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copied!<p>You cannot pre-allocate a MATLAB array without also changing it's size, at least not manually. However, MATLAB has improved automatic array growth performance a lot in recent versions, so you might not see a huge performance hit. Still, best practice would be to pre-allocate your array with <code>zeros</code> and index the rows with <code>A(i,:) = rowVec;</code> instead of appending a row (<code>A = [A; rowVec];</code>).</p> <h2>Pre-allocation</h2> <p>If you are determined to squeeze every bit of performance out of MATLAB, Yair Altman has a couple of excellent articles on the topic of memory pre-allocation:</p> <ul> <li><a href="http://undocumentedmatlab.com/blog/preallocation-performance/" rel="nofollow">Preallocation performance</a></li> <li><a href="http://undocumentedmatlab.com/blog/allocation-performance-take-2/" rel="nofollow">Preallocation performance and multithreading</a></li> </ul> <h2>Automatic Array Growth Optimization</h2> <p>If you really want to use dynamic array resizing by growing along a dimension, there are ways to do it right. See this this <a href="http://blogs.mathworks.com/steve/2011/05/20/more-about-automatic-array-growth-improvements-in-matlab-r2011a/" rel="nofollow">MathWorks blog post by Steve Eddins</a>. The most important thing to note is that you should <strong>grow along the last dimension for best performance</strong>. (i.e. add columns in your case). Yair also discusses dynamic array resizing in <a href="http://undocumentedmatlab.com/blog/array-resizing-performance/" rel="nofollow">another post on his blog</a>.</p> <p>Also, there are ways of allocating an array without initializing using some hairy MEX API acrobatics, but that's it.</p>

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