return 0;
}

[pwnedu46]

You also may want to use #include <complex> becasue sooner or later you will inevitably get one that requires this library. This is not really necessary, but could be useful when you have one of those tricky problems with imaginary numbers.

[JoR]

Also, global variables are considered as a bad style (for good reasons).

Better would be something like:

#include<iostream>
#include<math.h>
using namespace std;

void solvequadratic(double a, double b, double c, double& x1, double& x2){

int z=sqrt(pow(b,2)-4*a*c);

x1=(-b+z)/a/2;
x2=(-b-z)/a/2;

}

int main(){

int again;

int a,b,c,x1,x2;

cout << "This program solves quadratic equations!" << endl;

cout << endl;

cout << "ax^2 + bx + c = 0" << endl;

cout << endl;

cout << "Enter a: " << endl;
cin >> a;

while(a==0){
cout << "Please, enter a again: " << endl;
cin >> a;
}

cout << "Enter b: " << endl;
cin >> b;

cout << "Enter c: " << endl;
cin >> c;

solvequadratic(a,b,c,x1,x2);

cout << "x1 = " << x << endl;
cout << "x2 = " << x2 << endl;

cout << endl;

cout << "Do you want to try again?(1-yes/2-no)" << endl;
cin >> again;

while(again==1){
main();
again=0;
}

if(again==2){
exit(1);
}

getchar();
return 0;
}