create a specific matrix data ?

Question

MUKESH KUMAR 2018-1-3

编辑： Guillaume 2018-1-3

I have a matrix N=[2;3;4;2;4;1;4;3;2;1] (10*1 size) then from this i want to create a matrix like this that

for i=1:10
M(i)= i*ones(N(i),1)
end

I want this matrix M=[1;1;2;2;2;3;3;3;3;4;4;5;5;5;5;6;7;7;7;7;8;8;8;9;9;10]; but I did not get this???

Answer 1

Guillaume 2018-1-3

编辑：Guillaume 2018-1-3

The desired result is trivially obtained with repelem:

N=[2;3;4;2;4;1;4;3;2;1];
M = repelem((1:numel(N))', N)

If you were to use a loop to generate M, the most efficient way would probably be:

 startidx = cumsum([1; N]);
 M = zeros(sum(N), 1);
 for idx = 1:numel(N)
   M(startidx(idx) : startidx(idx)+N(idx)-1) = idx;
 end