probably a dumb question - logical indexing

Question

bmeyer 2015-7-8

1
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https://ww2.mathworks.cn/matlabcentral/answers/229135-probably-a-dumb-question-logical-indexing

回答： bmeyer 2015-7-8

Hello

Maybe I have forgotten the little I learned about logical indexing, but I can't think of a way to translate this into something using find function or any sort of logical indexing.

Here is what I am trying to do in terms of a loop:

for t = 1:numel(T)
    if abs(T(t+4000) - T(t)) <= 0.1
        tau = t;
        break;
    end
end

I want to compare temperature values taken throughout a given amount of time. If the difference between temperatures that are 4000 indices (timepoints in this case) apart is less than 0.1, it gives me the timepoint. This is a basic heating/cooling problem and I just want to find tau.

I feel like there is a much more computationally efficient way to do this, but other options are escaping me at the moment. Thanks

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Answer 1

Sean de Wolski 2015-7-8

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/229135-probably-a-dumb-question-logical-indexing#answer_185494

编辑：Sean de Wolski 2015-7-8

在 MATLAB Online 中打开

tau = find(T(4001:end)-T(1:end-4000)<=0.1,1,'first')

Something like this (not tested in ML)

Though this won't be faster for very large T where tau is small because it's doing the subtraction for the entire T-4000 elements array where as the loop only runs until it stops (which could be 1 iteration).