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PORetrieving element index in spfun, cellfun, arrayfun, etc. in MATLAB
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copied!<p>Is there any way to retrieve the <strong>index</strong> of the element on which a function called by <code>cellfun</code>, <code>arrayfun</code> or <code>spfun</code> acts? (i.e. retrieve the index of the element within the scope of the function). </p> <p>For the sake of simplicity, imagine I have the following toy example:</p> <pre><code>S = spdiags([1:4]',0,4,4) f = spfun(@(x) 2*x,S) </code></pre> <p>which builds a 4x4 sparse diagonal matrix and then multiplies each element by <code>2</code>.</p> <p>And say that now, instead of multiplying each element by the constant number <code>2</code>, I would like to multiply it by <strong>the index the element has in the original matrix</strong>, i.e. assuming that <code>linear_index</code> holds the index for each element, it would be something like this: </p> <pre><code>S = spdiags([1:4]',0,4,4) f = spfun(@(x) linear_index*x,S) </code></pre> <p>However, note the code above does not work (<code>linear_index</code> is undeclared).</p> <p>This question is partially motivated by the fact that <code>blocproc</code> gives you access to <code>block_struct.location</code>, which one could argue references the location (~index) of <em>the current element</em> within the <em>full object</em> (an image in this case):</p> <blockquote> <p><code>block_struct.location:</code> A two-element vector, [row col], that specifies the position of the first pixel (minimum-row, minimum-column) of the block data in the input image.</p> </blockquote>

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