Variable size changes on every iteration

Hi,

What is an efficient way of doing the following task.

departure_time(1) = 250;

file_arrival(1) = 0.0;

for i = 2:30

departure_time(i) = arrival_time(i)+transmission_time(i)+departure_time(i-1);

end

This code is working fine for me but the matlab script editor show the message that "the variable departure_time changes size on every loop iteration. I am sure that there will be another way of achieving the same task, right?

Thank you so much.

Answer 1

[José-Luis]

Pre-allocate. Before departure(1) = 250, place:

 departure_time = NaN .* ones(30,1);

[Aftab Ahmed Khan]

Hi, it works fine Thank you so much. Can you explain it to me that one line of yours? Plus i have few other lines as well with the same kind of situation, i think i can replace them as well, right ?

[José-Luis]

The thing with Matlab is if that you don't tell it what size a variable will be, it will increase its size as long as you keep appending stuff to it, as you do in that loop.

That is very inefficient computationally: since the elements of a matrix are placed contiguously in memory, said memory needs to be reallocated when you increase the size of your array.

A way to prevent that is to tell Matlab beforehand the size the final array will have: pre-allocation.

Please accept an answer if it helped you.

[Aftab Ahmed Khan]

Thanks for the explanation, Accepted.

Cheers.

[José-Luis]

My pleasure, thank you.