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POHow to avoid allocating memory for the returned value each time a function is called
 

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  1. POHow to avoid allocating memory for the returned value each time a function is called
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    <p>I have a function that returns a large vector and is called multiple times, with some logic going on between calls that makes vectorization not an option. </p> <p>An example of the function is</p> <pre><code>function a=f(X,i) a=zeros(size(X,1),1); a(:)=X(:,i); end </code></pre> <p>and I am doing</p> <pre><code>for i=1:n a=f(X,i); end </code></pre> <p>When profiling this (size(X,1)=5.10^5, n=100 ) times are 0.12s for the zeros line and 0.22s for a(:)=X(:,i) the second line. As expected memory is allocated at each call of f in the 'zeros' line.</p> <p>To get rid of that line and its 0.12s, I thought of allocating the returned value just once, and passing it in as return space each time to an appropriate function g like so:</p> <pre><code>function a=g(X,i,a) a(:)=X(:,i); end </code></pre> <p>and doing</p> <pre><code> a=zeros(m,1); for i=1:n a=g(X,i,a); end </code></pre> <p><strong>What is surprising to me is that profiling inside g still shows memory being allocated in the same amounts at the a(:)=X(:,i); line</strong>, and the time taken is very much like 0.12+0.22s.. </p> <p>1)Is this just <a href="http://blogs.mathworks.com/loren/2006/05/10/memory-management-for-functions-and-variables/" rel="nofollow noreferrer">"lazy copy on write"</a> because I am writing into a? 2)Going forward, what are the options? -a global variable for a (messy..)? -writing a matrix handle class (must I really?) (The <a href="https://stackoverflow.com/questions/1258761/do-i-conserve-memory-in-matlab-by-declaring-variables-global-instead-of-passing-t/1261429#1261429">nested function way</a> means some heavy redesigning to make a nesting function to which X is known (the matrix A with notations from that answer)..)</p>
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    1. COEventually I'll have to rewrite everything in C++, wrapping the n loop in a MEX file seems a lot of work atm, because the actual situation is like this: function fhandle=get_functor(X) fhandle=@f1; %X is actually a parameter in f1 function s=f1(i) s=sin(X./i); end end %begin script %X is defined in a variety of different ways ("code *factorization*") f=get_functor(X); So I imagine using a MEX file for the n-loop I will need to make X known to that C function.The actual situation being that I do this for a few 10's of different X's and like to add more/take some out just to test things..
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