How do I generate a vector of ordered integer without a loop or in faster way

I have to generate a vector 0123123451234567123456789 and the quickest idea was given by cyclist

       M=[0];
        for q=3:2:9

            M=[M,1:q];

        end

which is very slow. Does anyone have idea to make it faster? Thanks alot

Answer 1

[dpb]

Prealllocate and populate instead of concatenating for starters...

M=zeros(1,1+sum(q));
i1=2;
for q=3:2:9
  i2=i1-1+q;
  M(i1:i2)=[1:q];
  i1=i2+1;
end

I gotta' run at the moment but a kreative use of

doc kron  % _might_ bring nirvana, not sure otomh; haven't had time to think it thru...

[Oluwaseun Lijoka]

Thanks alot but is it possible to do away with for loop in this case or it is never avoidable?

[Oluwaseun Lijoka]

I also think matlab will complain about the first line M=zeros(1,1+sum(q)); where q is not defined.

[dpb]

Well, you have to define the range, yes...

Answer 2

[Stephen23]

You can adapt my answer to your other question:

X = floor(sqrt(0:(p+1)^2-1));
Y = diff(X);
Y(logical(Y)) = 0:-2:-2*nnz(Y)+1;
Y(Y==0) = 1;
Z = [0,cumsum(Y)];

This is hundreds of times faster than your original code (p=10000):

Elapsed time is 3.095720 seconds.
Elapsed time is 0.000041 seconds.

Note that 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.