Matrix with variable number of rows and fixed columns

Question

KE BR 2020-8-12

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回答： KE BR 2020-8-12

采纳的回答： Walter Roberson

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Hi

I am having trouble with some code. I have a mxn matrix BT where for each column i want to extract some data.

[H,W] = size(BT);
for i= 1:W
    [x(:,i),y(:,i)] = findpeaks((BT(:,i)),f1)
end

What this does is find the peaks for the first column of BT and store the amplitude into X and the frequency into Y and then move on to the second column. The problem is that lets say for the first column i get 19values wich are stored into X and Y. But for the second column i get 20values wich can not be stored into X and Y because the number of rows are not equal.

I was thinking to add zeros to a fixed number of rows for each column but i dont know how to do it for this code. Can anybody help me with this problem?

Kind regards

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Answer 1

Walter Roberson 2020-8-12

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This code uses nan padding instead of 0 padding, in case the peak value happens to be 0. If you know that cannot happen, then the code could be made a bit more simple.

[H,W] = size(BT);
x = []; y = [];
nr = 0;
for i = 1:W
    [thisx, thisy] = findpeaks((BT(:,i)),f1);
    nx = length(thisx);
    if nx <= nr
        thisx(end+1:nr) = nan;
        thisy(end+1:nr) = nan;
    else
        x(end+1:nx, :) = nan;
        y(end+1:nx, :) = nan; 
    end
    x(:, i) = thisx;
    y(:, i) = thisy;
    nr = size(x,1);
end

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Answer 2

KSSV 2020-8-12

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As you will have variable number of values in each column, save them in the cell.

[H,W] = size(BT);
x = cell(W,1) ; y = cell(W,1) ; 
for i= 1:W
    [xx,yy] = findpeaks((BT(:,i)),f1)
    x{i} = xx ;y{i} = yy ; 
end

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Answer 3

KE BR 2020-8-12

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stephen, KSSV, Walter all of your answers works perfectly! :) Thanks for your help and time! I had to accept Walter's answer as for future work it will be easier for me to work with a fixed number of rows and columns wich his code does! nevertheless thanks for the time and help! its much appreciated. :)