How do I create a faster ordered vector without for loop

I discover that this pseudo code within my code makes my program slow probably because of the for loop and the fact that JJ is not pre-allocated. I will appreciate a faster idea.

JJ=[];
for j=0:p
  JJ=[JJ; repmat(j,[1+((j+1)-1)*2,1])];
end

Answer 1

[Stephen23]

X = floor(sqrt(0:(p+1)^2-1))';

This is nearly fifty times faster (p=1000):

Elapsed time is 5.2233 seconds.
Elapsed time is 0.111006 seconds.

As p gets larger it will reach your memory limit very quickly: each magnitude increase in p causes a two-magnitude increase in the number of elements of the output vector.