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Basic indexing question Find ismember

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DIMITRY
DIMITRY on 8 Mar 2016
Commented: DIMITRY on 8 Mar 2016
Hi World,
I would like to create an index of a matrix based on a vector to resize this array by extracting the indexed rows:
X=(0.5:0.5:17)';
F=(1/52:1/52:30+1/52)';
Q=find(ismember(F,X));
But there are missing values from Q while I can see them in the workspace.
Can you help?

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Accepted Answer

Stephen Cobeldick
Stephen Cobeldick on 8 Mar 2016
Edited: Stephen Cobeldick on 8 Mar 2016
You need to learn about the practical limits of floating point numbers.
Basically the numbers that you see are only decimal approximations of the real binary values stored in the computer's memory. With calculations involving decimal fractions these values can differ by a small amount even though they look they same when you look at them in the command window or variable viewer. However they will fail an equality test, and so are not considered to be equal, thus ismember does not match them.
You could use ismembertol instead.
Information on Floating Point Numbers
And some external links on this topic:

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