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

User · Answer

As mentioned before  the use of x end 1    newElem has the advantage that it allows you to concatenate your vector with a scalar  regardless of whether your vector is transposed or not  Therefore it is more robust for adding scalars   However  what should not be forgotten is that x    x newElem  will also work when you try to add multiple elements at once  Furthermore  this generalizes a bit more naturally to the case where you want to concatenate matrices  M    M M1 M2 M3     All in all  if you want a solution that allows you to concatenate your existing vector x with newElem that may or may not be a scalar  this should do the trick    x end  1 numel newElem    newElem

User · Answer

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

User · Answer

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

[matlab] Octave/Matlab: Adding new elements to a vector

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