MATLAB Programming Fundamentals - MathWorks

Preallocation

for and while loops that incrementally increase the size of a data structure each time

through the loop can adversely affect performance and memory use. Repeatedly resizing

arrays often requires MATLAB to spend extra time looking for larger contiguous blocks of

memory, and then moving the array into those blocks. Often, you can improve code

execution time by preallocating the maximum amount of space required for the array.

The following code displays the amount of time needed to create a scalar variable, x, and

then to gradually increase the size of x in a for loop.

tic

x = 0;

for k = 2:1000000

x(k) = x(k-1) + 5;

end

toc

Elapsed time is 0.301528 seconds.

If you preallocate a 1-by-1,000,000 block of memory for x and initialize it to zero, then the

code runs much faster because there is no need to repeatedly reallocate memory for the

growing data structure.

tic

x = zeros(1, 1000000);

for k = 2:1000000

x(k) = x(k-1) + 5;

end

toc

Elapsed time is 0.011938 seconds.

Use the appropriate preallocation function for the kind of array you want to initialize:

• zeros for numeric arrays


• cell for character arrays

Preallocating a Nondouble Matrix

When you preallocate a block of memory to hold a matrix of some type other than

double, avoid using the method

Preallocation