Assign rank to numbers in a vector

Instead of sorting, I suggest you use unique:

[~, ~, ranking] = unique(x);

It also sorts the vector but maps identical values to the same index. This way identical elements in the original vector get the same rank. For instance, if x = [5 2 3 1 3], we get:

ranking =
   4   2   3   1   3

If you want an "average" rank, you could use accumarray in combination with the information obtained both from unique and from sort, so do the following instead:

[~, ~, idx_u] = unique(x);
[~, idx_s] = sort(x);
mean_ranks = accumarray(idx_u(:), idx_s(idx_s), [], @mean);
ranking = mean_ranks(idx_u);

In our example we would get:

ranking =
   1.0000
   2.0000
   3.5000
   5.0000
   3.5000

Note that both values 3 got the average rank of 3.5, as they shared ranks 3 and 4.