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

"(2b|!2b)==?" <>
Sun, 21 Dec 2008 20:51:08 +0000
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 ;
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;


Kai-Uwe Bux

Hi Kai, 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. 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 ...

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