Problem!!!

[bbobthorton10]

Hello! I am a beginning c++ programmer, and I was learning the language by practicing a problem. However, I have a problem in my code and I can't figure out what it is. The problem wants you to find the roots of the quadratic equation, but the output of the roots comes to a funny output like -1.#ND00. Can anybody help me debug this program? Thanks!

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cassert>

using namespace std;

int main (){
	
	cout<<fixed<<showpoint<<endl;
	
	cout<<"***********"<<endl;
	cout<<"WELCOME"<<endl;
	cout<<"***********"<<endl;

	cout<<"Please enter coefficients a, b and c from the quadratic expression: "<<endl;
	cout<<endl;
	double a;	
	cout<<"a: ";
	cin>>a;
	cout<<endl;
	double b;
	cout<<"b: ";
	cin>>b;
	cout<<endl;
	double c;
	cout<<"c: ";
	cin>>c;
	cout<<endl;

	double root1;
	double root2;

	root1 = (-b + pow(b*b-4*a*c,0.5))/2*a;
	assert (root2 = (-b - pow(b*b-4*a*c,0.5))/2*a);

	assert(cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl);

	if(root2=0){
		cout<<"The discriminant equals zero, therefore there is only a single (repeated) root"<<endl;
		cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;
		return 0;
	}

	else if(root2>0){
		cout<<"The discriminant is greater than zero, and therefore has two real roots"<<endl;
		cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;
		return 0;
	}

	else
		cout<<"The discriminant is less than zero, and therefore has two complex roots"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;

		return 0;
	}


	

[helios]

the output of the roots comes to a funny output like -1.#ND00

It means you tried to get the square root of a negative number, which in turn means the equation has no solutions.

The asserts on lines 35 and 37 make no sense.
On line 39 you're using the assignment operator (=) rather than the equality operator (==).
You're checking the sign of the second root, which is incorrect. You should be checking the sign of the discriminant. Furthermore, you should check that before calculating the square root.

[bbobthorton10]

Yea, I missed the equality operator and changed the asserts, but what do you mean I should calculate the discriminant before the square root? Am I not doing that now?

[helios]

but what do you mean I should calculate the discriminant before the square root?

I said "check", not "calculate".

[bbobthorton10]

and how do you check it?

[viliml]

and how do you check it?

LOL, you already did that! just copy-paste the checking to before calculating the roots!

[bbobthorton10]

Here is the changed code, but everytime the program runs it goes straight to the else statement (the discriminant is less than zero). Why is it still not working?

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cassert>

using namespace std;

int main (){
	
	cout<<fixed<<showpoint<<endl;
	
	cout<<"***********"<<endl;
	cout<<"WELCOME"<<endl;
	cout<<"***********"<<endl;

	cout<<"Please enter coefficients a, b and c from the quadratic expression: "<<endl;
	cout<<endl;
	double a;	
	cout<<"a: ";
	cin>>a;
	cout<<endl;
	double b;
	cout<<"b: ";
	cin>>b;
	cout<<endl;
	double c;
	cout<<"c: ";
	cin>>c;
	cout<<endl;

	double root1;
	double root2;

	root1 = (-b + pow(b*b-4*a*c,0.5))/2*a;
	root2 = (-b - pow(b*b-4*a*c,0.5))/2*a;

	if(root2==0)
		cout<<"The discriminant equals zero, therefore there is only a single (repeated) root"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;
		//return 0;
	

	else if(root2>0)
		cout<<"The discriminant is greater than zero, and therefore has two real roots"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;
	
	else
		cout<<"The discriminant is less than zero, and therefore has two complex roots"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;

	cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;

		return 0;
	}


	

[viliml]

why do you use the 2nd root as the discriminant?

[CoAstroGeek]

First, note your order of operations on the division. As written your code will execute as:

root1 = ((-b + pow(b*b-4*a*c,0.5))/2.0)*a;

use parentheses to force it to evaluate the denominator first:

root1 = (-b + pow(b*b-4*a*c,0.5))/(2.0*a);

I like to write my constants as floats unless I really intend them to be integer. Helps me spot int vs. float errors.

On to the problem you asked about! You need to check the sign of the discriminant (b^2-4ac) *before* you calculate the roots. Because the root calculation will bomb if the discriminant < 0 (sqrt of negative).

Something like this at line 33:

double disc = b*b - 4.0*a*c;
if(disc == 0)
{
 root1 = -b/(2.0*a);
 cout << "The discriminant equals zero, therefore there is only a single (repeated) root:\t" << root1 << endl;
}
else if(disc > 0)
{
 root1 = (-b + pow(disc,0.5))/(2.0*a);
 root2 = (-b - pow(disc,0.5))/(2.0*a);
 cout << "The discriminant  is greater than zero, and therefore has two real roots:\t" << root1 << "\t" << root2 << endl;
}
else
{
 cout<<"The discriminant is less than zero, and therefore has two complex roots"<<endl;
 // add code here for representing and calculating complex roots.
}

[bbobthorton10]

Okay, I see what I did wrong. Thanks for the help! However, I still have one problem. When I run the program for equations like 2x^2-11+5 (two real roots) or -4x^2+12x-9 (one real root), the roots of the equation don't come out in the output. What am I still doing wrong? Thanks again for your help! P.S. I know there is a lot of commented out stuff, but bear with me! thanks!

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cassert>

using namespace std;

int main (){
	
	cout<<fixed<<showpoint<<endl;
	
	cout<<"***********"<<endl;
	cout<<"WELCOME"<<endl;
	cout<<"***********"<<endl;

	cout<<"Please enter coefficients a, b and c from the quadratic expression: "<<endl;
	cout<<endl;
	double a;	
	cout<<"a: ";
	cin>>a;
	cout<<endl;
	double b;
	cout<<"b: ";
	cin>>b;
	cout<<endl;
	double c;
	cout<<"c: ";
	cin>>c;
	cout<<endl;

	double root1;
	double root2;

	//root1 = (-b + pow(b*b-4*a*c,0.5))/2*a;
	//root2 = (-b - pow(b*b-4*a*c,0.5))/(2*a);

	/*if(root2==0)
		cout<<"The discriminant equals zero, therefore there is only a single (repeated) root"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;
		//return 0;
	

	else if(root2>0)
		cout<<"The discriminant is greater than zero, and therefore has two real roots"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;
	
	else
		cout<<"The discriminant is less than zero, and therefore has two complex roots"<<endl;
		//cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;

	cout<<"The roots of the quadratic expression are: "<<root1<<" "<<root2<<endl;

		return 0;*/

double disc = b*b - 4.0*a*c;
if(disc == 0)
{
 root1 = -b/(2.0*a);
 cout << "The discriminant equals zero, therefore there is only a single (repeated) root:\t" << root1 << endl;
}
else if(disc > 0)
{
 root1 = (-b + pow(disc,0.5))/(2.0*a);
 root2 = (-b - pow(disc,0.5))/(2.0*a);
 cout << "The discriminant  is greater than zero, and therefore has two real roots:\t" << root1 << "\t" << root2 << endl;
}
else
{
 cout<<"The discriminant is less than zero, and therefore has two complex roots"<<endl;
 // add code here for representing and calculating complex roots.
}
	}


	

[CoAstroGeek]

Dunno - works for me:

$ ./a.out

***********
WELCOME
***********
Please enter coefficients a, b and c from the quadratic expression: 

a: 2

b: -11

c: 5

The discriminant  is greater than zero, and therefore has two real roots:       5.000000        0.500000

[bbobthorton10]

Okay, I see! It works now, my window just wasn't big enough and was cutting out the output! Thanks for your help!