Help when trying to exclude matrix values while indexing!

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SKYLAR SCHUTTER 2020-3-10

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评论： SKYLAR SCHUTTER 2020-3-10

I am trying to figure out how to index a matrix but exclude certain values. Specifically I have

y=grid.y;
latitudes = [-90 90];
I=find(y>=min(lat) & y<=max(lat));
II=find(y<=min(lat) & y>=max(lat));

The first find works (I), the second (II) does not and returns a NaN value. help?

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Ameer Hamza 2020-3-10

Can you attach a sample dataset so we can try to replicate the issue?

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Answer 1

Guillaume 2020-3-10

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"the second (II) does not and returns a NaN value"

Firstly, find never returns NaN. It can return an empty array, this is not the same thing at all as NaN.

I wouldn't be surprised that your second expression returns an empty array. numbers that are both smaller than a minimum while at the same time being larger than a maximum are ... very rare! You 2nd expression can be rewritten as:

II = find(y <= -90 & y >= 90);

which clearly is not going to be satisfied often.

In fact, only NaN numbers can satisfy this expression. So your find will return the indices of the NaN values in y or empty if there are no NaN.

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SKYLAR SCHUTTER 2020-3-10

Thank you for your comment! I think my confusion comes from misunderstanding the fundamentals of the "find" function. I was trying to use it to generate an index that represented the columns of my matrix but I see know that that was flawed. Thank you for your help!

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