Re: Multidimensional hash functions

"Victor Bazarov" <>
Thu, 24 May 2007 09:20:16 -0400
Mike wrote:

I have a multidimensional array that I would like to access as a hash
map for performance reasons. I cannot see anything in the STL (I mean
STL/TR1) that supports multi-dimensional hash access like eg :

hash_map<Key1, Key2, Value> md_hash;

V1 = md_hash.find(k1, k2); //etc

Do I really need to do this by making hash_maps for each of the rows
and then take a hash_map of all the maps ? Or is there an easier
way ?

I'd try

   hash_map<std::pair<Key1,Key2>, Value> md_hash;

and then make sure you have a proper hash calculation function.

Also, the set of {Key,Value} pairs is constant (it is a look up
table) and I would like to initialise the hash map from a 2
dimensional const array. It seems there is no way to do this directly
using STL type hashes but instead I need to create a wrapper class and
iterate through the initialiser array to initialise the hash maps.

Whatever works...

This has the downside of wasting the original table of values which is
discarded after the initialisation process, and also is a waste of

Keep pointers to const values in your hash_map instead.


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