Re: libstdc++/8195: n-th algorithm (STL) doesn`t work properly

[Wolfgang Bangerth <bangerth@ticam.utexas.edu>]

The following reply was made to PR libstdc++/8195; it has been noted by GNATS.

From: Wolfgang Bangerth <bangerth@ticam.utexas.edu>
To: "Belov, Eugeny" <Eugeny_Belov@stl.sarov.ru>
Cc: gcc-bugs@gcc.gnu.org, <gcc-gnats@gcc.gnu.org>
Subject: Re: libstdc++/8195: n-th algorithm (STL) doesn`t work properly
Date: Fri, 11 Oct 2002 10:50:51 -0500 (CDT)

 >   I am understanding that the principle of this algorithm (in simple 
 > terms) is to divide the sequence into 2 parts like this:
 > smaller elements , n-th element, bigger elements,  and in the general 
 > case both parts can be unsorted.
 
 Correct.
 
 >   But the real problem in this case is that smaller elements comes after 
 > n-th ("n-th" is the integer with value 12 - the biggiest of all other 
 > elements), i.e. the sequence after using the nth_element() is "4 4 5 6 6 
 > 6 9 _12_ 8 10" and in this case I think the n-th algorithm 
 > implementation make a mistake.
 
 Why? The signature of the function is
   void nth_element(RandomAccessIterator first, 
                    RandomAccessIterator nth,
                    RandomAccessIterator last);
 and you call it like
   nth_element(a, a+2, a+10);
 so the result is that elements 0 and 1 will be smaller than the one at 
 position 2, and that elements 3 through 9 will be larger. This is what 
 happens in your output.
 
 Regards
   Wolfgang
 
 PS-1: Note that a+2 points to "9" in your example, not "12" as you claim, 
 since indices are zero-based.
 
 PS-2: The algorithm sorts such that the values before and after the 
 _pointer_ are smaller and larger, respectively, not those elements that 
 are smaller and larger than the _element pointed to_! I think this is the 
 basic misunderstanding on your behalf. The standard on this reads
 
   After nth_element the element in the position pointed to by nth is the
   element that would be in that position if the whole range were sorted.
   Also  for  any iterator i in the range [first, nth) and any iterator j
   in the range [nth, last) it holds that: !(*i > *j) or comp(*j, *i)  ==
   false.
 
 -------------------------------------------------------------------------
 Wolfgang Bangerth              email:           bangerth@ticam.utexas.edu
                                www: http://www.ticam.utexas.edu/~bangerth