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POFinding islands of zeros in a sequence
 

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    <p>Imagine you have a very long sequence. What is the most efficient way of finding the intervals where the sequence is all zeros (or more precisely the sequence drops to near-zero values <code>abs(X)&lt;eps</code>):</p> <p>For simplicity, lets assume the following sequence:</p> <pre><code>sig = [1 1 0 0 0 0 1 1 1 1 1 0 1 0 0 0 1 1 1 1 1 1 1 1 0 0 1 1 1 0]; </code></pre> <p>I'm trying to get the following information:</p> <pre><code>startIndex EndIndex Duration 3 6 4 12 12 1 14 16 3 25 26 2 30 30 1 </code></pre> <p>then using this information, we find the intervals with duration >= to some specified value (say <code>3</code>), and returning the indices of the values in all these intervals combined:</p> <pre><code>indices = [3 4 5 6 14 15 16]; </code></pre> <p>That last part is related to a previous question:</p> <blockquote> <p><a href="https://stackoverflow.com/questions/2807270/matlab-vectorized-array-creation-from-a-list-of-start-end-indices">MATLAB: vectorized array creation from a list of start/end indices</a></p> </blockquote> <p>This is what I have so far:</p> <pre><code>sig = [1 1 0 0 0 0 1 1 1 1 1 0 1 0 0 0 1 1 1 1 1 1 1 1 0 0 1 1 1 0]; len = length(sig); thresh = 3; %# align the signal with itself successively shifted by one %# v will thus contain 1 in the starting locations of the zero interval v = true(1,len-thresh+1); for i=1:thresh v = v &amp; ( sig(i:len-thresh+i) == 0 ); end %# extend the 1's till the end of the intervals for i=1:thresh-1 v(find(v)+1) = true; end %# get the final indices v = find(v); </code></pre> <p>I'm looking to vectorize/optimize the code, but I'm open to other solutions. I have to stress that space and time efficiencies are very important, since I'm processing a large number of long bio-signals.</p>
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    1. COI like your usage of the word islands.
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    2. CO@ChaosPandion: searching islands of zeros in a sea of ones.. arrr :)
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