Calculating the root of a quadratic equation. C++

Question

I want to calculate the root of a quadratic equation ax2 + bx + c = 0. I think my algorithm is correct, but I am new to functions and I think my mistake is with calling them. Could you help me? Thank you in advance. Here is the code.

#include<iostream>
#include<cmath>
using namespace std;
const double EPS = 1.0e-14;
int quadraticEquation(int& a,int& b, int& c)

{
    if(abs(a) < EPS)
    {
        if(abs(b) < EPS)
        {
            cout << "No solution" << endl;
        }
        else
        {
            double x = - c /b ;
            cout << "One root: " << x << endl;
        }
    }
    else
    {
        double delta = b * b - 4 * a * c;
        if(delta < 0)
        {
            cout << "No solution:\n" ;
        }
        else
        {
            double square_delta = sqrt(delta);
            double root_1 = (-b + square_delta) / (2 * a);
            double root_2 = (-b - square_delta) / (2 * a);
            cout << "x1 = " << root_1 << endl;
            cout << "x2 = " << root_2 << endl;
        }
    }

}

int main()
{
    int a = 1;
    int b = 2;
    int c = 3;
    cout << "The solution is: " << quadraticEquation(int a,int b,int c);

    return 0;
}

As you implicitly note in your code, a quadratic equation generally has *two* solutions, and they're usually not integers. — Cheers and hth. - Alf, Dec 13 '14 at 19:08
@Cheersandhth.-Alf: Well, he's not returning anything anyway. Just printing both roots from inside the function. But he's also printing the undefined return value. — Fred Larson, Dec 13 '14 at 19:09

score 3 · Answer 1 · answered Dec 13 '14 at 18:37

3

It should be:

cout << "The solution is: " << quadraticEquation(a, b, c); // no types in calling a function

answered Dec 13 '14 at 18:37

erenon

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Wow, what stupid mistake have I done. Really thanks erenon! – rcson Dec 13 '14 at 18:45

score 2 · Answer 2 · answered Dec 13 '14 at 18:52

It is not really the question asked, and erenon and Paco Abato did answer the C++ problem.

But there are still some improvement to your algorythm :

abs(a) < EPS and abs(b) < EPS - if abc(c) < EPS every x is solution
if delta < 0 should be if delta <= (-EPS)
you forgot if abs(delta) < EPS) one double solution : double x = - b / a;

Paco Abato · Accepted Answer · 2014-12-13T19:14:35.440

-1

Define the function as:

int quadraticEquation(int a,int b, int c)

Without & signs to pass the parameters by value instead of by reference (althought as juanchopanza states on a comment it is not extrictly required).

And call it as:

cout << "The solution is: " << quadraticEquation( a, b, c);

Without int because in function calls you must not provide the type of the parameter.

edited Dec 13 '14 at 19:14

answered Dec 13 '14 at 18:48

Paco Abato

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The first part of the answer is both unnecessary and wrong (although it does make sense to pass integers by value in this case.) – juanchopanza Dec 13 '14 at 19:05
1

I'm not downvoting because I can't say that the answer is technically wrong, but restricting quadratic equations to those with a single integer root just sounds wrong. – Cheers and hth. - Alf Dec 13 '14 at 19:09
Edited my answer. I hope that it is more accurate now. – Paco Abato Dec 13 '14 at 19:15

Calculating the root of a quadratic equation. C++

3 Answers3