s = string({'A';'A';'B';'A';'B';'B';'C';'A';'C'});
out = zeros(numel(s),1);
[~,~,c] = unique(s);
for i = 1:max(c)
lo = c == i;
out(lo) = 1:sum(lo);
end
or
[~,~,c] = unique(s);
z = c == (1:max(c)); % or use here: z = bsxfun(@eq,c,1:max(c));
out = sum(cumsum(z).*z,2);
I'm use R2016b.
