CODEWITHSUNDEEP
arraysmatlabnan
LenaH
LenaH

Reputation: 323

How to assign NaN to values j < k in Matlab

I'm a Matlab newbie and I would like to assign NaN values to an array of size(j, k, l) wherever the dimension is j < k or j < l. How do I do this most efficiently?

Upvotes: 2

Views: 178

Answers (2)

rahnema1
rahnema1

Reputation: 15837

You can use bsxfun to do it efficiently:

J = (1:size(A,1)).';
K = 1:size(A,2);
L = reshape(1:size(A,3),1,1,[]);
A(bsxfun(@or,bsxfun(@lt,J,K),bsxfun(@lt,J,L))) = NaN;

In MATLAB r2016b or Octave you can simply write:

J = (1:size(A,1)).';
K = 1:size(A,2);
L = reshape(1:size(A,3),1,1,[]);
A(J<K|J<L)=NaN;

Result of a test on a matrix A = rand(500,400,300):

 ________________________________
| METHOD   | MEMORY   | SPEED    |
|==========|==========|==========|
| MESHGRID | 1547 MB  | 1.24 Secs|
|----------|----------|----------|
| BSXFUN   | 57 MB    | 0.18 Secs|
|__________|__________|__________|

Upvotes: 4

Ander Biguri
Ander Biguri

Reputation: 35525

Use fancy vecotization:

% this may be memory expensive for big matrices:
[j,k,l]=meshgrid(1:size(A,1),1:size(A,2),1:size(A,3));

% Tada!
A(j<k | k<l)=NaN;

If you do not have enough RAM (or do not want to use it for this), then the best option is just loopy:

for jj=1:size(A,1)
   for k=1:size(A,2)
       for l=1:size(A,3)
           if (jj<k | k<l)
               A(jj,k,l)=NaN;
           end
       end
   end
end

This will likely be slower, but doesn't need any extra memory.

Upvotes: 3

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