MATLAB Programming

Using Memory Efficiently
and stores the array data in that block. It also stores information about the
array data, such as its data type and dimensions, in a separate, small block of
memory called a header. The variable that you assign this data to is actually
a pointer to the data; it does not contain the data.
If you add new elements to an existing array, MATLAB expands the existing
array in memory in a way that keeps its storage contiguous. This might
require finding a new block of memory large enough to hold the expanded
array, and then copying the contents of the array from its original location to
the new block in memory, adding the new elements to the array in this block,
and freeing up the original array location in memory.
If you remove elements from an existing array, MATLAB keeps the memory
storage contiguous by removing the deleted elements, and then compacting its
storage in the original memory location.
Working with Large Data Sets. If you are working with large data sets,
you need to be careful when increasing the size of an array to avoid getting
errors caused by insufficient memory. If you expand the array beyond the
available contiguous memory of its original location, MATLAB has to make
a copy of the array in a new location in memory, as explained above, and
then set this array to its new value. During this operation, there are two
copies of the original array in memory, thus temporarily doubling the amount
of memory required for the array and increasing the risk of your program
running out of memory during execution. It is better to preallocate sufficient
memory for the array at the start. See “Preallocating Arrays” on page 11-7.
Copying Arrays
Internally, multiple variables can point to the same block of data, thus
sharing that array’s value. When you copy a variable to another variable (e.g.,
B = A), MATLAB makes a copy of the pointer, not the array. For example,
thefollowingcodecreatesasingle500-by-500matrixandtwopointerstoit,
A and B:
A = magic(500);
B = A;
11-13