Hash Map Usage

Hey guys, long time no see!

First of all... can someone tell me if hash_maps are actually a part of STL, and if not, where I get/how I include the files to use them?  I've used them back in university

Can someone provide me with some short sample code for using a hash_map (a simple, but full/compileable file)?

Thanks :D

-John
LVL 12
w00teAsked:
Who is Participating?

Improve company productivity with a Business Account.Sign Up

x
 
Infinity08Connect With a Mentor Commented:
>> First of all... can someone tell me if hash_maps are actually a part of STL

No, they are an extension that several platforms have made available (including popular compilers like GNU C++ and Visual C++).


>> where I get/how I include the files to use them?

Depends what compiler you are using ... ?


>> Can someone provide me with some short sample code for using a hash_map (a simple, but full/compileable file)?

If you're using the SGI implementation, then the reference page for it will help (it includes sample code) :

        http://www.sgi.com/tech/stl/hash_map.html
0
 
evilrixConnect With a Mentor Senior Software Engineer (Avast)Commented:
Google have an excellent implementation of various hash map containers. They are portable, working on Windows and Linux and are available as packages on most Linux platforms.

http://google-sparsehash.googlecode.com/svn/trunk/doc/index.html
0
 
sarabandeCommented:
for many problems a normal std::map would also be a good and easy-to-use choice.

you can add entries by statements like

    mymap[key] = value;

Sara
0
Free Tool: ZipGrep

ZipGrep is a utility that can list and search zip (.war, .ear, .jar, etc) archives for text patterns, without the need to extract the archive's contents.

One of a set of tools we're offering as a way to say thank you for being a part of the community.

 
Deepu AbrahamR & D Engineering ManagerCommented:
0
 
LevantCommented:
As [sarabande] pointed out - the std::map class in STL has exactly the hash table functionality of storing values according to the key.

Boris
0
 
evilrixSenior Software Engineer (Avast)Commented:
>> exactly the hash table functionality of storing values
A map is not a hash map. They have the same basic functionality (in terms of storing key/value with retrieval by key) but don't confuse them. They are implemented very differently and have very different runtime complexities in terms of speed and memory usage. There are also functional difference, for example items in a map are stored in a way that allow for easy retrieval in a sorted order, this is not possible with a hash map.
0
 
w00teAuthor Commented:
Thanks for the answers guys, the SGI page was exactly what I was looking for.  Got a nice example coded on our system now :)  The google pages were an interesting read too!

I just needed something with a little more speed than a normal map (as evilrix pointed out in the last comment) and wasn't sure how to get access to the C++ implementation of a hashmap.  This should work perfectly :)
0
 
sarabandeCommented:
a std::map has logarithmic speed for searches. it normally is implemented as balanced sorted tree (that's why you get a sorted list for free when iterating). std::map is relatively slow for inserts when it requires new balancing.

the performance of a hash_map is highly dependent on the goodness of the hash algorithm on the given keys and on the relation of number of keys to number of hash keys. for example if you have string keys which all begin with same prefix "xxxxx_" you may get very lousy performance cause too many strings were mapped to the same hash. you normally get good performance with an equal-distributed or normal-distributed numeric key.

so you can't say that hash_map is faster than std::map before you measured it. but you can make more mistakes with hash_map than with std::map.

Sara
0
 
w00teAuthor Commented:
Hi Sara,

I understand, I've actually done quite alot with hashing algorithms in other languages - It's just been a while (college) since I touched them in C++, and we run on some proprietary OS's here which I was having trouble finding the headers on.

It turns out that the OS we compile to didn't have the header, but the unix system we cross-compile from did and it was messing with me a little.  It's all good now though :)  I worked it out once I saw the sgi page.

I apprecaite all the insight though :D  Have a good day!

-w00te
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.