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.

Can you give us an example of incorrect output, along with what the correct output should be? — baum, Jun 12 '15 at 01:13
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: R1 -3.333, R2 3.2, R3 6.33, R4 5.33 but this is a wrong answer becuase this system doesn't have solution — user2461687, Jun 12 '15 at 01:17
Why don't you also post the entirety of your code. Even if the parts you removed aren't important to your problem, I can't effectively run (and replicate) your errors without the full code. — baum, Jun 12 '15 at 01:36
the full code is in the link, is a notepage, here´s the link, you can modify the code if you want, https://shrib.com/7uymq2yi in the page — user2461687, Jun 12 '15 at 01:43
Might be easier for other readers. Also, it won't be easily accidentally editable. — baum, Jun 12 '15 at 01:52
I agree. Please use proper indentation too. Also, translating important comments from Spanish to english would probably be good. — John M, Jun 12 '15 at 01:53
Please note that code with good comments and proper indentation will always get more help than code with bad comments and poor indentation :) — John M, Jun 12 '15 at 01:56

samgak · Accepted Answer · 2015-06-12T02:46:52.810

2

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*/

edited Jun 12 '15 at 02:46

answered Jun 12 '15 at 02:42

samgak

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Thanks so much now it's woking as expected :D thaaaanks – user2461687 Jun 12 '15 at 02:45

score 1 · Answer 2 · answered Jun 12 '15 at 01:58

1

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.

answered Jun 12 '15 at 01:58

chux - Reinstate Monica

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I'm doing the changes to run on the arduino, now the complete code is there, thanks so much – user2461687 Jun 12 '15 at 02:03
I made the changes but I getting the results yet :/ – user2461687 Jun 12 '15 at 02:05
@user2461687 Did you change `abs()` --> `fabs()` in 3 places? – chux - Reinstate Monica Jun 12 '15 at 02:06
`String num=""; ,,, num=(String)numero; lgtd = num.length();` hints that code is standard not C - maybe C++ or an arduino extension. If so, that `abs()` may be a false lead. good luck. – chux - Reinstate Monica Jun 12 '15 at 02:13
I use for measure the lenght of the string for place the dot – user2461687 Jun 12 '15 at 02:20
@user2461687 If "can't use it " means `fabs()` then you can use it, but `abs()` may be overloaded and `abs(double)` does the same thing as `fabs(double)`. I am not familiar enough with arduino to know. Did not see that tag until late. – chux - Reinstate Monica Jun 12 '15 at 02:21
at the end of the entrada(); function I joint together numero1 and numero2 and I get a float. I don't see why does it affects – user2461687 Jun 12 '15 at 02:24
I refer to 'can´t use it' to the String num="" ,,,,, num=(String)numero; lgtd = num.length(); not to the fabs – user2461687 Jun 12 '15 at 02:26

Linear equation solver Arduino,printing wrong answers

2 Answers2