After reading a config file during installation, I am saving the web services url into a Hashtable to test the connectivity to those services.
Before going through all the values I saved, I want to test just the first value. The keys I am using are the whole xml node containing the service url so it's unknown to me.
I didn't know much about Hashtable first, so I tried accessing it using an index. Assuming ht is a populated Hashtable, I tried this:
Dim serviceUrl as String = ht(0).Value
Which obviously failed since there's no key equal to 0, and the serviceUrl is just a Nothing.
Then I tried to access first element using:
Dim firstEntry as DictionaryEntry = ht(ht.Keys(0).ToString())
' Also tried this:
' Dim firstEntry as DictionaryEntry = ht(ht.Keys(0))
In both cases I got an error:
System.InvalidCastException: Specified cast is not valid.
I ended up using a For Each and exiting the loop directly after first iteration.
For Each entry As DictionaryEntry In ht
Dim serviceUrl as String = entry.Value
'Use it and exit for.
Exit For
Next
Well, this looks really awful.
After some time debugging and looking around, I used an array to hold the keys values:
Dim arr as Object() = new Object(100){}
'Copy the keys to that array.
ht.Keys.CopyTo(arr,0)
'Now I can directly access first item from the Hashtable:
Dim serviceUrl as String = ht(arr(0))
I am not sure if this is the right approach.
Is there any direct/clean way to access first item from a Hashtable?
The Keys property is an ICollection, not an IList, so it can't be indexed. ICollection is basically just IEnumerable with a Count property, so you should treat is the same way as an IEnumerable. That means enumerating it to get the first item. You can use LINQ:
Dim firstKey = myHashtable.Keys.Cast(Of Object)().FirstOrDefault()
or you can go old-school:
Dim firstKey As Object
For Each key In myHashtable.Keys
firstKey = key
Exit For
Next
If the collection may be empty, you can use the Count property to test first.
Related
Are there any problems with iterating over a dictionary in the following manner?
Dim dict As New Dictionary(Of String, Integer) From {{"One", 1}, {"Two", 2}, {"Three", 3}}
For i = 0 To dict.Count - 1
Dim Key = dict.Keys(i)
Dim Value = dict.Item(Key)
'Do more work
dict.Item(Key) = NewValue
Next
I have used it a lot without any problems. But I recently read that the best way to iterate over a dictionary was using a ForEach loop. This led me to question the method that I've used.
Update: Note I am not asking how to iterate over a dictionary, but rather if the method that I've used successfully in the past is wrong and if so why.
Are there any problems with iterating over a dictionary in the following manner?
Yes and no. Technically there's nothing inherently wrong with the way you're doing it as it does what you need it to do, BUT it requires unnecessary computations and is therefore slower than simply using a For Each loop and iterating the key/value-pairs.
Iterating keys, then fetching value
The Keys property is not a separate collection of keys, but is actually just a thin wrapper around the dictionary itself which contains an enumerator for enumerating the keys only. For this reason it also doesn't have an indexer that lets you access the key at a specific index like you are right now.
What's actually happening is that VB.NET is utilizing the extension method ElementAtOrDefault(), which works by stepping through the enumeration until the wanted index has been reached. This means that for every iteration of your main loop, ElementAtOrDefault() also performs a similar step-through iteration until it gets to the index you've specified. You now have two loops, resulting in an O(N * N) = O(N2) operation.
What's more, when you access the value via Item(Key) it has to calculate the hash of the key and determine the respective value to fetch. While this operation is close to O(1), it's still an unnecessary additional operation compared to what I'm talking about below.
Iterating key/value-pairs
The dictionary already has an internal list (array) holding the keys and their respective values, so when iterating the dictionary using a For Each loop all it does is fetch each pair and put them into a KeyValuePair. Since it is fetching directly by index this time (at a specific memory location) you only have one loop, thus the fetch operation is O(1), making your entire loop O(N * 1) = O(N).
Based on this we see that iterating the key/value-pairs is actually faster.
This kind of loop would look like (where kvp is a KeyValuePair(Of String, Integer)):
For Each kvp In dict
Dim Key = kvp.Key
Dim Value = kvp.Value
Next
See here:
https://www.dotnetperls.com/dictionary-vbnet
Keys. You can get a List of the Dictionary keys. Dictionary has a get accessor property with the identifier Keys. You can pass the Keys to a List constructor to obtain a List of the keys.
It cites an example similar to yours:
Module Module1
Sub Main()
' Put four keys and values in the Dictionary.
Dim dictionary As New Dictionary(Of String, Integer)
dictionary.Add("please", 12)
dictionary.Add("help", 11)
dictionary.Add("poor", 10)
dictionary.Add("people", -11)
' Put keys into List Of String.
Dim list As New List(Of String)(dictionary.Keys)
' Loop over each string.
Dim str As String
For Each str In list
' Print string and also Item(string), which is the value.
Console.WriteLine("{0}, {1}", str, dictionary.Item(str))
Next
End Sub
End Module
I used to have the following working code
Set result = DecodeJson(MyRequest.responseText)
Dim keys() As String
keys = GetKeys(result.issues)
Now, my approach has changed and instead of having result being the object, I receive a Collection based upon Array of class objects as class member in VBA
Dim resultsFromQueries As Collection
Set resultsFromQueries = GetAllJSonObjects(searchString) ' this calls DecodeJson(MyRequest.responseText) as per the code snippet above
Dim i As Integer
For i = 0 To resultsFromQueries.Count
Dim keys() As String
Dim item As Object
Set item = resultsFromQueries.item(i + 1) ' I guess it's not 0 based?
keys = GetKeys(item.result.issues) 'KABOOM
The issue I have now is I keep getting the following exception
Run time error '424':
Object required
Checking in the watch window, item shows as type Variant/String and has the value of "[object Object]"
Do I need to cast it?
I had that problem just a few weeks ago, the problem was the following:
The collection was collected with the follwing code:Collection.Add(OBJECT)
This however leads to the addition of the value of the Object into the Collection and can be solved as simple as leaving the parathesis.Collection.add OBJECT
So my advice is to go into the GetAllJSonObjects(searchString) function and
search for the addition process and make sure that it is without parethesis.
Like this the Collection will contain Objects and not variants and your code with item(i+1) should work properly.
Good luck!
I try to download files from gridview .. I save files in database and then I display in grid-view I try this
I save files in database table not in folder so I try to download files
when i do this document is download but there is problem when i debug the code and check then in this line
Dim row = db_stu.dResult.Tables(0).Rows(i)
dResult shows
docid document docname docextension
1014 System.Byte[] Book2.xlsx .xlsx
and then when i further proceed docname shows "1912218726836.xlsx" this and also file download as a corrupt
These two lines together are wrong:
Dim binary() As Byte = TryCast(structDb.dstResult.Tables(0).Rows(i).Item("document"), Byte())
Dim ms As MemoryStream = New MemoryStream(binary)
The reason to use TryCast is that the object that you're trying to cast may not be the type you're trying to cast it as. In that case, TryCast will return Nothing. Use of TryCast should ALWAYS be followed by a test for Nothing, which you haven't done. You're using the result as though you're sure that there will be an object of that type. If you know that then you should be using DirectCast rather than TryCast.
Even if you do know that the reference will not be to an object of a different type and you use DirectCast though, if you cast a null reference, i.e. Nothing, then you're still going to get Nothing back. So, you first need to determine whether structDb.dstResult.Tables(0).Rows(i).Item("document") can refer to an object of a type other than Byte(). If it can't then use DirectCast rather than TryCast. Either way, it appears that that expression can produce Nothing so you need to check for Nothing either way, e.g.
Dim binary() As Byte = TryCast(structDb.dstResult.Tables(0).Rows(i).Item("document"), Byte())
If binary IsNot Nothing Then
Dim ms As MemoryStream = New MemoryStream(binary)
'...
End If
EDIT: If the column is nullable then you need to first test whether the row contains null and then only use the data if there is some:
Dim row = structDb.dstResult.Tables(0).Rows(i)
If Not row.IsNull("document") Then
'There is data so go ahead and use it.
Dim binary = DirectCast(row("document"), Byte())
'...
I'm surprised at how hard this has been to do but I imagine it's a quick fix so I will ask here (searched google and documentation but neither helped). I have some code that adds items to a collection using keys. When I come across a key that already exists in the collection, I simply want to set it by adding a number to the current value.
Here is the code:
If CollectionItemExists(aKey, aColl) Then 'If key already has a value
'add value to existing item
aColl(aKey).Item = aColl(aKey) + someValue
Else
'add a new item to the collection (aka a new key/value pair)
mwTable_ISO_DA.Add someValue, aKey
End If
The first time I add the key/value pair into the collection, I am adding an integer as the value. When I come across the key again, I try to add another integer to the value, but this doesn't work. I don't think the problem lies in any kind of object mis-match or something similar. The error message I currently get is
Runtime Error 424: Object Required
You can't edit values once they've been added to a collection. So this is not possible:
aColl.Item(aKey) = aColl.Item(aKey) + someValue
Instead, you can take the object out of the collection, edit its value, and add it back.
temp = aColl.Item(aKey)
aColl.Remove aKey
aColl.Add temp + someValue, aKey
This is a bit tedious, but place these three lines in a Sub and you're all set.
Collections are more friendly when they are used as containers for objects (as opposed to containers for "primitive" variables like integer, double, etc.). You can't change the object reference contained in the collection, but you can manipulate the object attached to that reference.
On a side note, I think you've misunderstood the syntax related to Item. You can't say: aColl(aKey).Item. The right syntax is aColl.Item(aKey), or, for short, aColl(aKey) since Item is the default method of the Collection object. However, I prefer to use the full, explicit form...
Dictionaries are more versatile and more time efficient than Collections. If you went this route you could run an simple Exists test on the Dictionary directly below, and then update the key value
Patrick Matthews has written an excellent article on dictionaries v collections
Sub Test()
Dim MyDict
Set MyDict = CreateObject("scripting.dictionary")
MyDict.Add "apples", 10
If MyDict.exists("apples") Then MyDict.Item("apples") = MyDict.Item("apples") + 20
MsgBox MyDict.Item("apples")
End Sub
I think you need to remove the existing key-value pair and then add the key to the collection again but with the new value
I need your help, guys! :|
I've got myself a CSV file with the following contents:
1,The Compact,1.8GHz,1024MB,160GB,440
2,The Medium,2.4GHz,1024MB,180GB,500
3,The Workhorse,2.4GHz,2048MB,220GB,650
It's a list of computer systems, basically, that the user can purchase.
I need to read this file, line-by-line, into an array. Let's call this array csvline().
The first line of the text file would stored in csvline(0). Line two would be stored in csvline(1). And so on. (I've started with zero because that's where VB starts its arrays). A drop-down list would then enable the user to select 1, 2 or 3 (or however many lines/systems are stored in the file). Upon selecting a number - say, 1 - csvline(0) would be displayed inside a textbox (textbox1, let's say). If 2 was selected, csvline(1) would be displayed, and so on.
It's not the formatting I need help with, though; that's the easy part. I just need someone to help teach me how to read a CSV file line-by-line, putting each line into a string array - csvlines(count) - then increment count by one so that the next line is read into another slot.
So far, I've been able to paste the numbers of each system into an combobox:
Using csvfileparser As New Microsoft.VisualBasic.FileIO.TextFieldParser _
("F:\folder\programname\programname\bin\Debug\systems.csv")
Dim csvalue As String()
csvfileparser.TextFieldType = Microsoft.VisualBasic.FileIO.FieldType.Delimited
csvfileparser.Delimiters = New String() {","}
While Not csvfileparser.EndOfData
csvalue = csvfileparser.ReadFields()
combobox1.Items.Add(String.Format("{1}{0}", _
Environment.NewLine, _
csvalue(0)))
End While
End Using
But this only selects individual values. I need to figure out how selecting one of these numbers in the combobox can trigger textbox1 to be appended with just that line (I can handle the formatting, using the string.format stuff). If I try to do this using csvalue = csvtranslator.ReadLine , I get the following error message:
"Error 1 Value of type 'String' cannot be converted to '1-dimensional array of String'."
If I then put it as an array, ie: csvalue() = csvtranslator.ReadLine , I then get a different error message:
"Error 1 Number of indices is less than the number of dimensions of the indexed array."
What's the knack, guys? I've spent hours trying to figure this out.
Please go easy on me - and keep any responses ultra-simple for my newbie brain - I'm very new to all this programming malarkey and just starting out! :)
Structure systemstructure
Dim number As Byte
Dim name As String
Dim procspeed As String
Dim ram As String
Dim harddrive As String
Dim price As Integer
End Structure
Private Sub csvmanagement()
Dim systemspecs As New systemstructure
Using csvparser As New FileIO.TextFieldParser _
("F:\folder\programname\programname\bin\Debug\systems.csv")
Dim csvalue As String()
csvparser.TextFieldType = Microsoft.VisualBasic.FileIO.FieldType.Delimited
csvparser.Delimiters = New String() {","}
csvalue = csvparser.ReadFields()
systemspecs.number = csvalue(0)
systemspecs.name = csvalue(1)
systemspecs.procspeed = csvalue(2)
systemspecs.ram = csvalue(3)
systemspecs.harddrive = csvalue(4)
systemspecs.optical = csvalue(5)
systemspecs.graphics = csvalue(6)
systemspecs.audio = csvalue(7)
systemspecs.monitor = csvalue(8)
systemspecs.software = csvalue(9)
systemspecs.price = csvalue(10)
While Not csvparser.EndOfData
csvalue = csvparser.ReadFields()
systemlist.Items.Add(systemspecs)
End While
End Using
End Sub
Edit:
Thanks for your help guys, I've managed to solve the problem now.
It was merely a matter calling loops at the right point in time.
I would recommend using FileHelpers to do the reading.
The binding shouldn't be an issue after that.
Here is the Quickstart for Delimited Records:
Dim engine As New FileHelperEngine(GetType( Customer))
// To Read Use:
Dim res As Customer() = DirectCast(engine.ReadFile("FileIn.txt"), Customer())
// To Write Use:
engine.WriteFile("FileOut.txt", res)
When you get the file read, put it into a normal class and just bind to the class or use the list of items you have to do custom stuff with the combobox. Basically, get it out of the file and into a real class asap, then things will be easier.
At least take a look at the library. After using it, we use a lot more simple flat files since it is so easy, and we haven't written a file access routine since (for that kinda stuff).
http://msdn.microsoft.com/en-us/library/microsoft.visualbasic.fileio.textfieldparser.aspx
I think your main problem is understanding how arrays work (hence the error message).
You can use split and join functions to convert strings into and out of arrays
dim s() as string = split("1,2,3",",") gives and array of strings with 3 elements
dim ss as string = join(s,",") gives you the string back
Firstly, it's actually really good that you are using the TextFieldParser for reading CSV files - most don't but you won't have to worry about extra commas and quoted text etc...
The Readline method only gives you the raw string, hence the "Error 1 Value of type 'String' cannot be converted to '1-dimensional array of String'."
What you may find easier with combo boxes etc is to use an object (e.g. 'systemspecs') rather than strings. Assign the CSV data to the objects and override the "ToString" method of the 'systemspecs' class to display in the combo box how you want with formatting etc. That way when you handle the SelectedIndexChanged event (or similar) you get the "SelectedItem" from the combo box (which can be Nothing so check) and cast it as the 'systemspecs' to use it. The advantage is that you are not restricted to display the exact data in the combo etc.
' in "systemspecs"...
Public Overrides Function ToString() As String
Return Name ' or whatever...
End Function ' ToString
e.g.
dim item as new systemspecs
item.ID = csvalue(1)
item.Name = csvalue(2)
' etc...
combobox1.Items.Add(item)
Let me know if that makes sense!
PK :-)