Applying function on few elements each time, using a for loop

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Zuha Yousuf 2020-4-13

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编辑： Rik 2020-4-13

So lets say I have an array, which has 540000 values. I want to apply a max function over this array, but only over 30000 values each time, using a for loop. Is there a way I could do this??

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Answer 1

Ameer Hamza 2020-4-13

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编辑：Ameer Hamza 2020-4-13

You can do it without for loop in MATLAB

x = rand(1,540000); % random data replace it with your vector
y = max(reshape(x, 30000, []));

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Zuha Yousuf 2020-4-13

Hey, thanks for answering! So what I meant was, that I need to apply this function one at a time over this chosen sample window each time.

The code you gave me gives me an error that 'Array indices must be positive integers or logical values'.

Ameer Hamza 2020-4-13

Yes, This code applies the max function using a window of 30000 numbers. Then move the window to the next block and apply max. Can you make sure there is no variable named 'max' in your workspace? Otherwise, paste your code here, which gives this error.

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Answer 2

Rik 2020-4-13

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https://ww2.mathworks.cn/matlabcentral/answers/517473-applying-function-on-few-elements-each-time-using-a-for-loop#answer_425693

编辑：Rik 2020-4-13

If you don't mean what Ameer posted, then you could mean a sliding window maximum. You could write it yourself with a loop, but it is more efficient to use the builting movmax function.

If you are using an older release (or insist on a loop) you can use something like this:

x = rand(1,540000); % random data
window_size=30000;
output=zeros(size(x));
window=ceil(window_size*[-0.5 0.5]);
for n=1:numel(x)
    start=max(1,n+window(1));%ensure positive starting index with max(1,__)
    stop=min(n+window(2),numel(x));%ensure value stays within bounds
    output=max(x(start:stop));
end
%test:
isequal(output,movmax(x,abs(window))) %returns true