Re: hash_map

"Maxim Yegorushkin" <>
29 Sep 2006 08:37:07 -0400
tat wrote:


My question is kind of off-topic. I am trying to learn hash_map. I have
the below. I expect that I get the following output
The key is in there
362:369 10.329

I instead got
362:369 0

Can someone explain me why? What's wrong with my code? I compiled and
ran this code on Debian GNU/Etch with g++ version 4.0.4.


#include <cstdlib>
#include <iostream>
#include <string>
#include <ext/hash_map>

int main(int argc, char* argv[])
   __gnu_cxx::hash_map<const char*, float,
   __gnu_cxx::hash<const char*> > sourceMatrix;

   const std::string& p1 = "362:369";
   const std::string& p2 = "369:362";
   const std::string& p3 = "335:376";
   const std::string& p4 = "376:335";
   const std::string& p5 = "427:450";
   const std::string& p6 = "450:427";

   sourceMatrix[p1.c_str()] = sourceMatrix[p2.c_str()] = 10.329;
   sourceMatrix[p3.c_str()] = sourceMatrix[p4.c_str()] = 19.142;
   sourceMatrix[p5.c_str()] = sourceMatrix[p6.c_str()] = 20.383;

   const std::string& key = "362:369";

   if (sourceMatrix.find(key.c_str()) != sourceMatrix.end()) {
     std::cout << " The key is in there\n";

   std::cout << key << "\t" << sourceMatrix["362:369"]<< "\n";
   return EXIT_SUCCESS;

The hash functor you are using is fine, it does the right thing for
c-strings. You only need to use a proper compare functor:

struct equal_string
    bool operator()(char const* a, char const* b) { return 0 ==
strcmp(a, b); }

__gnu_cxx::hash_map<const char*, float, __gnu_cxx::hash<const char*>,
equal_string> sourceMatrix;

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