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POWhat's the fastest way to remove or change large number of entries in arrays in MATLAB?
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I want to change a number of values in a 4D array M_ ijkl to NaN using MATLAB. I use <code>find</code> to get the indices i and j that meet a certain condition for k = 2 and l = 4 (in my case it's the y component of a position at time t_4). I now want to set all the entries for these i and j combinations and for all k and l to NaN. I used this method to do it (example by nkjt): <pre><code>% initialise M = zeros(10,10,2,4); % set two points in (:,:,2,4) to be above threshold. M(2,4,2,4)=5; M(6,8,2,4)=5; % find and set to NaN [i,j] = find(M(:,:,2,4) > 4); M(i,j,:,:)= NaN; % count NaNs sum(isnan(M(:))) % returns 32 </code></pre> This method is is very slow as this example illustrates: <pre><code>M = rand(360,360,2,4); threshold = 0.5; % slow and wrong result [i,j] = find(M(:,:,2,4) > threshold); tic; M(i,j,:,:) = NaN; toc; Elapsed time is 54.698449 seconds. </code></pre> Note that the <code>tic</code> and <code>toc</code> don't time the <code>find</code> so that is not the problem. With Rody's and njkt's help I also realized that my method doesn't actually do what I want. I only want to change entries with the combinations i and j i found with <code>find</code> (for all k and l), i.e. <code>[2,4,:,:]</code> and <code>[6,8,:,:]</code>, but not <code>[2,8,:,:]</code> and <code>[6,4,:,:]</code>. In the first example <code>sum(isnan(M(:)))</code> should return 16.
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<arrays><performance><matlab><nan>
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What's the fastest way to remove or change large number of entries in arrays in MATLAB?
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