Preallocating a vector, what is the correct way?

Hi

Trying to learn how to do preallocation to save on memory, I copy pasted the example code from the help page into the editor.

x = zeros(1, 1000000);
for k = 2:1000000
   x(k) = x(k-1) + 5
end

This is advertised as taking 0.011938 seconds or some such.

Upon running this code, the output is a massive quantity of zeros, that as of ten minutes afterwards, is still being produced. (it looks to me as if it's writing out a million zeros a million times) What is wrong with this code and how do I correct it?

Also, how do I terminate a process within matlab without shutting down the entire thing? It just keeps on going!

Answer 1

[Guillaume]

1. Preallocation does not save any memory. It will save time

2. Put a semicolon at the end of your x(k) = x(k-1)+5 line and it'll process it near instantly:

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

The reason it's so slow is because of the lack a semicolon, matlab prints the whole matrix (1 million numbers) at each iteration of the loop.

3. To terminate a running calculation or script, press CTRL + C