Hashtable or Datatable?

  • Thread starter Thread starter Andrius B.
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A

Andrius B.

Hi all.

I used Hashtable to store data loaded from Access (the db can contain from
1000 till 100000 records), and that worked very well, because of fast data
retrieving from Hashtable using key.
The main problem with Hashtable is sorting. I have to convert all the
Hashtable to array list or smth, sort it with the help of ICompairer
interface, and then converting back to hashtable. That takes much time.
Now I am trying to modify the app code and started to use Datatable objects
instead of Hashtable. Datatable has a much faster sorting mechanism, and
that is very important for me, because I have to display the items from the
db in Listview - not all of them, just some (up to 50), but the items must
be ordered by certain key, t.i. the user should have possibility ti order
the items in the way he wishes. That's why fast sorting is neccessary.
ListView is in Virtual mode.
But on the other hand, datatable is not the one I am looking for. The item
retrieving from datatable (using datatable.rows.find (by key) is much slower
than getting items from hashtable. When for sorting and displaying, it is
not very bad, because, as I have said, I need at a moment dislpay only up to
50 items in Listview. But also my program has to make some calculations and
analysis with the data, so when it tries to retrieve data from datatable,
the speed of analyse decreases very markebly, in compairing to hashtable.

So, what could I do, if want to have fast sorting and also fast data
retrieving in my app? Storing the data both in datatable and in hashtable
could be a solution, but it consumes to much memory. Is there a class or
smth, that could provide this? Or should I try to write my own class
implementing many interfaces (for sorting etc.)?
I use VB.Net 2005.

Thanks for any help.
 
Thanks Mr. Arnold!
I tried to realize Your idea to use only List (of type) to store the data
and then to sort it using to List.Sort(IComparer). Sorting became a little
bit faster then using Hashtable and the indirect sorting (by convert to
arraylist adn back to Hashtable, as I explained in my question). Of course,
the new conception coud not reach the speed of sorting produced by DataTable
:) I thing I could find some changes in then inner code witch does the
comparing (inside the Compare function) to make the sorting faster.

Thanks again for the suggestion. Nevertheless, it would be better to have
some kind of class that could combine the good things of DataTable and
generic objects :)

Regards.
 
Andrius said:
Hi all.

I used Hashtable to store data loaded from Access (the db can contain from
1000 till 100000 records), and that worked very well, because of fast data
retrieving from Hashtable using key.
The main problem with Hashtable is sorting. I have to convert all the
Hashtable to array list or smth, sort it with the help of ICompairer
interface, and then converting back to hashtable. That takes much time.
Now I am trying to modify the app code and started to use Datatable objects
instead of Hashtable. Datatable has a much faster sorting mechanism, and
that is very important for me, because I have to display the items from the
db in Listview - not all of them, just some (up to 50), but the items must
be ordered by certain key, t.i. the user should have possibility ti order
the items in the way he wishes. That's why fast sorting is neccessary.
ListView is in Virtual mode.
But on the other hand, datatable is not the one I am looking for. The item
retrieving from datatable (using datatable.rows.find (by key) is much slower
than getting items from hashtable. When for sorting and displaying, it is
not very bad, because, as I have said, I need at a moment dislpay only up to
50 items in Listview. But also my program has to make some calculations and
analysis with the data, so when it tries to retrieve data from datatable,
the speed of analyse decreases very markebly, in compairing to hashtable.

So, what could I do, if want to have fast sorting and also fast data
retrieving in my app? Storing the data both in datatable and in hashtable
could be a solution, but it consumes to much memory. Is there a class or
smth, that could provide this? Or should I try to write my own class
implementing many interfaces (for sorting etc.)?
I use VB.Net 2005.

Thanks for any help.
Click to expand...

Why not try a SortedList or a SortedDictionary? Just a thought!
 
By the way, I thing the binary search using IComparer interface (witch is
using for sorting such objects as Arraylist) is not the fastest one. Is
there
smth better? I mean, some kind of faster algorithm for sorting custom data
types?
 
By the way, I thing the binary search using IComparer interface (witch is
using for sorting such objects as Arraylist) is not the fastest one. Is
there
smth better? I mean, some kind of faster algorithm for sorting custom data
types?
Click to expand...

The standard sort methods do not use Binary search - they use QuickSort.
 
Andrius said:
By the way, I thing the binary search using IComparer interface (witch is
using for sorting such objects as Arraylist) is not the fastest one. Is
there
smth better? I mean, some kind of faster algorithm for sorting custom data
types?
Click to expand...

The BinarySearch method is used for locating an item in an already
sorted collection, and it's very fast. For a collection containing a
million items you don't need more than 20 comparisons to find an item.

You might get slightly better performance using a Dictionary, but that
depends on how efficient the implementation of the hashing and
comparison algorithms are.
 
Thats a good idea, I tried it, but the problem is, that I must have a
possibility to sort the Items by different key.
E.G. there are 3 ListViewItems (Text and two subitems, let's imagine that we
see Details - View with 3 columns) :

Name: ID: Town:

Item #1: AAA 333 OOO
Item #2: BBB 111 SSS
Item #3: AAA 222 EEE

So, if want to sort those items by ID, it is not difficult. I will create a
New SortedList(Of String, ListViewItem), and add all the items, specifying
the Item.Subitems(1).text as a key. The sorting will be performed as I
expect (t.i., after sort: Item2, Item3, Item1), because the "ID's" are
unique.

But if I want to sort the Items by "Name", things get worse. "Name" is not
unique. So I have to "whisper" to the SortedList, how I would like to order
items, if they have equal "Name" - then ID's schould be compared; if ID's
are also equal - compare the "Town". And how to tell that to SortedList?
SortedList has only one key, not two or three.
So, I have to combine the neccessary subitems when creating a key for
sorting by "Name"

key = Item.text & Item.Subitems(1).text & Item.Subitems(2).text.

Thats works for me (sorting is adequate), but that's the problem - keys
become quite "long". And, of course, when adding a new value to the
SortedList, as far as I know, the key of the new value is being compared
with the keys of values already in SortedList. Am I not right with the idea,
that the comparing of "long" keys takes more time than of "short" ones? And
of course it is not very "logic" to compare by all subitems' text: if the
one item's "Name" differs from the second item's Name, the Sortedlist should
not pay any attention to the ID's or Town's of the Items.
 
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