Re: FYI: nth_element

[Shai Ayal]

On Thu, Oct 15, 2009 at 9:15 AM, Jaroslav Hajek <address@hidden> wrote:
> hi all,
>
> Octave has just got a new function: nth_element:
> http://hg.savannah.gnu.org/hgweb/octave/rev/aea3a3a950e1
>
> the docstring:
>
>  -- Built-in Function:  nth_element (X, N)
>  -- Built-in Function:  nth_element (X, N, DIM)
>     Select the n-th smallest element of a vector, using the ordering
>     defined by `sort'.  In other words, the result is equivalent to
>     `sort(X)(N)'.  N can also be a contiguous range, either ascending
>     `l:u' or descending `u:-1:l', in which case a range of elements is
>     returned.  If X is an array, `nth_element' operates along the
>     dimension defined by DIM, or the first non-singleton dimension if
>     DIM is not given.
>
>     nth_element encapsulates the C++ STL algorithms nth_element and
>     partial_sort.  On average, the complexity of the operation is
>     O(M*log(K)), where `M = size(X, DIM)' and `K = length (N)'.  This
>     function is intended for cases where the ratio K/M is small;
>     otherwise, it may be better to use `sort'.
>
>     See also: sort, min, max
>
> In short, it allows extracting a small portion of sort(X) without
> actually doing the full sort.
> This is sometimes useful for statistics when computing quantiles and
> the like... statisticians can surely tell better.
>
> Maybe it can be used to boost some existing functions. As an example,
> it can be readily used to speed up the "median" function:
> http://hg.savannah.gnu.org/hgweb/octave/rev/b7b89061bd0e
>
It can also help hist quite a lot.

Shai