RE: Hash table & threading
- From: "Ted Warner" <Ted Warner@xxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 7 Jul 2005 12:03:01 -0700
Hash Tables use more memory, but in almost every case a Hash Table will have
better performance than an array. The only case where it's best to simply use
an array is when the keys are all consecutive integers.
Hash Tables are roughly O(1) to access a value with its key, while arrays
are usually O(N) or similar depending on your algorithm for lookup.
If you want to compromise to save memory, one thing you can do is use a
tree-based map. That will have slower access time [usually O(log N)] but it
will use very low memory overhead.
Another way to lower memory usage is to set the load factor of your hash
table to a high value. Lower load factors mean faster lookup, but they also
mean more memory. In general, the memory a hash table uses is 1.5 /
load-factor * number-of-entries. Setting your load factor too high can cause
your hash table to perform like an array. Setting it too low can cause it to
resize too often. It is usually good to keep it around 0.7. Unfortunately the
Hashtable class provided in the .NET Platform doesn't have an easy way to
change load factor that I know of. I would just rely on the Hashtable
implementation to handle your data efficiently.
There is almost no case where the memory savings of an Array will make up
for the performance hit. It's almost certainly better to use a Hash Table.
"Ravi" wrote:
> Hi,
> I am working on a winform app. I need to use an object which can store
> some information(key/value pairs) and also can be acessed by multiple
> threads(read/write). From what I heard Hash table is best suited for it. My
> question is
> Why hash table. why can't we use an array.
> I thought hash table uses more memory than array. Correct me if am wrong.
>
>
.
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