Octave/Matlab: Adding new elements to a vector

Question

Having a vector x and I have to add an element (newElem) .

Is there any difference between -

x(end+1) = newElem;  

and

x = [x newElem]; 

?

Answer 1

x(end+1) = newElem is a bit more robust.

x = [x newElem] will only work if x is a row-vector, if it is a column vector x = [x; newElem] should be used. x(end+1) = newElem, however, works for both row- and column-vectors.

In general though, growing vectors should be avoided. If you do this a lot, it might bring your code down to a crawl. Think about it: growing an array involves allocating new space, copying everything over, adding the new element, and cleaning up the old mess...Quite a waste of time if you knew the correct size beforehand :)

Answer 2

Just to add to @ThijsW's answer, there is a significant speed advantage to the first method over the concatenation method:

big = 1e5; tic; x = rand(big,1); toc  x = zeros(big,1); tic; for ii = 1:big     x(ii) = rand; end toc  x = [];  tic;  for ii = 1:big     x(end+1) = rand;  end;  toc   x = [];  tic;  for ii = 1:big     x = [x rand];  end;  toc     Elapsed time is 0.004611 seconds.    Elapsed time is 0.016448 seconds.    Elapsed time is 0.034107 seconds.    Elapsed time is 12.341434 seconds. 

I got these times running in 2012b however when I ran the same code on the same computer in matlab 2010a I get

Elapsed time is 0.003044 seconds. Elapsed time is 0.009947 seconds. Elapsed time is 12.013875 seconds. Elapsed time is 12.165593 seconds. 

So I guess the speed advantage only applies to more recent versions of Matlab