MATLAB: Can’t seem to properly debug “In an assignment A(:) = B…” error

a/berrorerror in debuggingfor loopif loopimage analysisin an assignmentMATLAB

I am doing some image analysis and getting the following error message:
In an assignment  A(:) = B, the number of elements in A and B must be the same.
Error in FINAL (line 37)
      bottomLine(col) = find(croppedImage(:, col), 1, 'last');
I have made sure that my column vector (col) is the proper length, but it doesn't seem to be working. For some context, the purpose of this set of code is to analyze a video file of a droplet on a surface. As the droplet freezes, I see the freezing line move up.
I have also watched this video: https://blogs.mathworks.com/videos/2014/11/20/matlab-in-an-assignment-ai-b-the-number-of-elements-in-b-and-i-must-be-the-same/
When I evaluate my "problem" line of code (line 37) all of my variables are of acceptable length, the columns vector is 236 and so is the columns of the croppedImage matrix. The for loops seems to stop at column number 148 every time which I don't understand.
Here is my code:
nstart = 260;
nend = 700;
Height = (90:370);                          %Temporary


Width1 = (94:796);                          %Temporary
Width2 = (1060:1775);                         %Temporary
load('Crop1.mat')
%%Processing for Droplet 1
numberOfCells = length(Crop1);
freezingLine1 = zeros(1, numberOfCells);
L1 = round((length(Width1)/3)):round((2*length(Width1)/3));
for k = 1:numberOfCells
  thisCell = Crop1{k};
  if ~isempty(thisCell)
    subplot(2, 2, 1);
    imshow(thisCell);
    axis on;
    caption = sprintf('Frame %d of %d', k, numberOfCells);
    title(caption, 'FontSize', fontSize);
    % Extract columns 200-300
    croppedImage = thisCell(1:length(Height), L1);
    subplot(2, 2, 2);
    imshow(croppedImage);
    axis on;
    title(caption, 'FontSize', fontSize);
    % Get the bottom most pixel
    [rows, columns] = size(croppedImage);
    bottomLine = zeros(1, columns);
    for col = 1 : columns
      bottomLine(col) = find(croppedImage(:, col), 1, 'last');
    end
    subplot(2, 2, 3);
    plot(bottomLine, 'b-', 'LineWidth', 2);
    ylim([1, rows]);
    xlim([1, columns]);
    title('Bottom Pixels', 'FontSize', fontSize);
    xlabel('Column', 'FontSize', fontSize);
    ylabel('Row', 'FontSize', fontSize);
    grid on;
    % Throw out outliers by looking at the median absolute deviation.
    medianValue = median(bottomLine);
    mad = bottomLine - medianValue;   
    goodIndexes = mad <= 3;  % Good locations have a mad of less than or equal to 3.
    % Find the mean of the good indexes.
    freezingLine1(k) = mean(bottomLine(goodIndexes));
    subplot(2, 2, 4);
    plot(freezingLine1, 'b-', 'LineWidth', 2);
    grid on;
    title('Freezing Line Row1', 'FontSize', fontSize);
    xlabel('Frame', 'FontSize', fontSize);
    ylabel('Row', 'FontSize', fontSize);
    drawnow;
  end
end
and attached is a .mat file that needs to be loaded to run the code.

Best Answer

I am guessing this error is due to null matrices appearing as a result of this call:
find(croppedImage(:, col), 1, 'last');
Try this instead:
 for col = 1 : 236
     try
        bottomLine(col) = find(croppedImage(:, col), 1, 'last');
     catch
        bottomLine(col) = NaN;
     end
 end
This would mean some columns in the image simply doesn't have a non-zero value. I am dealing with them as NaNs here but you can assign them whatever you want.
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