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copied!<p><strong>Image coordinates vs plot coordinates</strong> Used to get me every time.</p> <pre><code>%# create an image with one white pixel img = zeros(100); img(25,65) = 1; %# show the image figure imshow(img); %# now circle the pixel. To be sure of the coordinate, let's run find [x,y] = find(img); hold on %# plot a red circle... plot(x,y,'or') %# ... and it's not in the right place %# plot a green circle with x,y switched, and it works plot(y,x,'og') </code></pre> <p><em>Edit 1</em> </p> <p><strong>Array dimensions</strong></p> <p>Variables have at least two dimensions. Scalars are size <code>[1,1]</code>, vectors are size <code>[1,n]</code> or <code>[n,1]</code>. Thus, <code>ndims</code> returns 2 for any of them (in fact, <code>ndims([])</code> is 2 as well, since <code>size([])</code> is <code>[0,0]</code>). This makes it a bit cumbersome to test for the dimensionality of your input. To check for 1D arrays, you have to use <code>isvector</code>, 0D arrays need <code>isscalar</code>.</p> <p><em>Edit 2</em></p> <p><strong>Array assignments</strong></p> <p>Normally, Matlab is strict with array assignments. For example</p> <pre><code>m = magic(3); m(1:2,1:3) = zeros(3,2); </code></pre> <p>throws a</p> <pre><code>??? Subscripted assignment dimension mismatch. </code></pre> <p>However, these work:</p> <pre><code>m(1:2,1:2) = 1; %# scalar to vector m(2,:) = ones(3,1); %# vector n-by-1 to vector 1-by-n (for newer Matlab versions) m(:) = 1:9; %# vector to 'linearized array' </code></pre> <p><em>Edit 3</em></p> <p><strong>Logical indexing with wrongly sized arrays</strong> Good luck debugging this!</p> <p>Logical indexing seems to make a call to <code>find</code>, since your logical array doesn't need the same amount of elements as there are indices!</p> <pre><code>>> m = magic(4); %# a 4-by-4 array >> id = logical([1 1 0 1 0]) id = 1 1 0 1 0 >> m(id,:) %# id has five elements, m only four rows ans = 16 2 3 13 5 11 10 8 4 14 15 1 %# this wouldn't work if the last element of id was 1, btw >> id = logical([1 1 0]) id = 1 1 0 >> m(id,:) %# id has three elements, m has four rows ans = 16 2 3 13 5 11 10 8 </code></pre>

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