Using inputted variables in multiple functions in C++

Question

I just started learning C++ from scratch last week and on my assignment I have to create a quadratic equation solver through a few files: assign1.cpp which contains my main(), funcs.cpp which contains every function, and a makefile. I'm having trouble pulling everything together.

I have to get the coefficients from a user and return them to a struct with three fields, and use their inputted information in a few functions to solve the equation.

My programming is all over the place, and I got most of the errors to go away with coeff input; in every function and input.variable_name_here for a, b, and c.

Also, he wants us to use parameters.

I'm hardly half way done with the program.

Here's what I've got:

//Program to solve quadratic equation

#include <iostream>
#include <cmath>
using namespace std;

//Structure for coefficients to be used
struct coeff {
    double a;
    double b;
    double c;
};

//Prototypes?
double readCoeffs(), equSolver(), discr(), outResults();

//Read coefficients from user
double readCoeffs() {

    coeff input;

    cout << "Enter coefficient a: ";
    cin >> input.a;
    cout << "Enter coefficient b: ";
    cin >> input.b;
    cout << "Enter coefficient c: ";
    cin >> input.c;

}

//Solve quadratic equation from user's input
double equSolver() {

    coeff input;

    /*
    need to somehow get the discrim variable from discr() to this function

    discr();
    */

    double solution1 = (pow(input.b, 2) + sqrt(discrim)) / (2 * input.a);
    double solution2 = (pow(input.b, 2) - sqrt(discrim)) / (2 * input.a);

}

//Solves discriminent of quadratic equation
double discr() {

    coeff input;

    double discrim = pow(input.b, 2) - (4 * input.a * input.c);

}

/*
//Display on the screen results of the calculation
outResults() {

    if (//root exists)
        cout << "Quadratic equation with the following coefficients: \n";
        cout << "a: " << value << "; b: " << value << "; c: " << value << "\n" << endl;
        cout << "has the following roots ";
        cout << "Root1: " << value << "; Root2: " << value << "\n" << endl;

    else
        cout << "Quadratic equation with the following coefficients: ";
        cout << "a: " << value << "; b: " << value << "; c: " << value << "\n" << endl;

}

*/

It might help if you could actually ask a question. Try to be as precise as possible, e.g., “the compiler complains that discr does not return a value, why does it not just pick the last value computed?” – you may be able to help yourself much faster than anyone else could, with a little training and effort. — Christopher Creutzig
– Christopher Creutzig, Commented Jan 31, 2014 at 2:31

chrisb2244 · Accepted Answer · 2014-01-31 02:04:52Z

2

Your readCoeffs() function declares the object input. This means that input only exists within the function, so you can't use it elsewhere - it falls out of scope when the function finishes. Also, your function is declared with a double return-type, but doesn't return anything.

Consider taking an argument as a reference to a coeff struct, and declaring it as void?

void readCoeffs(coeff &input)
{
    cout << "enter...";
    cin >> input.a;
    cin >> input.b;
    cin >> input.c;
}

Then do the same for other functions (passing the struct), and declare your struct within your main() function.

Edit: Added an example of a function with a return.

double discr(coeff &input)
{
double discrim = pow(input.b, 2) - (4 * input.a * input.c);
return discrim;
}

edited Jan 31, 2014 at 2:04

answered Jan 31, 2014 at 1:31

chrisb2244

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17 Comments

Auchindoun Over a year ago

When I try that it's saying the readCoeffs it's an incomplete type and is not allowed, the the coeff type name is not allowed, and that it expects a ) and a ;

chrisb2244 Over a year ago

Did you also change the prototype to void, and add the argument?

Auchindoun Over a year ago

If void readCoeffs(); is the correct way to instantiate a prototype then yes. It says that the declaration is incompatible. I tried it without the prototype as well

chrisb2244 Over a year ago

You'd need readCoeffs(coeffs &input); as your prototype if you want to pass the argument too (Which you'll need to if you want to change the struct variables)

chrisb2244 Over a year ago

Although if you don't have the prototype for readCoeffs, provided your struct is defined before your function, and your function before you use it, that shouldn't matter...

|

Andrew L · Accepted Answer · 2014-01-31 02:27:08Z

0

I couldn't resist so I rewrote and reorganized the code to solve the issue. This compiles and does what you ask. I also fixed the quadratic formula because it should be:

(-b +/- (b^2 - 4ac)) / 2a

and you had

(b^2 +/- (b^2 - 4ac)) / 2a

#include <iostream>
#include <cmath>
using namespace std;

//Structure for coefficients to be used
struct coeff {
    double a;
    double b;
    double c;
    double sol1;
    double sol2;
};

//Declarations
void readCoeffs(coeff &x);
void equSolver(coeff &x);
double discr(coeff x);
void outResults(coeff x);

//Read coefficients from user
void readCoeffs(coeff &input) {

    cout << "Enter coefficient a: ";
    cin >> input.a;
    cout << "Enter coefficient b: ";
    cin >> input.b;
    cout << "Enter coefficient c: ";
    cin >> input.c;

}

//Solve quadratic equation from user's input
void equSolver(coeff &input) {

    double solution1 = ((-1 * input.b) + sqrt(discr(input))) / (2 * input.a);
    double solution2 = ((-1 * input.b) - sqrt(discr(input))) / (2 * input.a);
    input.sol1 = solution1;
    input.sol2 = solution2;

}

//Solves discriminent of quadratic equation
double discr(coeff input) {

    double d;
    d = pow(input.b, 2) - (4 * input.a * input.c);
    return d;

}

void outResults(coeff input) {

      if( std::isnan(input.sol1) || std::isnan(input.sol2)) {
        cout << "Quadratic equation with the following coefficients could not be solved: \n";
        cout << "a: " << input.a << "; b: " << input.b << "; c: " << input.c << "\n" << endl;
      } else {
        cout << "Quadratic equation with the following coefficients: \n";
        cout << "a: " << input.a << "; b: " << input.b << "; c: " << input.c << "\n" << endl;
        cout << "has the following roots ";
        cout << "Root1: " << input.sol1 << "; Root2: " << input.sol2 << "\n" << endl;
      }


}

int main() {

 coeff Coeff;
 readCoeffs(Coeff);
 equSolver(Coeff);
 outResults(Coeff);

}

edited Jan 31, 2014 at 2:27

answered Jan 31, 2014 at 2:20

Andrew L

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3 Comments

Auchindoun Over a year ago

I'll definitely take a look at this. Thank you for taking the time to help!

chrisb2244 Over a year ago

Regarding something being 'off', perhaps problems occur if descr returns a negative number, since equSolver has no knowledge of complex numbers? (Although perhaps that's what your isnan(...) section handles?)

Andrew L Over a year ago

The formula was off, I fixed it now.

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