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arraysmatlabelementextractnan
Windy Day
Windy Day

Reputation: 173

how to extract some elements from array and make rest NaN in matlab?

I have an array of 13x14 double. I want to extract few elements which are not in any particular order and make rest of the elements NaN.

eg, if 

A = [ 0.2 0.3 0.6 0.4
      0.1 0.5 0.2 0.8
      0.7 0.1 0.5 0.9
      0.2 0.5 0.6 0.7]

I want to extract element having index (2,1), (2,3) (3,3) and (4,2) and make rest as NaN. So the final output should be : 

[ NaN NaN NaN NAN
 0.1 NaN 0.2 NaN
 NaN NaN 0.5 NaN 
 NaN 0.5 NaN NaN ]

I tried logical indexing but it gives me a vector which I don't want because then you cannot reshape it and make it 2-D array. I want a 2-D array. Thanks.

Upvotes: 0

Views: 118

Answers (2)

Nras
Nras

Reputation: 4311

You can do this by creating a matrix of the same size of A full of NaNs and then use linear indexing to overwrite the NaNs by the values wanted:

A = [ 0.2 0.3 0.6 0.4
     0.1 0.5 0.2 0.8
     0.7 0.1 0.5 0.9
     0.2 0.5 0.6 0.7 ]

%(2,1), (2,3) (3,3) and (4,2) % // reads as...
rows = [2, 2, 3, 4];
cols = [1, 3, 3, 2];
idx = sub2ind(size(A), rows, cols) % // pair of indices as linear indices
out = NaN(size(A)); % // Matrix full of NaN (same size as A)
out(idx) = A(idx); % // Overwriting with values from A at given indices

Upvotes: 2

Dan
Dan

Reputation: 45752

Preallocate first:

result = NaN(size(A))
result(2,1) = A(2,1)
result(2,3) = A(2,3)

or if you have your subscripts as an n-by-2 matrix then you can use sub2ind to do it in one shot:

result = NaN(size(A))
subs = [2,1
        2,3
        3,3
        4,2];
ind = sub2ubd(size(A), subs(:,1), subs(:,2));
result(ind) = A(ind)

Upvotes: 0

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