Sorting duplicate incremental array elements

I was curious if there is a more efficient method for performing the following result:

>> x = 1:4;
>> xx = sort(x([1:end,1:end]))

       1     1     2     2     3     3     4     4

If an alternative method is applicable to the previous example, consider also the more general case:

>> x = 1:4;
>> n = 2;
>> xn = sort(x(repmat([1:end],[1,n])))

To me it seems like the use of the sort command is inefficient and could cause notable performance reductions when working with larger incremental lists.

I would appreciate any suggestions.

Answer 1

[José-Luis]

x = 1:4; %sort(x); %if needed
xx = [x;x];
xx = xx(:);

[David]

That is definitely more efficient.

It would be a little more tricky to achieve for an input, x, with an arbitrary shape (a column vector would be one such example). It seems that no matter how you approach such a problem, you will need some preliminary code to handle the dimensionality of x, such that its first dimension is of singleton length so that the duplicates can be concatenated along this dimension.

Cheers