Why can't my perceptron determine if a point is exactly on the line?

[TSLexi]

When I use my perceptron, it classifies everything appropriately, except it always says points that are exactly on the line are above it. Any ideas as to why?

Thanks.

[2kaud]

    float error = desired - guess;

You are assuming that this will give 0 if desired is the same as guess. There is often an issue with floating point numbers doing comparison due to the way these numbers are stored internally. Two numbers that display the same may not be stored internally exactly the same so subtracting them won't give an absolute zero.

Consider something like

    float error = (abs(desired - guess) < 0.0001) ? 0.0 : desired - guess;

Where the 'magic number' 0.0001 should be chosen to be appropriate.

See
http://www.cygnus-software.com/paper...ringfloats.htm
http://stackoverflow.com/questions/1...ble-comparison
http://stackoverflow.com/questions/4...compare-floats

[TSLexi]

Fixed!

Changed the conditional in feedback () to:

if (sum == 3)
	return 0;
	else if (sum < 3 && sum > 0)
	return 1;
	else 
	return 0;

Now it gets that (2,5) is on the line, and (2,4) are below the line, but it says any point with a y-value above 5 is on the line.

[Paul McKenzie]

When I use my perceptron, it classifies everything appropriately, except it always says points that are exactly on the line are above it. Any ideas as to why?

http://www.parashift.com/c++-faq/floating-pt-errs.html

Unless the floating point number is a sum of inverse powers of 2, there is no exact binary representation of a decimal fractional amount. As the link states, the issue you're facing has to do with numerical analysis and computer science, not programming. If you used Java, C#, C, Basic, Delphi/Pascal, etc. you will find the same problem with floating point numbers.

Regards,

Paul McKenzie

[Paul McKenzie]

Fixed!

Changed the conditional in feedback () to:

if (sum == 3)

What is "sum"? Is it a floating point variable? If so, if the value of "sum" is computed, then there is no guarantee that sum will be exactly 3.

but it says any point with a y-value above 5 is on the line.

I think it's time to use the debugger. Learning to debug your programs is a mandatory step in learning how to program.

Regards,

Paul McKenzie

[2kaud]

sum is type float - so shouldn't really be compared directly with 3 (see earlier posts from myself and Paul). However, looking at the code, it seems to return 1 only if sum is < 3 and > 0. This can be re-coded as

return (sum > 0.0 && sum < 3.0);

[TSLexi]

I actually think it's a algorithm issue.

[GCDEF]

Fixed!

Changed the conditional in feedback () to:

if (sum == 3)
	return 0;
	else if (sum < 3 && sum > 0)
	return 1;
	else 
	return 0;

Now it gets that (2,5) is on the line, and (2,4) are below the line, but it says any point with a y-value above 5 is on the line.

Not that it matters much, but that's more complicated than it needs to be. This line does the same thing

return (sum > 0 && sum < 3);

[2kaud]

Not that it matters much, but that's more complicated than it needs to be. This line does the same thing

return (sum > 0 && sum < 3);

See my post #6

[GCDEF]

See my post #6

Oops.