Address of local variable ’x’ returned

[uhlajs]

Hello, please help me. I can't fing mistake but I know that there is! :D
I have a simple app. This app contain two file arith.h and main.cpp

arith.h
@#ifndef ARITH_H
#define ARITH_H

#include <math.h>

float* quadratic_equation(float coefficient[]) {
// promene
float d, x[2];
// vypocet
d = pow(coefficient[1],2)-4coefficient[0]coefficient[2];
x[0]=((-coefficient[1])+ sqrt(d))/(2coefficient[0]);
x[1]=((-coefficient[1])- sqrt(d))/(2coefficient[0]);
return x;
}
float* two_equations(float coefficient_1[], float coefficient_2[]) {
// promene
float cal_1 = coefficient_1[1], cal_2=coefficient_2[1], x[2];
// vypocet
for (int i=0; i <=2; i++)
{
coefficient_1[i]=cal_2;
coefficient_2[i]=cal_1;
coefficient_1[i]-=coefficient_2[i];
}
x[0]=coefficient_1[2]/coefficient_1[0];
x[1]=(coefficient_2[2]-coefficient_2[0]x[0])/coefficient_2[1];
return x;
}
float three_equations(float coefficient_1[], float coefficient_2[], float coefficient_3[]) {
//promene
float *pointer = NULL;
float cal_1[4], cal_2[4], x[3];
pointer = cal_2;
// vypocet
for (int I = 0; I < 2; I++)
{
for (int i = 0; i <= 3; i++)
{
cal_1[i] = coefficient_1[i]*pointer[0];

    }
    for (int i = 0; i <= 3; i++)
    {
        pointer[i] *= coefficient_1[0]*(-1);
        pointer[i] += cal_1[i];
    }
    pointer = coefficient_3;
}
for (int i = 0; i <= 3; i++)
{
    cal_2[i] = coefficient_2[i]*coefficient_3[1];
}
for (int i = 0; i <= 3; i++)
{
    coefficient_3[i] *= coefficient_2[1]*(-1);
    coefficient_3[i] += cal_2[i];
}
x[0] = coefficient_3[3]/coefficient_3[2];
x[1] = (coefficient_2[3]-coefficient_2[2]*x[0])/coefficient_2[1];
x[2] = (coefficient_1[3]-coefficient_1[2]*x[0]-coefficient_1[1]*x[1])/coefficient_1[0];
return x;

}

#endif // ARITH_H
@

main.cpp
@#include <iostream>
#include "arith.h"
using namespace std;

int main()
{
cout << "rovnice";
float pole[3], pole1[3];
pole[0]=4;
pole[1]=3;
pole[2]=6;
pole1[0]=2;
pole1[1]=1;
pole1[2]=4;
cout << two_equations(pole, pole1)[1];
cout << two_equations(pole, pole1)[0];
cout << two_equations(pole, pole1)[1];

return 0;

}

@

Problem is that the third call of function two_equations give me "nan" result. But it not true. So where i do mistake?

[mcosta]

Hi,

in two_equations you return a pointer to a local variable (x); this is wrong because outside of your function, the memory address could be reused and then you access to wrong data.

Usually, if you need to return a value you MUST choose one of these techinques:

return by value (stack value)

return a dinamically allocated value (heap)

modify a input parameter address

[andreyc]

Local variables are destroyed when control flow leaves the function, so the result is lost and you are getting some random data.
I would suggest to allocate memory inside the function and then deallocate it in the main or pass pointer or reference to x to the function and fill it with the result.