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matlabfloating-point
patrik
patrik

Reputation: 4558

how to solve floating point errors in matlab

I have a question that I somehow surprising cannot find an answer to. The issue is about floating point errors. It is not a "why does a==b give 0" question, but rather if there are a way to fix floating point errors. The part of the code where I am right now is sensitive and I need to find a way to solve floating point errors. I have tried with

round(100000*myDouble)/100000;

and with

double(int64(100000*myDouble))/100000

but the output does still have floating point errors for some numbers (not all of them, but a few is enough to mess up my code). The problem is that the function I use in matlab is a polygon clipper which I use to calculate the union of many polygons. The function looks for common points and if there are even a small difference, this will mess everything up. The problem should really not be a problem anyway since it is a union and partly overlapping polygons should not cause trouble. However due to some issues with the function I need to make sure to not have these overlaps. The function works really fine in most cases but to speed up I have added a vector of nan separated polygons with holes and since the function is not expected to handle cases like this there are sometimes problems.

There are no point to use int64 for the calculation instead since the function end up with a mex file and is thus not possible to make it work for int64.

Upvotes: 0

Views: 1013

Answers (1)

Daniel
Daniel

Reputation: 36710

There is not a single answer to solve all floating point precision issues. Some strategies are:

  • Usage of vpa (or symbolic variables).
  • Make the errors deterministic. If the same formula is used to calculate the same number twice, the result should be equal.
  • Don't round using decimal factors. Use 2^x instead. In this case, try 2^16 instead of 100000. This way you round the fraction and keep the exponent.

Upvotes: 1

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