Concatenation in Matlab (with or without copying elements)

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Mateusz
Mateusz 2012-1-14
Assume we have X = [A, B] where A and B are two vectors (or matrices). Is concatenation operation copies elements in A? If so is it possible to do concatenation without copying elements?
Possible answer to the second. What if I do the following in order to avoid copying elements: X(1:length(A)) = A; X(length(A)+1:length(A)+length(B)) = B; Does 'copy-on-write' work in this context?
Follow-up question: How can I benchmark these two codes with respect to memory usage? Any ideas?

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Jan
Jan 2012-1-14
If you concatenate two arrays, the values of both must be copied. The memory used to store the values of an array need to be stored in a continuos block. Therefore even A = [A, B] has to copy all elements of A and B.
The copy-on-write strategy does not matter in this case.
If you have to store large arrays and really need to avoid a copy, use a CELL as a container of the subarrays. Of course any operations using this new object must be re-programmed and will be complicated (slow!), e.g. a matrix-multiplication.
This is even worse:
X(1:length(A)) = A;
X(length(A)+1:length(A)+length(B)) = B;
The indexed copy duplicates A at first. But in the 2nd line, a new larger array is allocated for X and the values of A are copied a second time before B is appended!
It is very hard to define a "memory footprint" exactly, and in consequence the measurement is not trivial also. The JIT might free the memory used by A if it is not used anymore in a function. But if this freed memory is reused to create X depends on the interaction with the operating system. E.g. the current processor load can influence if the memory is cleared and re-used, or if a new block is allocated and the garbage collection happens later.

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the cyclist
the cyclist 2012-1-14
This page will provide you with a lot of information about memory management in MATLAB: http://www.mathworks.com/support/tech-notes/1100/1106.html
When you do ...
>> X = A;
MATLAB doesn't make a copy of A, until you change an element of X. (Then it copies the entire array.) However, I am pretty sure that that does not apply to concatenation. I don't think there is a way to carry out
>> X = [A,B];
without actually creating the new array in memory.

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