Finding the n'th element satisfying a condition

Question

Nirmal 2012-5-7

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I have a huge array A (about 30 million elements) and I'm trying to extract the n'th element satisfying a condition A == b. Currently, I' using the following:

x = max(find(A == b,n))

Is there a more efficient way to do this since I need to do this several times in my code?

Thanks.

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Answer 1

Sean de Wolski 2012-5-7

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That looks good to me. The only thing that might make it faster would be to index instead of use max():

idx = find(A==B,n,'first')
idx(end)

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Nirmal 2012-5-7

Using this made the code considerably faster, actually..

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Answer 2

Thomas 2012-5-7

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Seems to be the fastest way of doing this.. Index might make it faster (though I doubt it..) try timing it..

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Answer 3

Walter Roberson 2012-5-7

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I am not sure why you are using max() ?

t = find(A == b, n, 'first');
t(end)

If you are doing this repeatedly, what is varying, and what is known at the time you want to start the first search? If you know all the values you will be searching on, then the three-output version of ismember() might be appropriate.