Would the following:
Dim stringlist As List(Of String)
Dim stringlisthas = stringlist.Contains("thing1")
be any slower than
Dim stringlist As List(Of String)
Dim stringlisthash As New HashSet(Of String)(stringlist)
Dim stringlisthas = stringlisthash.Contains("thing1")
Is a hashset needed for contains?
Is a hashset needed for contains?
Needed? No.
Would [List<T>.Contains] be any slower than [HashSet<T>.Contains]?
Probably. It depends on how List<T>.Contains is implemented (its probably a linear search).
I'll answer a question you didn't ask.
Does it matter?
It depends. You have to code up both, profile it, and see if it's a bottleneck in your application. If it's not, just stick with List<T>.Contains.
Related
Please see the code below:
Public Function ExecuteDynamicQuery(ByVal strSQL As String, ByVal list As List(Of clsType), ByVal tyType As clsType) As List(Of clsType) Implements IGenie.ExecuteDynamicQuery
Dim objParameterValues As New clsParameterValues
Dim iConnectionBLL As iConnectionBLL
Dim objCon As DbConnection
Dim objDR As DbDataReader
Dim paramValues() As DbParameter
objParameterValues = New clsParameterValues
iConnectionBLL = New clsConnectionBLL()
objCon = iConnectionBLL.getDatabaseTypeByDescription("Genie2")
Using objCon
paramValues = objParameterValues.getParameterValues
objDR = clsDatabaseHelper.ExecuteReader(objCon, CommandType.Text, strSQL, paramValues)
Do While objDR.Read
Dim tyType2 As clsType = tyType
tyType.PopulateDataReader(objDR)
list.Add(tyType2)
Loop
objDR.Close()
Return list
End Using
End Function
An SQL statement is passed to the function along with clsType (the base type). A list of types is returned e.g. a list of Persons. For example, in this case strSQL = "SELECT * FROM Persons". A list of 500 persons is returned but they are all the same person (the last person added to the list). I realise this is because the list is referncing the same object for each entry. How do I change this?
This is a situation where making the method generic would be useful. For instance:
Public Function MyGenericMethod(Of T As New)() As List(Of T)
Dim results As New List(Of T)()
For i As Integer = 0 To 9
Dim item As New T()
' Populate item ...
results.Add(item)
Next
Return results
End Function
For what it's worth, though, I see people trying do this kind of thing often, and it never sits well with me. I'm always the first one in line to suggest that common code should be encapsulated and not duplicated all over the place, but, I've never been convinced that creating some sort of data access layer that encapsulates the calls to ADO, but doesn't also encapsulate the SQL, is a good idea.
Consider for a moment that ADO, is in-and-of-itself an encapsulation of that part of the data-access layer. Sure, it can take a few more lines of code than you might like to execute a simple SQL command, but that extra complexity is there for a reason. It's necessary in order to support all of the features of the data source. If you try to simplify it, inevitably, you will one day need to use some other feature of the data source, but it won't be supported by your simplified interface. In my opinion, each data access method should use all of the necessary ADO objects directly rather than trying to some how create some common methods to do that. Yes, that does mean that many of your data access methods will be very similar in structure, but I think you'll be happier in the long run.
I've reduced your original code. The following sample is functionally equivalent to what you posted. Without knowing more about your types, it will hard to give you anything more than this, but maybe the reduction will make the code clear enough for you to spot a solution:
Public Function ExecuteDynamicQuery(ByVal sql As String, ByVal list As List(Of clsType), ByVal type As clsType) As List(Of clsType) Implements IGenie.ExecuteDynamicQuery
Dim paramValues() As DbParameter = New clsParameterValues().getParameterValues()
Using conn As DbConnection = iConnectionBLL.getDatabaseTypeByDescription("Genie2"), _
rdr As DbDataReader = clsDatabaseHelper.ExecuteReader(conn, CommandType.Text, sql, paramValues)
While rdr.Read()
type.PopulateDataReader(rdr)
list.Add(type)
End While
Return list
End Using
End Function
There are a few additional bits of advice I can give you:
You must have some way to accept parameter information for your query that is separate from the query itself. The ExecuteReader() method that you call supports this, but you only ever pass it an empty array. Fix this, or you will get hacked.
A implementation that uses Generics (as posted in another answer) would be much simpler and cleaner. The Genie interface you're relying doesn't seem to be adding much value. You'll likely do better starting over with a system that understands generics.
The problem of re-using the same object over and over can be fixed by creating a new object inside the loop. As written, the only way to do that is with a New clsType (and it seems you may have Option Strict Off, such that this could blow up on you at run time), through some messy reflection code, a switch to using generics as suggested in #2, or a by accepting a Func(Of clsType) delegate that can build the new object for you.
Is it possible to have a string array as the value in a dictionary? I need to save the description (Hour2) as the key and as value being able to access both the price (elements_PR(4)) and the time offset (1). Is there a good way to do that?
Dim pr_val(2) As String
Dim PR As New Dictionary(Of String, pr_val)
PR.Add("Hour2", {elements_PR(4), "1"})
There is no reason why you can't do it - you could also use values of type Tuple(Of String, String), if the number of elements is fixed in advance - in this case, 2. It'd be easier to perform comparisons on, and would also be immutable, which is often a good idea.
You sure can. Try something like this:
Dim PR As New Dictionary(Of String, IEnumerable(Of String))
PR.Add("Hour2", {elements_PR(4), "1"})
It seems to me that you could create a class representing the price and the time offset. Therefore, you could do something like PR.Add("Hour2", instanceOfClass).
Depending on the meaning of the description in your situation, you could even include it in your class. It would allow you to use another approach with a List(Of YourClass) containing a list of items with the description, the price and the time offset. To retrieve an item by "key", you could then use some Linq.
Short answer - yes. Assuming the following was declared somewhere:
Dim elements_PR(4) As String : elements_PR(4) = "Hello"
Dim PR As New Dictionary(Of String, String())
You can either do:
PR.Add("Hour2", {elements_PR(4), "1"})
or
Dim pr_val() As String = {elements_PR(4), "1"}
PR.Add("Hour1", pr_val)
i want to know how to manipulate the data from a
Public ListD As New List(Of Dictionary(Of String, String))
meaning read/write. Can you help me please?
I've tried with
ListD.Add("string_as_key", var_as_value)
but it haven't worked
Thanks!
In the case of
("string_as_key", var_as_value)
To add that to your ListD you would first have to either create a dictionary and add that item to it, or add that item to one of the existing dictionaries.
For example:
'Create dictionary
Dim dic as new Dictionary(Of String, String)
dic.Add("string_as_key", var_as_value)
'Add it to list
ListD.Add(dic)
To read a single item from your ListD would look something like:
Dim dic As Dictionary(Of String, String) = ListD(0)
Dim var_as_value As String = dic("string_as_key")
A dictionary is a group of multiple items in itself. So if you have several pairs of things with unique items you can use as a key, a dictionary is a good choice.
I may be way off here since I don't know what you are using it for, but I get the impression that rather than a list of Dictionary you may be better of with just a dictionary(Of String, String)
Your question can't really be 'answered' but to give you a start have a look at the online documentation, specifically:
List(Of T) Class
Dictionary(Of TKey, TValue) Class
How to loop in VB.NET <- Start here and work upwards when you understand each one.
People often tell me not to use ArrayList for making my arrays in VB.NET.
I would like to hear opinions about that, why shouldn't I? What is the best method for creating and manipulating array contents, dimensions etc?
Thanks.
Use generic lists instead. ArrayList is not typed, meaning that you can have a list with strings, numbers, +++. Rather you should use a generic list like this:
Dim list1 As New List(Of String) ' This beeing a list of string
The lists-class also allows you to expand the list on the fly, however, it also enforces typing which helps write cleaner code (you don't have to typecast) and code that is less prone to bugs.
ArrayList is gennerally speaking just a List(Of Object).
ArrayLists are not type checked so you will need to do a lot of boxing/unboxing. Use a .net collection instead that support generics like List.
Because List does not have to unbox your objects it boasts a surprisingly better performance than Arraylist.
ArrayLists are less performant and memory-extensive:
Dim list1 As New ArrayList
For i As Integer = 1 To 100000000
list1.Add(i)
Next
' --> OutOfMemoryException after 13.163 seconds, having added 67.108.864 items
Dim list2 As New List(Of Integer)
For i As Integer = 1 To 100000000
list2.Add(i)
Next
' --> finished after 1.778 seconds, having added all values
Because its not strongly typed. Use a List(Of T) which T is your type.
Can I have a List containing one string and two numbers? Or I can only have one type of element?
If that's the kind of functionality you want, then I would look at the non-generic System.Collections.ArrayList class.
Update
For those of you who aren't going to read the huge comment chain...it looks like Adam Robinson is on to something using List<object> over ArrayList. Both will work but on large collections it seems like List<object> is measurably faster than ArrayList.
You can. A list of Objects can do that. But, you lose type safety with that and also design time intelliSense.
What do you want to do? You could also use a class with 3 members.
No, containers like List(Of T) store exactly one type T of elements. You can, though, make this one type consist of one string and two numbers.
Structure Foo
Public Desc As String
Public x As Integer, y As Integer
End Structure
Dim List = New List(Of Foo)
Yes, you can.
dim myVehicles as new list(of object)
dim myCar as new car
dim myBike as new bike
dim mySecondCar as new car
myVehicles.add(myCar)
myVehicles.add(myBike)
myVehicles.add(mySecondCar)