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 21:00:28 +0100

Message-ID:

<494ea05c$0$17069$6e1ede2f@read.cnntp.org>

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