Re: hashtable or map? (map inserts not behaving as I expect - and I cant find a decent simple example for hashtable)

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 21 Dec 2008 22:25:57 +0100

Message-ID:

<494eb465$0$17067$6e1ede2f@read.cnntp.org>

(2b|!2b)==? wrote:

Kai-Uwe Bux wrote:

Obnoxious User wrote:

On Sun, 21 Dec 2008 16:44:33 +0000, (2b|!2b)==? wrote:

I have a list of items that I want to ignore during processing. I read
a list of items from file and populate a map variable. However, only a
subset of the items are being inserted in the map.

The struct for IgnoreItem looks like this:

struct IgnoreItem
{
//ctors and op== omitted for the sake of brevity

bool operator<(const IgnoreItem& key) const {
if ( _stricmp(syb.c_str(), key.syb.c_str()) < 0)
return true;
else if (_stricmp(key.syb.c_str(), syb.c_str()) < 0)
return false;
else if ( xid < key.xid )
return true ;
else if ( key.xid < xid )
return false;
else if ( icid < key.icid)
return true ;
else
return key.icid < icid ;
}

I think, you messed up on the third criterion: if the syb and xid compare
equal, you return true whenever icid of *this and key differ.

std::string syb;
unsigned char xid ;
long icid ;
};

That's just messed up.

Pseudo code:

if(syb < key.syb) return true;
if(syb == key.syb) {
if(xid < key.xid) return true;
if(xid == key.xid) {
if(icid < key.icid) return true;
}
}
return false;

Do you see the pattern?

Well, there is another pattern, which is closer to the code posted:

  if ( lhs.syb < rhs.syb ) return true;
  if ( rhs.syb < lhs.syb ) return false;
  if ( lhs.xid < rhs.xid ) return true;
  if ( rhs.xid < lhs.xid ) return false;
  if ( lhs.icid < rhs.icid ) return true;
  if ( rhs.icid < lhs.icid ) return false;
  return false;

Best

Kai-Uwe Bux

Hi Kai,

It's Kai-Uwe. I do not have a middle name.

thanks for pointing out my pretzel logic. I updated my code to
reflect the changes you mentioned - however the effect is still the same
- I only have 21 items in the map instead of the original list of 84,
and the syb fields of the keys are unique.

Then the bug might not be hinding in the comparison function. Please post
the smallest complete program that exhibits the problem.

This is a problem because the
rows (i.e. composite fields) in my list that are unique ... I am tempted
to write a hashing dunction that takes a string, char, long and returns
a long, so that I use that as the key instead ... but that is simply
avoiding the issue of storing my IgnoreItem keys which are unique, in a
std::map ...

True.

Best

Kai-Uwe Bux

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