memory management - MATLAB : Appending to pre-allocated matrix

Question

I have some MATLAB code with mxn matrix.
Initially, I put first row in it and then the code runs through a for loop which appends remaining m-1 rows one by one; one for each iteration of the loop.
As expected, MATLAB recommends me to pre-allocate the matrix because it is expanding with every iteration of loop. So, if I pre-allocate zeros in all m rows, MATLAB most probably will append rows after the m rows(starting from m+1 for 1st appended row) because m rows are already filled(even though with zeros!)
Is there any way of pre-allocating matrix in this scenario for improving speed?

Answer 1

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 zeros and index the rows with A(i,:) = rowVec; instead of appending a row (A = [A; rowVec];).

Pre-allocation

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:

• Preallocation performance
• Preallocation performance and multithreading

Automatic Array Growth Optimization

If you really want to use dynamic array resizing by growing along a dimension, there are ways to do it right. See this this MathWorks blog post by Steve Eddins. The most important thing to note is that you should grow along the last dimension for best performance. (i.e. add columns in your case). Yair also discusses dynamic array resizing in another post on his blog.

Also, there are ways of allocating an array without initializing using some hairy MEX API acrobatics, but that's it.