MATLAB: Concatenation in Matlab (with or without copying elements)

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?

Best Answer

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.

Related Solutions

MATLAB: Out of memory when assigning values to existing arrays

This is the expected behavior. Matlab uses a copy-on-write strategy:

a = rand(1e9, 1);
b = a;     % Shared data

Now the data pointer of b points to the same memory as a. But when any element of b is modified, the data are duplicated:

b(1) = 1;  % Data duplication

This strategy is useful when data are provided to functions. Then a shared data copy saves time and memory as long as no values are modified.

MATLAB: Write a function called “makemat” that will receive two row vectors as input arguments, and from them create and return a matrix with two rows. You may not assume that the length of the vectors is known. Also, the vectors may be of different length

function [C] = ArmStrong(A, B)
if length(A)==length(B)
    X = A ;
    Y = B ;
elseif length(B)>length(A)
  X = zeros(size(B)) ;
  X(1:length(A)) = A ;
  Y = B ;
elseif length(A)>length(B)
    Y = zeros(size(A)) ;
    Y(1:length(B)) = B ;
    X = A ;
end
C(1,:)=X ;
C(2,:)=Y ;
end

There are multiple ways to achieve the above. In simple:

L = [length(A), length(B)] ;
C = zeros(2,max(L)) ;
C(1,1:L(1)) = A ;
C(2,1:L(2)) = B ;

Best Answer

Related Solutions

MATLAB: Out of memory when assigning values to existing arrays

MATLAB: Write a function called “makemat” that will receive two row vectors as input arguments, and from them create and return a matrix with two rows. You may not assume that the length of the vectors is known. Also, the vectors may be of different length

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