How to deal with error "Input #3 expected to be a cell array, was double instead." using cellfun?

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Fabian Lürßen 2021 年 6 月 25 日
コメント済み: Fabian Lürßen 2021 年 6 月 25 日
Hello everyone,
as I'm trying to lean how to code with vectorization and do this magic with little lines and incredible performence there are a few hurdles for me to overcome. First I need to learn how to use cellfun correctly. Though I think the descriptions MathWorks gives are often usefull they are just examples and not always show me the solutions to my issues. Please note I am here to learn. There are a lot of things to learn for me. So I am thankful for ever advice to improve my coding. And I am not a native english speaker so there is the possibility that I just don't get right what the discription is supposed to tell me.
I have the following code:
[~,locs{1},~,~] = findpeaks(Mbx,t,'MinPeakHeight',maxPeakx,'MinPeakWidth',MinPeakWi,'MinPeakDistance', MinPeakDis ,'Annotate','extents');
[~,locs{2},~,~] = findpeaks(-Mbx,t,'MinPeakHeight',maxPeakx,'MinPeakWidth',MinPeakWi,'MinPeakDistance', MinPeakDis,'Annotate','extents');
[~,locs{3},~,~] = findpeaks(Mby,t,'MinPeakHeight',maxPeaky,'MinPeakWidth',MinPeakWi,'MinPeakDistance', MinPeakDis ,'Annotate','extents');
[~,locs{4},~,~] = findpeaks(-Mby,t,'MinPeakHeight',maxPeaky,'MinPeakWidth',MinPeakWi,'MinPeakDistance', MinPeakDis,'Annotate','extents');
In this case Mbx, Mby and t are arrays and the code works. It gives me the locations of the peaks in terms of t (time vector).
But I want to be able to get the same result in just one line without using a loop.
So I formed a cellarray with:
Mb = {Mbx -Mbx Mby -Mby};
and tried to perform a cellfun like this:
[~,locs,~,~] = cellfun(@(x)findpeaks(x,t,'MinPeakHeight',maxPeakx,'MinPeakWidth',MinPeakWi,'MinPeakDistance', MinPeakDis ,'Annotate','extents'),Mb,'UniformOutput',false);
I thought x is the input for my function and it performes findpeaks with all given name and value pairs on each cell of it.
But a get the error:
Error using cellfun
Input #3 expected to be a cell array, was double instead.
So what is Input #3 in this case? And how do I hand over the name value pairs for findpeaks correctly to the cellfun?
kind regards
Fabian

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回答 (1 件)

David Hill 2021 年 6 月 25 日
Loops are not bad. cellfun is performing a loop.
Mb = {Mbx -Mbx Mby -Mby};
for k=1:4
[~,locs{k},~,~] = findpeaks(Mb{k},t,'MinPeakHeight',maxPeakx,'MinPeakWidth',MinPeakWi,'MinPeakDistance', MinPeakDis ,'Annotate','extents');
end
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Fabian Lürßen 2021 年 6 月 25 日
Okay sorry I believe you. I am barely six weeks into MATLAB and think I got something a little wrong and confused vectorisation with those one-liners where you call an anonymous function. It is kind of hard to understand all the descriptions with all those specific foreign words memorize and implement them.
But do you have any advice on how to write clearer loops and avoid things that slow down the performance? I thing those calls on find in a loop are sure a bad idea for the performance. But I simply did the work with the tools I know.

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