C++ STL hash ???

bigearsbilly · 07-04-2007, 04:55 AM

I want a simple hash in C++
i was trying the STL and on SGI web site is this:

Code:

int main()
{
  hash<const char*> H;
  cout << "foo -> " << H("foo") << endl;
  cout << "bar -> " << H("bar") << endl;
}

which will not compile

Code:

g++    -c -o 1.o 1.cc
1.cc: In function `int main()':
1.cc:8: error: `hash' was not declared in this scope
1.cc:8: error: expected primary-expression before "const"
1.cc:8: error: expected `;' before "const"
make

great example eh?
I added the std namespace

any clues as to what to #include?

I've tried <hash> <hash.h> hash_map

searched here and on koders.org etc...

also tried

Code:

#include <ext/hash_map>
#include <ext/hash_set>
#include <ext/hash_fun.h>

bigearsbilly · 07-04-2007, 05:19 AM

doh!

Quote:

This class is an SGI extension; it is not part of the C++ standard.

always read the whole page

nx5000 · 07-04-2007, 05:46 AM

There is the standard map but the last time I used c++ there was no standard hash (the sorting can be a problem)
SGI does an extension to this:
http://www.sgi.com/tech/stl/hash_map.html
That's not so clear on their website, I remember it now

Code:

#include <ext/hash_map>
#include <iostream>
#include <string.h>

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

int main()
{
 using namespace std;
 using namespace __gnu_cxx;

  hash_map<const char*, int, hash<const char*>, eqstr> months;
  
  months["january"] = 31;
  months["february"] = 28;
  months["march"] = 31;
  months["april"] = 30;
  months["may"] = 31;
  months["june"] = 30;
  months["july"] = 31;
  months["august"] = 31;
  months["september"] = 30;
  months["october"] = 31;
  months["november"] = 30;
  months["december"] = 31;
  
  cout << "september -> " << months["september"] << endl;
  cout << "april     -> " << months["april"] << endl;
  cout << "june      -> " << months["june"] << endl;
  cout << "november  -> " << months["november"] << endl;
}

atulsvasu · 07-04-2007, 10:16 AM

In g++, extensions are available in
__gnu_cxx namespace not in std.

bigearsbilly · 07-05-2007, 02:21 AM

thanks,
yes I ended up using a map

nx5000 · 07-05-2007, 05:26 AM

The STL is really good but some functions add a very high complexity (O(n)..) to your program. Iterators or find algo are such ones, it can happen that they recreate a complete class on every iteration.
In case you don't know these libraries, it's worth mentionning: http://www.boost.org
These are used in big environments that have strong requirements in term of speed.

bigearsbilly · 07-05-2007, 05:43 AM

cheers,

complexity or re-invent the wheel?
I am lazy first and foremost.

It's a test for a job-interview, an arithmetical expression parser.
So I want to show I know how to use STL, multiple inheritance, exceptions, polymorph etc.

I ain't done C++ since college (and I can see why)

God it's a hideous language.

as ugly as perl and half as much fun.