Error in simple calculations

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Ahmed
Ahmed 2015-4-17
评论： Ahmed 2015-4-18
Hi everyone I'm really having a very weird and silly problem with Matlab. A very simple calculation which is z=(x-y). That's all, but when x= 0.327999967200000 and y=0.319999968000000 then z=0.00799999920000000 which is correct. But when x=0.311999968800000 and y=0.303999969600000 then z=0.00799999920000005 which is wrong as the answer should stay 0.00799999920000000. It is a simple silly stupid operation nevertheless the Matlab is unable to solve it correctly. I used everything, the format the cast the eps() non of them worked and the error is still persistent. What shall I do so the Matlab would perform such calculation without errors. Is it a bug or something? I'm using Matlab 2014b.
Thanx in advanced

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回答（1 个）

Sebastian Castro
Sebastian Castro 2015-4-17
Definitely not a bug -- just floating-point error: http://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero
If you care about that level of precision, it might be feasible to work with fixed-point data. This way, you can choose a good fixed-point implementation that can exactly represent a particular set of numbers.
- Sebastian
  1 个评论
Ahmed
Ahmed 2015-4-18
Dear Sebastian I'm really grateful for the answer but I wonder if there is a simpler way to do it. Actually these number are indecies for voxel cells and they are in millions multiply that by 8 for each cube. Using such approach yields the execution time to explode. I want to avoid this. Besides, the geniuses and huge minds that created such sophisticated scientific package should address and fix this annoying problem. Any calculator, windows calc or a free calc, can perform such operation flawlessly. Also, we as user should have full confidence in Matlab's results not wondering if there was a bug or round-off error may cause the results to be like this. I know it is a precision and word length problem but honostly I saw this for the first time with Matlab and I'm a C++ user.
Any way is there another approach less time consuming and more direct to solve it? I would appreciate it.
Thanx alot

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