hash_map problems...

[juanpast]

Hi Gurus!!


Thanks in advance.


I need to construct a hash table, I use STL hash_map template but I have some errors. I am using Visual Studio 2003 and it is my code:

#include <hash_map>
#include <vector> 	

struct eqstr {
	bool operator()(char* s1, char* s2) {
		return strcmp(s1, s2) == 0;
	}
};

class CSimplificada  {
public:
	BOOL nTipo; // 0 S1 1 S2
	char sKey[256];
	vector <int> vnPosInFile;

	CSimplificada() {
		sKey[0] = '\0';
		nTipo = -1;
		vnPosInFile.clear();
	}
	const CSimplificada & operator = (const CSimplificada &s) {
		strcpy(sKey, s.sKey);
		nTipo = s.nTipo;
		vnPosInFile = s.vnPosInFile;
		return *this;
	}
	bool operator < (const CSimplificada &s) {
		if(strcmp(sKey, s.sKey) < 0) return true;
		return false;
	}
	bool operator == (const CSimplificada &s) {
		if(strcmp(sKey, s.sKey) == 0 && nTipo == s.nTipo) return true;
		return false;
	}
};

When I try to declare hash_map, for example:

hash_map <char *,CSimplificada,hash_compare <char*>, eqstr> hashMapS1;

I get compiler errors. Is right the code? what is happening?


Very thanks!!!

[juanpast]

I change the code and now compile:

struct eqstr {
	bool operator()(const char* s1, const char* s2) const {
		return strcmp(s1, s2) == 0;
	}
};

and

hash_map <char *,CSimplificada,hash_compare <char*, eqstr> > hashMapS1;

But when I search for one key not found it:

hash_map <char *,CSimplificada,hash_compare <char*, eqstr> >::iterator itBusca;
CSimplificada sS1;
for(int i=0;i<nRecords;i++) { // nRecords is the number of records where a have the data
	itBusca = hashMapS1.find(sKey); // sKey is declared as: char sKey[256]; and fill which data of the i-record
	if(itBusca == itS1End) { // not found, add the new key and data
		sS1.nTipo = S1; // S1 is: #define S1 0
		sS1.vnPosInFile.push_back(i);
		hashMapS1[sKey] = sS1;
	} else { // encontrado 
		itBusca->second.vnPosInFile.push_back(i);
	}
}

the term: if(itBusca == itS1End) allways evaluated as true but I can find records with the same key.

What is happening now?

[GNiewerth]

Is there a specific reason why you use char[256] instead of string?
Did you make sure the char* used as hash_map key never goes out of scope and remains constant?
Each time you change the sKey string to something different it also affects the elemt already stored in your hash_map, because its key points to sKey. You always refer to the same memory address, that´s why find always returns a valid iterator.

[a1ex07]

the term: if(itBusca == itS1End) allways evaluated as true but I can find records with the same key.
What is happening now

I haven't worked with Studio 2003 for a long time, but I hope that hash_map implementation in Studio 2005 is the same. The problem is that Microsoft hash_map requires keys to be less than comparable, so instead of eqstr you need less_str (or greater_str, whatever you like). For example

struct less_str {
   bool operator()(const char* x, const char* y) const {
      if ( strcmp(x, y) > 0)
         return true;

      return false;
   }
};

Now your hash_map looks like

typedef stdext::hash_map <char *,CSimplificada, stdext::hash_compare <char*, less_str> > HM;

and HM::find(key) will return expected results

[juanpast]

thanks!!!


I create a simple project to check this tag. It is a V2003 project:


By carefull with the file path....

"D:\\tono\\Fuentes\\Hash_mapExample\\Hash_mapExample\\List.txt"


And


"D:\\tono\\Fuentes\\Hash_mapExample\\Hash_mapExample\\Output.txt"



Thanks a lot

[juanpast]

I obtain a good results!!!!


Here you are the solution. I need to change CString by string because CString not have size_t operator.


Thanks in any case.


Regards