intersect misses common elements of 2 matrices

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Jack
Jack 2016-12-16
编辑： James Tursa 2016-12-17
I've been using intersect to identify the common elements of two 2D grids. However, when I try it against the following code, it misses some of the common elements.
[x1Grid_1,x2Grid_1] = meshgrid(-70:0.5:70, 1:0.05:17);
xGrid_1 = [x1Grid_1(:), x2Grid_1(:)];
[x1Grid_2,x2Grid_2] = meshgrid(-56:0.5:35, 6:0.05:16);
xGrid_2 = [x1Grid_2(:), x2Grid_2(:)];
[c1,c2,c3] = intersect(xGrid_1,xGrid_2,'rows');
The vector xGrid_2 should be made entirely of elements from xGrid_1. xGrid_2 has 36783 rows, but after running intersect, vectors c1-c3 only have 30195 elements, meaning they miss some of the common elements. I can't understand why this would be happening. Any input would be helpful.
I'm using 2016a.
Thanks, Jack

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Image Analyst
Image Analyst 2016-12-16
I think this might be a case of floating point numbers, arrived at via different operations, not matching when you think they should. You need to check within a tolerance. See the FAQ: http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F
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Image Analyst
Image Analyst 2016-12-16
True, as are all negative powers of 2: 2^(-1) and 2^(-2), etc. (0.5, 0.25, 0.125, etc.).
James Tursa
James Tursa 2016-12-17
编辑：James Tursa 2016-12-17
Or if you are on a Windows system where the underlying sprintf algorithm is different, you can use num2strexact instead:
>> num2strexact(0.05)
ans =
5.000000000000000277555756156289135105907917022705078125e-2

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Jack
Jack 2016-12-16
Rounding it did the trick - I just used
xGrid_1 = round(xGrid_1*100)/100;
xGrid_2 = round(xGrid_2*100)/100;
to cutoff the decimals after 0.01. That made it work.
Thanks

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