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Re: FYI: nth_element
From: |
Shai Ayal |
Subject: |
Re: FYI: nth_element |
Date: |
Thu, 15 Oct 2009 14:22:59 +0200 |
On Thu, Oct 15, 2009 at 12:36 PM, Jaroslav Hajek <address@hidden> wrote:
> On Thu, Oct 15, 2009 at 9:56 AM, Shai Ayal <address@hidden> wrote:
>> 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.
>>
>
> I don't see how. Can you clarify?
You are right -- I was confused. hist needs the number of elements
smaller than a given value (and uses sort to find out), while
nth_element gives the value of the nth element.
Writing hist in c++ is not hard and would really help since it uses
(M+K)log(M+K) sort in order to avoid a M*K loop just because looping
is slow in the scripting language. One of these days I'll get down to
doing that ...
Shai
Re: FYI: nth_element, Jordi Gutiérrez Hermoso, 2009/10/17