Re: Should I implement this using string hash or multimap..

From: (Carl Barron)
16 Sep 2006 07:58:01 -0400
Jonay Aloat <> wrote:

I need to implement the following. Shoul I use multimap or write a string
hash class? ie

Brand Product
Samson Television
Samsung Television
Samsung VCR
Samsung DVD Player
Samsunk Television

The brand is the key and the product is the data.

If I use multimap, then I can implement it using something like this

typedef std::multimap<string, string> mmap;
typedef std::multimap<string, string>::iterator mapIter;

mmap product_map;
product_map.insert( pair<string, string> string a(Samsung), string
To find the product in brand, all I have to do is iterate using lower and
upper bound.

mapIter lowerB = product_map.lower_bound("Samsung");
mapIter upperB = product_map.upper_bound("Samsung");

for (lowerB; lowerB != upperB; ++lowerB) {
    string p = (*lowerB).second;
    if ( strcmp (p.c_str(), "VCR") == 0 ) ) {
            //do something
    } else {
            //do something

Assume that I have a lot of data. Is this implementation efficient? I know
using string as the key is not a good idea. Is there a better way to do
this? Is the performance better using hash? How do I implent the hash

  How big is a lot??? 4066 of each? 1 meg of each ??

Asunning the # of companies no_companies < = numeric_limits<unsigned
short> ::max() and the no_products <= numeric_limits<unsigned
short>::max() typically 32k or larger.

and sizeof(unsigned short) *2 <= sizeof(unsigned long) then a simple
shift and or provides a hash.

    unsiged long hash(std::pair<unsigned short,unsigned short> x)
          unsigend long k = x.first;
          return k << (sizeof(unsiged short)*CHAR_BIT) | x.second;

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