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    <p>I (think) I see what you are trying to achieve. You are writing your syntax like a mathematical function definition. Matlab is interpreting <code>f</code> as a 2-dimensional data type and trying to assign the value of the expression to data indexed at <code>x,y</code>. The values of <code>x</code> and <code>y</code> are not integers, so Matlab complains.</p> <p>If you want to plot the output of the function (we'll call it <code>z</code>) as a function of <code>x</code> and <code>y</code>, you need to define the function quite differently . . . </p> <pre><code>f = @(x,y)(x-3).^2 - (y-2).^2; x=2:.2:4; y=1:.2:3; z = f( repmat(x(:)',numel(y),1) , repmat(y(:),1,numel(x) ) ); surf(x,y,z); xlabel('X'); ylabel('Y'); zlabel('Z'); </code></pre> <p>This will give you an output like this . . . <img src="https://i.stack.imgur.com/sfMjY.png" alt="enter image description here"></p> <p>The <code>f = @(x,y)</code> part of the first line states you want to define a function called <code>f</code> taking variables <code>x</code> and <code>y</code>. The rest of the line is the definition of that function.</p> <p>If you want to plot <code>z</code> as a function of both <code>x</code> and <code>y</code>, then you need to supply all possible combinations in your range. This is what the line containing the <code>repmat</code> commands is for.</p> <p><strong>EDIT</strong></p> <p>There is a neat Matlab function <code>meshgrid</code> that can replace the repmat version of the script as suggested by @bas (welcome bas, please scroll to bas' answer and +1 it!) ...</p> <pre><code>f = @(x,y)(x-3).^2 - (y-2).^2; x=2:.2:4; y=1:.2:3; [X,Y] = meshgrid(x,y); surf(x,y,f(X,Y)); xlabel('x'); ylabel('y'); zlabel('z'); </code></pre>
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