math.sqrt returns 0

Question

I have a c# function that contains formula to calculate euclidean distance of some points. I got the point's position defined by R(rx,ry) and L(lx,ly).

at first, I tried to write the code like this:

double dRightLeft = Math.Sqrt((Math.Pow(rx - lx, 2) + Math.Pow(ry - ly, 2)));

it returns 0.0.

then I tried to split the variable to check where did I do wrong, like this:

 double rl = (Math.Pow(rx - lx, 2) + Math.Pow(ry - ly, 2));
 double dRightLeft = Math.Sqrt(rl);

the rl variable returns a valid value of its operation. but then when I tried to get the square root out of it, the dRighLeft variable still returns 0.0. I tried both assigned and unassigned dRightLeft like this:

//assigned
dRightLeft = 0;
//unassigned
dRightLeft;

they both still returns 0.0 value.

here's my short but complete program where I get the rx, ry, lx, and ly value:

public Bitmap getDetectedImage()
{
int rx, rx, lx, ly, ...;
double dRightLeft = 0;
...
//righteyeloop
for (int x = fo.rightEye.X; x < (fo.rightEye.X + fo.rightEye.Width); x++)
{
    for (int y = fo.rightEye.Y; y < (fo.rightEye.Y + fo.rightEye.Height); y++)
    {    //segmentation...//   
         rPixel++;
         result.byteImage[x, y].R = 0;
         result.byteImage[x, y].G = 255;
         result.byteImage[x, y].B = 0;
//to get the the first pixel detected//
         if (rPixel == 1)
         {
             result.byteImage[x, y].R = 255;
             result.byteImage[x, y].G = 0;
             result.byteImage[x, y].B = 0;

rx = x + (fo.rightEye.Width / setting.featureWidth * setting.eyeHeight / setting.eyeWidth);
ry = y + (fo.rightEye.Height / setting.featureWidth * setting.eyeHeight / setting.eyeWidth);
         }
    }
}
//lefteyeloop basically the same type as righteyeloop//
.....
//this to count the distance of righteye and lefteye
    double rl = ((rx - lx) * (rx - lx) + (ry - ly) * (ry - ly));
    double dRightLeft = Math.Pow(rl, 0.5);
}

Please show a short but *complete* program demonstrating the problem. — Jon Skeet, Jun 26 '15 at 13:48
Seems that `rx == lx` and `ry == ly`. Just check values in debugger — Dmitry Bychenko, Jun 26 '15 at 13:52
this is my example: R(212,179) and L(291,180) the value of rl = 6242.0 — rfa, Jun 26 '15 at 13:55
@rfa What are the values and types of `ry`, `ly`, `rx` and `lx`? — DavidG, Jun 26 '15 at 14:00
Something else you're not showing is happening: https://dotnetfiddle.net/2SDnXc — Preston Guillot, Jun 26 '15 at 14:01
@rfa Using those values, the output is `79.0063288604147`, there is something else you are not showing us. Can you show us the entire method? — DavidG, Jun 26 '15 at 14:09
That is not a complete program, I can not copy and paste it and run it on my machine. Go to https://dotnetfiddle.net/, write the absolute smallest program that runs on that site and demonstrates the problem (Bitmap is not needed to demonstrate your problem, just write a function that only includes the math portion and outputs the numbers) then copy the code in to your question and maybe even include a link to the runnable dotnetfiddle program. — Scott Chamberlain, Jun 26 '15 at 14:29
solved. the blame is on me. I should put a getter variable right below them to get the value. — rfa, Jun 26 '15 at 15:07

score 0 · Answer 1 · edited May 23 '17 at 11:51

0

I suspect the problem is that Math.Pow deals with doubles, which have low precision (see this SO question for more discussion). The immediate instinct is to just replace Math.Pow by writing out the multiplication:

double rl = ((rx - lx) * (rx - lx) + (ry - ly) * (ry - ly));
double dRightLeft = Math.Sqrt(rl);

From the Math.Pow reference, it seems that, with an exponent of two, the only way to return 0 is if your base (i.e. rx - lx or ry - ly) is also 0.

edited May 23 '17 at 11:51

Community

1
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answered Jun 26 '15 at 13:55

ajiang

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is integer divsion to blame? in C# (and many other languages), writing 3/5 returns 0. it seems unlikely, especially given the values you provided – ajiang Jun 26 '15 at 14:03
but I got the valid rl value. rx = 212, ry = 179, lx = 291, ly = 180. – rfa Jun 26 '15 at 14:08
I think it's best to post the entire method/class where you are having the issue, since, at this point, we've determined that Math.Sqrt() and Math.Pow() are not causing the problem. – ajiang Jun 26 '15 at 14:22

math.sqrt returns 0

1 Answers1