空函数 (c++)

Question

I want to create a program that solve quadratic equation (Ax²+Bx+C=0) using 2 void functions: one to insert the values of A,B,C, and the second for solving the equation. This is what I did:

#include <iostream>
#include <math.h>

using namespace std;

void add_nmbr(int a, int b, int c){

    int *pa,*pb,*pc;
    cout << "Entrer le nombre A " <<endl;
    cin >> a;
    cout << "Entrer le nombre B " <<endl;
    cin >> b;
    cout << "Entrer le nombre C " <<endl;
    cin >> c;
    pa = &a;
    pb = &b;
    pc = &c;
    cout << a <<"x2 + "<<b<<"x + "<<"c = 0"<<endl;

}

void resoudre(int a,int b, int c){

    double delta;
    double x1,x2;
    delta= b*b-4*a*c ;

    if (delta<0){
        cout << "Pas de solution !"<<endl;
    }else{
        x1=(-b-(sqrt(delta)))/(2*a);
        x2=(-b+(sqrt(delta)))/(2*a);
    }
    cout << a <<"x2 + "<<b<<"x + "<<"c = 0"<<endl;
    cout << "la solution est : " << x1 << endl;
    cout << "la solution est : " << x2 << endl;
}

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

    add_nmbr(a,b,c);
    resoudre(a,b,c);

    return 0;

}

Answer 1

When you declare a function like void add_nmbr(int a, int b, int c) you are passing parameters by value which means you pass a copy of value into the function. You can change the value inside add_nmbr for a but that value stays inside the function. In you case, the variable a in function main stays un-initialized.

The same thing for resoudre . To fix it, you can use reference , like this

void add_nmbr(int &a, int &b, int &c) {...}    

Answer 2

Look at this;

void add_nmbr(int& a, int& b, int& c){

    cout << "Entrer le nombre A " <<endl;
    cin >> a;
    cin.ignore(); //Use it after cin because of you hitting enter after getting the value.
    cout << "Entrer le nombre B " <<endl;
    cin >> b;
    cin.ignore();
    cout << "Entrer le nombre C " <<endl;
    cin >> c;
    cin.ignore();

    cout << a <<"x2 + "<<b<<"x + "<<"c = 0"<<endl;
}

But yeah, you should try to read up on call by refence and call by value.

Answer 3

Why you dont use reference ?
Like that

void add_nmbr(int& a, int& b, int& c)
{
    cout << "Entrer le nombre A " << endl;
    cin >> a;
    cout << "Entrer le nombre B " << endl;
    cin >> b;
    cout << "Entrer le nombre C " << endl;
    cin >> c;
    cout << a << "x2 + " << b << "x + "<<"c = 0"<< endl;
}
void resoudre(const int &a,const int &b, const int &c)
{
    double delta;
    double x1,x2;
    delta= b*b-4*a*c ;
    if (delta<0){
        cout << "Pas de solution !"<< endl;
    }else{
        x1=(-b-(sqrt(delta)))/(2*a);
        x2=(-b+(sqrt(delta)))/(2*a);
    }
    cout << a <<"x2 + "<< b << "x + "<< "c = 0"<< endl;
    cout << "la solution est : " << x1 << endl;
    cout << "la solution est : " << x2 << endl;
}

Do attention, you need a test in a because you divided by 0.