How to give index to each replicated number?

For example, when I have a=[1,1,1,1,3,3,4,4,4], I would like the output to be b=[1,2,3,4,1,2,1,2,3].

[Matt J]

What if a number's repetitions are interrupted like in the following,

a=[1,1,1,1,  3,3,  1,1]

Should the result be

b = [1,2,3,4,  1,2,  5,6]

[Qingfang Liu]

Yes correct!

Answer 1

[Matt J]

  G=cumsum(diff([0,a])~=0);
  
  b=cell2mat(splitapply(@(g){1:numel(g)}, G,G));

Answer 2

[Matt J]

Yes correct!

In that case,

S=sparse(  1:numel(a),   findgroups(a),   1);

S(S>0)=nonzeros(   cumsum(sort(S))  );

b=full(sum(S,2)).';