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== comparison not working

Started by KingofNoobs Dec 13, 2012 at 9:39 AM 8 replies 3.4k views

Original Post

Dec 13, 2012 at 9:39 AM

Hello all,

I have the following code that works in case (2) but not in case (1). Can anyone explain this to me? This could be the cause of many of my more subtle bugs.


 int main()
 {
 sf::Window gameWindow(sf::VideoMode::GetMode(0), "Block Buster",sf::Style::Fullscreen);
 uint32_t VideoModesCount = sf::VideoMode::GetModesCount();
 for(uint32_t i = 0; i < VideoModesCount; ++i)
 {
 sf::VideoMode Mode = sf::VideoMode::GetMode(i);
 float sixteenByNine = (float)16/9;
 float aspectRatio = (float)Mode.Width / Mode.Height;
 
 (1) if((float)((float)Mode.Width / Mode.Height) == ((float)16/9))
 (2) if(aspectRatio == sixteenByNine)
 std::cout << "Video Mode " << i << ":\tWidth: " << Mode.Width
 <<"\tHeight: " << Mode.Height << "\tBpp: " << Mode.BitsPerPixel
 << std::endl;
 }
 // ...
 return EXIT_SUCCESS;
 }

Is there some weird truncation that is happening when I am directly comparing the variables inside the if() statement?

Thanks for your help.

- Dave Ottley

Ectara

Dec 13, 2012 at 9:50 AM

Floating point inaccuracy; one should not directly test for equality between floating point numbers.

C0lumbo

Dec 13, 2012 at 9:54 AM

TBH, the two options look equivalent to me, so I don't know why your results are inconsistent, but I'd recommend that you never try to compare the result of floating point calculations using ==. In general, instead of:

if (fA == fB)

You should use something like.

if (fabsf(fA - fB) < 0.0001f)

Because you almost always have precision errors in any floating point calculation and even if it works in debug, it might not work in release because the floating point operations can be rearranged to produce slightly different results.

KingofNoobs

Dec 13, 2012 at 9:54 AM

However, it works in case 2. Why should this work any better than in case 1?

KingofNoobs

Dec 13, 2012 at 9:55 AM

I see, I just read your post Columbo. I will try your code to see if it works.

KingofNoobs

Dec 13, 2012 at 9:59 AM

FYI, it works! Thank you very much kind sirs.

caldiar

Dec 13, 2012 at 10:02 AM

Ectara's right. Removing the poorly thought out suggestion.

Ectara

Dec 13, 2012 at 10:05 AM

You could also check the width and height to see if they're multiples of 16 and 9 respectively such as:

if( ((Mode.Width % 16) == 0) && ((Mode.Height % 9) == 0)) { std::cout << "Video Mode " << i << ": Width: " << Mode.Width <<" Height: " << Mode.Height << " Bpp: " << Mode.BitsPerPixel << std::endl; }

Almost, but the important part is the ratio: 32 by 9 is not a 16:9 ratio.

caldiar

Dec 13, 2012 at 10:13 AM

Almost, but the important part is the ratio: 32 by 9 is not a 16:9 ratio.

Well spotted! Perhaps a sign for me to turn off the computer for the night and go to bed.

Ectara

Dec 13, 2012 at 10:25 AM

Well spotted! Perhaps a sign for me to turn off the computer for the night and go to bed.

Happens to everyone every now and again.

Personally, I used an enum for preset aspect ratios, with an extra value to signify that it should calculate it from the width and height.

I think that if you know the ranges for the width and height, and you need determinism, you could try fixed point math.

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