Functions, passing by reference

[Ed4434]

Hey, just a beginner.. as the forum would imply!

I'm just writing a quadratic equation solver and am wondering if someone could give me a few "pointers" on what is going wrong with this function?

The compiler is telling me that variables r1 and r2 aren't being initialised so I'm unsure of what to do..

//This is my qSolv.h header file for the function.
#include <iostream>
#include <cmath>

double qSolv(double &a, double &b, double &c)

{
	double r1, r2;

	if ((pow(b,2) - (4*a*c)) < 0) {
		std::cout<< "No real roots to this quadratic.";
		std::cin.ignore();
		return -1;
	}

	else {
	r1 = (-b + sqrt(pow(b,2) - (4*a*c)))/(2*a);
	r2 = (-b - sqrt(pow(b,2) - (4*a*c)))/(2*a);
	}
}

//And this is just.. my "program"
#include <iostream>
#include "qSolv.h"

using namespace std;

int main()

{
	double a, b, c;
	double r1, r2;

	cout<< "Please enter your coefficients: ";
	cin>> a >> b >> c;

	qSolv(a, b, c);

	cout<< "Your roots are: " << r1 << " and " << r2;
	cin.ignore();

	cin.ignore();
	return 0;
}

Apologies if this is terrible and thanks for the info to whoever replies! :)

Ed


Nobody?? :(

[Ed4434]

Is nobody gonna reply? T_T

[cire]

In qSolv, r1 and r2 are local variables that stop existing when the function ends. Despite the fact that they share a name with two different variables declared in main, they are in no way related to them.

Is nobody gonna reply? T_T

The people who use this forum volunteer their own time to do so. Waiting 20 minutes to bump a thread because you didn't get a reply is a level of impatience we don't see here often. (And from your edit, I see you couldn't even wait that long for an answer.) You would do well not to display it again.

[abhishekm71]

The variables r1 and r2 in your main function are local to your main function only. They are not visible to your qSolv function.

The variables r1 and r2 in your qSolv function are again just local to the qSolv function only and they are not visible to your main function. If you change these variables, the variables in main do not get changed.

Since you are trying to output the values of r1 and r2 from your main function, the compiler is warning you that you haven't as yet assigned any values to them.

Instead in your qSolv:

double qSolv(double &a, double &b, double &c)

{
	double r1, r2;

	if ((pow(b,2) - (4*a*c)) < 0) {
		std::cout<< "No real roots to this quadratic.";
		std::cin.ignore();
		return -1;
	}

	else {
	r1 = (-b + sqrt(pow(b,2) - (4*a*c)))/(2*a);
	r2 = (-b - sqrt(pow(b,2) - (4*a*c)))/(2*a);
	}
        std::cout << "Your roots are: " << r1 << " and " << r2; // You will have to include iostream in this file.
}

your program:

int main()

{
	double a, b, c;
	double r1, r2;

	cout<< "Please enter your coefficients: ";
	cin>> a >> b >> c;

	qSolv(a, b, c);

	//cout<< "Your roots are: " << r1 << " and " << r2; 
	cin.get();

	return 0;
}

[Ed4434]

Okay, apologies for my impatience!

And.. what would this mean then? I'm unsure how to fix it :s

Thanks for your reply,
Ed

[Ed4434]

Ah thank you very much! So.. this is a scope problem??

Thanks,
Ed

[keskiverto]

Unsure of what?

[abhishekm71]

>So.. this is a scope problem??

Yes it is. The solution I suggested is the simplest one with minimal changes to your code.

However, there are other, probably better ways to resolve such scope issues. One way is to pass r1 and r2 to your qSolv function by reference. (like your other variables).

[Ed4434]

Okay, but.. if r1 and r2 are essentially being created by qSolv, why would I pass any values for r1 and r2 into the function??

Thanks,
Ed

[abhishekm71]

In that case you will NOT declare them locally.

double qSolv(double &a, double &b, double &c, double &r1, double &r2 )
{
	//double r1, r2;

	if ((pow(b,2) - (4*a*c)) < 0) {
		std::cout<< "No real roots to this quadratic.";
		std::cin.ignore();
		return -1;
	}

	else {
	r1 = (-b + sqrt(pow(b,2) - (4*a*c)))/(2*a);
	r2 = (-b - sqrt(pow(b,2) - (4*a*c)))/(2*a);
	}
}

[cire]

Okay, but.. if r1 and r2 are essentially being created by qSolv, why would I pass any values for r1 and r2 into the function??

Because they should not be created by qSolv.

#include <cmath>
#include <iostream>

bool qSolv(double a, double b, double c, double& r1, double& r2)

{
    const double discriminant = b*b - 4*a*c;

    if (discriminant < 0)
        return false;

    r1 = (-b + std::sqrt(discriminant)) / (2 * a);
    r2 = (-b - std::sqrt(discriminant)) / (2 * a);
    return true;
}

int main()
{
    double a, b, c;

    std::cout << "Please enter your coefficients: ";
    std::cin >> a >> b >> c;

    double root1, root2;
    if (qSolv(a, b, c, root1, root2))
        std::cout << "Your roots are: " << root1 << " and " << root2 << '\n';
    else
        std::cout << "No real roots to this quadratic.\n";
}

You should also return values from functions which you promise will return values (such as your version of qSolv which should return a value of type double.) Don't put function definitions in header files unless they are inline.

[ne555]

You don't care about their value, you expect them to be assigned in the function.
In other languages you would specify

function qSolv(
   a,b,c : in,
   r1, r2 : out
)

That said, ¿why are you passing a,b,c by reference? You have no advantage and you make illegal to do something as qSolv( 1, 0, 1 );


Also, the `no errors' path returns garbage, but you don't use the return value at all

[Ed4434]

Well.. if I, as the user, am inputting a, b and c myself, do I just pass by value because those three variables aren't actually being changed or what?

Sorry, been working at C++ for about 3 days so I'm not very good atm!

Thanks,
Ed