Good ole gnu::hash_map, I'm impressed

Mirco Wahab <>
Wed, 16 Jul 2008 22:53:52 +0200

recently I tried to mimic a simple word frequency
counter in C++ that uses hashing.

Assumed we have a somehow big text file (14 MB) that
contains >93,000 *different* words. To get word
count and frequencies, one would use:

[Perl, don't boggle]

   my $fn = 'fulltext.txt';

   print "start slurping\n";
   open my $fh, '<', $fn or die "$fn - $!";
   my $data; { local $/; $data = <$fh> }

   print "start hashing\n";
   my %hash;
   ++$hash{$1} while $data=~/(\w\w*)/g;

   print "done, $fn (" . int(length($data)/1024)
        . " KB) has " . (scalar keys %hash) . " different words\n"

This is just the "first part". Build a hash with
all different words (simplified) and count their
frequencies at first.

This runs through on a test machine (P4/2666MHz, Linux, Perl 5.8.8)
in ~4s (real). I'd never expect a "complicated looking" C++
solution to beat that one.

After some fiddling with the gnu::hash_map (which works
also as a drop-in replacement for std::map), I'm stunned:

[File size: 14MB; total words: 2,335,569; different words: 93,405]
    std::map C++ implementation - 0m5.739s real
    perl %hash implementation - 0m3.981s real
    gnu::hash_map C++ implementation - 0m3.597s real ***

(I did three runs for each and took the best one.)

The hash_map implementation did work on gcc from 3.4.4
through 4.3.2 on Linux and Windows (Cygwin).

Q1: Does anybody else (besides me) like to "hash something"?
How do you do that? Boost (I didn't get this working
*without* their (experimental) /tr1 branch on Cygwin
and older Linuxes).

Q2: Which "future" can be expected regarding "hashing"?

Thanks & Regards


PS.: I'll add the gnu::hash_map variant below. In
order to use std::map here, remove the "namespace hack"
on the beginning and modify the typedef. Thats it.


#include <boost/regex.hpp>
#include <ext/hash_map> // Gnu gcc specific, switch to <map>
#include <iostream>
#include <fstream>
#include <string>

// allow the gnu hash_map to work on std::string
namespace __gnu_cxx {
    template<> struct hash< std::string > {
       size_t operator()(const std::string& s) const {
          return hash< const char* >()( s.c_str() );
    }; /* */

// this is what we would love:
typedef __gnu_cxx::hash_map<std::string, int> Hash; // change to std::map

char *slurp(const char *fname, size_t* len);
int word_freq(const char *block, size_t len, Hash& hash);

  int main()
  using namespace std;
  size_t len;

  const char *fn = "fulltext.txt";
  cout << "start slurping" << endl;
  char *block = slurp(fn, &len);

  Hash hash;
  cout << "start hashing" << endl;
  int n = word_freq(block, len, hash);
  delete [] block;

  cout << "done, " << fn << " (" << len/1024
       << "KB) has " << n << " different words" << endl;

  return 0;

  char *slurp(const char *fname, size_t* len)
  std::ifstream fh(fname); // open
  fh.seekg(0, std::ios::end); // get to EOF
  *len = fh.tellg(); // read file pointer
  fh.seekg(0, std::ios::beg); // back to pos 0
  char* data = new char [*len+1];, *len); // slurp the file
  return data;

  int word_freq(const char *block, size_t len, Hash& hash)
  using namespace boost;
  match_flag_type flags = match_default;
  static regex r("\\w\\w*");
  cmatch match;

  const char *from=block, *to=block+len;
  while( regex_search(from, to, match, r, flags) ) {
     hash[ std::string(match[0].first, match[0].second) ]++;
     from = match[0].second;
  return hash.size();


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