Linear equation solver Arduino,printing wrong answers

Question

Please help me any help will be appreciated. I wrote a code in C to solve equations, but I don't know why some equations that I input into this calculator, give me wrong values, the algorithm is correct because I execute it in C and give me the correct answers but when I use it on the arduino sometimes it doesn't give me the correct answer, well here's the full code

and any help will be appreciated.

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long jordan(){/*this do the gauss elimination for solve the equation*/
long FilaMax=0,k=0;
long double maxEl=0,tmp=0,fracc=0;
   lcd.clear();
for (colum=0; colum<cant-1; colum++) {
    /* search the maximun colum*/
    maxEl = abs(a[colum][colum]);
    FilaMax = colum;
    for (k=colum+1; k<cant; k++) {
        if (abs(a[k][colum]) > maxEl) {
            maxEl = abs(a[k][colum]);
            FilaMax = k;
        }
    }
    /* change the maximunby the actual row*/
    for (k=colum; k<cant+1;k++) {
        tmp = a[FilaMax][k];
        a[FilaMax][k] = a[colum][k];
        a[colum][k] = tmp;
    }
    /*lower cero's triangular matrix it's done here*/
    for (k=colum+1;k<cant; k++) {
        fracc = -a[k][colum]/a[colum][colum];
        for (fila=colum; fila<cant+1; fila++) {
            if (colum==fila) {
                a[k][fila] = 0;
            }else{
                a[k][fila] += fracc * a[colum][fila];
            }
        }
             } 
}
    char sr=' ';
    lcd.setCursor(0,0);
if(a[cant-1][cant-1]==0){
    lcd.print("No solucion"); /* if there is no solution print this*/ 
    do{
    sr=keypad.waitForKey();
    }while(sr!='\n');
}else{                       /*is there values to print*/
for (colum=cant-1; colum>=0; colum--) {
    res[colum] = a[colum][cant]/a[colum][colum];
    for (k=colum-1;k>=0;k--) {
        a[k][cant] -= a[k][colum]*res[colum];
    }
}
colum=0;
    do{     
            lcd.setCursor(0,0);
            lcd.print("R");
            lcd.setCursor(1,0);
            lcd.print(colum+1);
            lcd.setCursor(0,1);
    lcd.print(res[colum],DEC);
            sr=keypad.waitForKey();
            if(sr=='#') colum++;
            if (colum==cant) colum=0;
            if(sr=='\n') break;
    }while(1);
   }
   colum=0;
   fila=0;
   cant=0;
   sr=' ';
  return 0;
 }

Try the code with this:

 input:
    # size
    4
    elements:
    1 -2 1 1 2
    3 0 2 -2 -8
    0 4 -1 -1 1
    5 0 3 -1 -3 
 output:
 some numbers

 it should print without solution.

I first run the code in c on the pc, and the algorithm work perfect but when I copy to the arduino it doesn't work as expected the most of the time give the right answer to the linear system but somtimes fail

any help will be appreciated.

Answer 1

It's a floating point error, the final value you are getting is very close to zero. Demo.

Add a small epsilon value to your final test to allow for floating point inaccuracies:

if(fabs(a[cant-1][cant-1]) < 0.000001){
    lcd.print("No solucion"); /* if there is no solution print this*/

Answer 2

Without much review - going out on a limb as I've had this error too.

Use floating point function rather than int

// maxEl = abs(a[colum][colum]);
maxEl = fabs(a[colum][colum]);
// other places too

Other issues may exist.