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From: Ioannis Vranos <ivr@remove.this.grad.com>
Newsgroups: comp.lang.ada
Subject: Re: Teaching new tricks to an old dog (C++ -->Ada)
Date: Wed, 23 Mar 2005 12:53:16 +0200
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Date: 2005-03-23T12:53:16+02:00
List-Id: <comp.lang.ada>

Vinzent 'Gadget' Hoefler wrote:

> Yeah, right. Binary searches are not much slower. Only about 10 or 20
> times. Seem, you just rendered hashing algorithms obsolete. 


OK, one can use a third-party hashed container then, like a hash_map:


#include <hash_map>
#include <iostream>

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

    hash_map<double, double> id;

    id[-400.1]= 7.1;

    id[-2500.6]= -1;

    id[10.43]= 9;

    id[-300.65]= 7.1;

    unsigned counter= 0;

    for(hash_map<double, double>::const_iterator p= id.begin();
                                                        p!=id.end(); ++p)
    {
        if(p->second== 7.1)
        {
            cout<<"7.1 was found at index "<<p->first<<"\n";

            ++counter;
        }
    }

    cout<<"\n7.1 was found "<<counter<<" times in total.\n";
}


and using std::count_if in a integral indices case:


#include <hash_map>
#include <iostream>
#include <algorithm>


class comp: public std::unary_function<std::pair<int, double>, bool>
{
     const double searchValue;

     public:

         comp(const double value):searchValue(value) {}

         bool operator() (const std::pair<int, double> &arg) const
         {
             if(arg.second==searchValue)
               return true;

             return false;
         }
};



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

     hash_map<int, double> id;

     id[-400]= 7.1;

     id[-2500]= -1;

     id[10]= 9;

     id[-300]= 7.1;

     cout<<"\n7.1 was found "<<count_if(id.begin(), id.end(), comp(7.1))
         <<" times in total.\n";
}



C:\c>temp

7.1 was found 2 times in total.

C:\c>



We can also check the entire range of indices, in which case the map 
(and the third party hash_map) returns 0 (of the value type) on case 
that there is no entry.

In other words only the entries occupy space while the no-entries occupy 
no space.


Thus if not the entire range is used, the iterator/count_if approach is 
faster than the entire range check.



-- 
Ioannis Vranos

http://www23.brinkster.com/noicys