using logical arrays to select certain elements into another matrix

Question

I believe the answer to this is simple but my brain will not function.

Say I have a m x n matrix which is of type double & lets call it A. I also have a matrix B, which is m x n and is all NaN's.

I then want to find out which numbers are say equal to some number, let say 100. I can do the following,

  A_index = A == 100;

So I now have a logical array, A_index. This is all fine.

My question is how do I select the elements from A where A_index is true into matrix B?

Some made up matrices

A= [ 50 100 75 90 100; 0 50 60 30 10; 100 25 80 250 100; 5 100 0 100 90];
A_index = A == 100;
B= zeros(4,5) * NaN;

Since A and B are the same size and shape, try B(A==100) = ... (replacing the ... by whatever expression you want on the rhs). Then read the documentation on the topic of logical indexing. — High Performance Mark
– High Performance Mark, Commented Nov 1, 2016 at 8:28
Here is a link to the SO documentation : indexing in Matlab. — Hoki
– Hoki, Commented Nov 1, 2016 at 8:36

Graham · Accepted Answer · 2017-09-27 20:03:35Z

1

Something like:

A= [ 50 100 75 90 100; 0 50 60 30 10; 100 25 80 250 100; 5 100 0 100 90];
A_index = A == 100;

B= zeros(4,5) * NaN;

B(A_index) = 100

This way you will get 100 in the entries of B where A is equal to 100

See the section on logical indexing in the MATLAB docs

Graham

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answered Nov 1, 2016 at 8:31

João Almeida

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