Need help plotting 100 Strings onto grid (FOR loop not working)!

Question

Delvin 2013-4-22

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MATLAB has been working on this code for over 30 minutes and still hasnt processed it:

 %
hold on
axis on
for i = 1:100
    for j = 1:100
        annotation('textbox', [i/100 j/100 0.01 0.01], 'String', image(i,j));
    end
end

Image() is a matrix 100x100 which contains a number at each position.

I read online that matlab isnt very efficient at for loops, and since i am iterating 10,000 times, that explains it.

I dont know how to vectorise this code either. PCOLOR, SURF, MESH etc dont deal with plotting "Strings".

Would appreciate your help,

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Answer 1

Walter Roberson 2013-4-22

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[I, J] = ndgrid(1:100, 1:100);
strs = cellstr(num2str(imagedata(:)));
text( J(:)./100, I(:)./100, strs );

Note the change of variable name -- "image" is the name of an important MATLAB graphics function.

Please note the exchange of I and J. This is needed if what you are doing is labeling blocks that you have displayed using image() or imagesc() or imshow(). For example,

data = floor(rand(3,5) * 100);
strs = cellstr(num2str(data(:)));
[I,J] = ndgrid(1:size(data,1),1:size(data,2));
imagesc(data);
t = text(J(:)./1, I(:)./1, strs);   %in this example coordinates are integer

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Answer 2

Matt Kindig 2013-4-22

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I would use a text() function rather than a textbox annotation, since the former is a lot lighter than the latter.

You can then call it without using a loop at all by calling it as:

[ii,jj]=meshgrid( 1:size(image,1)/100, 1:size(image,2)/100);    %get x,y values
ii = reshape(ii, [], 1); 
jj = reshape(jj, [], 1);
img = reshape(image, [], 1);
text(ii, jj, num2str(img));

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Answer 3

Kelly Kearney 2013-4-22

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The slowdown is probably because you're trying to create 10000 annotation objects, and Matlab tends to get bogged down with too many graphics objects. You can achieve the numbers in a grid effect a lot more efficiently using the text command along with appropriate grid lines:

 n = 100;
 im = round(rand(n)*10);
 xgrid = linspace(0,1,n+1);
 xcent = (xgrid(1:end-1)+xgrid(2:end))./2;
 [x,y] = meshgrid(xcent);
 axes('position', [0 0 1 1], 'xlim', [0 1], 'ylim', [0 1], ...
      'xtick', xgrid,  'ytick', xgrid, 'xgrid', 'on', ...
      'ygrid', 'on', 'gridlinestyle', '-');
 text(x(:), y(:), cellstr(num2str(im(:))), 'horiz', 'center');

A few side notes:

- Using image as the name of your array is a bad idea, since that will overshadow the image function.

- Matlab is pretty good at loops these days; most of the comments relating to slow iteration over loops refers to older versions.

- Are you sure you want to display a 100x100 grid of numbers? Even with my single-digit example above, that gets a bit crowded even for a maximized figure on a decent-sized monitor.