Following code was supposed to sort an ArrayList of strings while keeping
one "fixed" element at the last position.
However, the code hangs in .NET 1.1. I think that answering these questions
should be an interesting and instructive task for .NET learners:
1. why the code hangs in .NET 1.1?
2. why it does not hang in .NET 2.0?
3. why it does not hang in .NET 1.1 when you keep the "fixed" element at the
first (instead of last) position:
if ( ys == "fixed" )
return 1;
if ( xs == "fixed" )
return -1;
4. how to fix the code of the comparer so that sorting works in .NET 1.1?
Regards,
Wiktor Zychla
using System;
using System.Collecti ons;
class TestComparer : IComparer
{
public int Compare(object x, object y)
{
if (!( x is string )) return 0;
if (!( y is string )) return 0;
string xs = x as string;
string ys = y as string;
if ( ys == "fixed" )
return -1;
if ( xs == "fixed" )
return 1;
return xs.CompareTo( ys );
}
}
class Test
{
public static void Main()
{
ArrayList test = new ArrayList();
test.Add( "test2" );
test.Add( "test1" );
test.Add( "fixed" );
test.Add( "test5" );
test.Add( "test4" );
test.Add( "test3" );
foreach ( string s in test )
Console.WriteLi ne( s );
Console.WriteLi ne();
test.Sort( new TestComparer() );
foreach ( string s in test )
Console.WriteLi ne( s );
}
}
Jan 26 '06
18  1726 
my knowledge of quicksort and sorting algorithms isn't accomplished
enough to work it out. Please reveal all!
Chris S. wrote: my knowledge of quicksort and sorting algorithms isn't accomplished
enough to work it out. Please reveal all!
Okay, another hint: the problem is in the comparator, and the fact that
quicksort is being used isn't really relevant. Look at what the
contract for IComparer, and think about different possibilities for
what might be passed as the parameters...
(Bill: the algorithm has been tweaked. I don't believe it was tweaked
specifically to get around this flaw, but it's the kind of flaw which
is quite data-sensitive; depending on the data given, 2.0 either
notices the problem or happens to get the result right.)
Jon
> (Bill: the algorithm has been tweaked. I don't believe it was tweaked specifically to get around this flaw, but it's the kind of flaw which
I do not even believe that they were aware of that. Seems that the tweaked
version is by chance better in handling such inconsistent comparers but is a
little controversial for other reasons: http://tinyurl.com/7qab5 (I had a short discussion on that with Ryan
Byington from the dev team and I hope that we could expect it to be tuned
someday)
Wiktor Zychla
Jon Skeet [C# MVP] wrote: Chris S. wrote: my knowledge of quicksort and sorting algorithms isn't accomplished
enough to work it out. Please reveal all!
Okay, another hint: the problem is in the comparator, and the fact that
quicksort is being used isn't really relevant. Look at what the
contract for IComparer, and think about different possibilities for
what might be passed as the parameters...
Oh I know I know! I think...
rot13
Jung unccraf vs Pbzcner vf pnyyrq jvgu k naq l OBGU rdhny gb "svkrq"?
Gur erghea inyhr fubhyq or mreb va guvf pnfr ohg vg ergheaf -1, fb gur
fbeg vf crecrghnyyl gelvat gb fbeg svkrq gb gur yrsg bs vgfrys, be
fbzrguvat yvxr gung.
/rot13
--
Larry Lard
Replies to group please
> rot13
Jung unccraf vs Pbzcner vf pnyyrq jvgu k naq l OBGU rdhny gb "svkrq"?
Gur erghea inyhr fubhyq or mreb va guvf pnfr ohg vg ergheaf -1, fb gur
fbeg vf crecrghnyyl gelvat gb fbeg svkrq gb gur yrsg bs vgfrys, be
fbzrguvat yvxr gung.
/rot13
although you does not seem to be fluent in ancient gnomish yet ("fubhyq" is
an irregular verb and the "guvf" noun should not be used in such content),
your explanation is fine ;)
Wiktor Zychla
Larry Lard wrote: Oh I know I know! I think...
rot13
Jung unccraf vs Pbzcner vf pnyyrq jvgu k naq l OBGU rdhny gb "svkrq"?
Gur erghea inyhr fubhyq or mreb va guvf pnfr ohg vg ergheaf -1, fb gur
fbeg vf crecrghnyyl gelvat gb fbeg svkrq gb gur yrsg bs vgfrys, be
fbzrguvat yvxr gung.
/rot13
Spot on. Nice idea to rot13, too - it hadn't even occurred to me
(despite having seen it similarly used before). Doh!
Jon
Hi,
"Jon Skeet [C# MVP]" <sk***@pobox.co m> wrote in message
news:11******** **************@ g49g2000cwa.goo glegroups.com.. . Larry Lard wrote: Oh I know I know! I think...
rot13
Jung unccraf vs Pbzcner vf pnyyrq jvgu k naq l OBGU rdhny gb "svkrq"?
Gur erghea inyhr fubhyq or mreb va guvf pnfr ohg vg ergheaf -1, fb gur
fbeg vf crecrghnyyl gelvat gb fbeg svkrq gb gur yrsg bs vgfrys, be
fbzrguvat yvxr gung.
/rot13
Spot on. Nice idea to rot13, too - it hadn't even occurred to me
(despite having seen it similarly used before). Doh!
First time for me, luckily wikipedia clarified it: http://en.wikipedia.org/wiki/ROT13
I have one question though, IIRC the pivot should not be included in either
list when you divide your starting list. This is to avoid comparing a value
to itself (which is the cause of the problem in the particular example) ,
would it be that the quicksort argorithm has a bug after all?
Of course that even with a correct QS in place you may have errors, but only
when you have two "fixed" elements in the list, with one it should not be a
problem
Am I correct in the above?
It's early in the morning and I have had no coffe yet ;)
I may be missing something
--
Ignacio Machin,
ignacio.machin AT dot.state.fl.us
Florida Department Of Transportation
"Jon Skeet [C# MVP]" <sk***@pobox.co m> wrote in message
news:11******** **************@ f14g2000cwb.goo glegroups.com.. . (Bill: the algorithm has been tweaked. I don't believe it was tweaked
specifically to get around this flaw, but it's the kind of flaw which
is quite data-sensitive; depending on the data given, 2.0 either
notices the problem or happens to get the result right.)
I don't mean that the algorithm was tweaked to fix this problem, but rather a slight optimization
accidentally fixed a broken IComparer. Similar to the standard C++ idiom to prevent self assignment.
I can explain what I mean more fully, but I am still being semi-cryptic.
Bill This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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