concatenating vectors generated with loop

I would like to run a for loop like

   for i=1:N
          output=[1 i]
          output2 = [output2 output]
    end

But in this example it seems that I need to define output2 at the beginning. Is there any compact way to write this operation?

[Stephen23]

Expanding the array on each iteration will (in general) slow down your code. As David Goodmanson wrote, it is good practice preallocate the output array before the loop. Read this to know more:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[David Goodmanson]

Hi alpedhuez, define output2 by putting

output2 = [];

just before the do loop.

Unless you are using the intermediate results somehow, concatenation in this manner is not recommended. For N = 10000 or less it's all right in terms of execution time. But with execution time growing rapidly, eventually it makes more sense (and is better practice anyway) to define an array by output2 = ones(1,2*N) and then fill all the even entries with output2(2:2:2*N) = 1:N

Answer 2

[David Fletcher]

Not really more compact, but it does away with the loop

   N=10
   output=reshape([ones(1,N); 1:N],1,2*N)