Problem
I am testing the following simple VB.Net code:
For i = 0 To 5
Dim f As Integer
If i = 3 Then
f = 1
End If
Debug.WriteLine(f)
Next
and the output is as follows:
0
0
0
1
1
1
but i was expecting:
0
0
0
1
0
0
Solution
The simple way to solve it is to replace Dim f As Integer by Dim f As Integer = 0
Question
Is this the expected behavior and if so why?
This is documented in the Visual Basic Programming Guide:
Even if the scope of a variable is limited to a block, its lifetime is still that of the entire procedure. If you enter the block more than once during the procedure, each block variable retains its previous value. To avoid unexpected results in such a case, it is wise to initialize block variables at the beginning of the block.
That precisely describes the behaviour you're seeing.
I don't know why the language was designed that way - I prefer a language that doesn't let you use a variable without it being definitely assigned a value - but I wouldn't be surprised if it's just a matter of history and backward compatibility at this stage.
Related
i have 2 array list, dateListDead and dateListNotMinggu. Both is DateTime List of Array. This is the ilustration of the date value in list of array
The arrayList value
its supposed to remove specific element that exist in other array list.
so far i tried, this code it's not working.
Dim d, x As Integer
For x = 0 To dateListDead.Count - 1
For d = 0 To dateListNotMinggu.Count - 1
If dateListNotMinggu(d) = dateListDead(x) Then
dateListNotMinggu.RemoveAt(d)
End If
Next
Next
the error is : index out of range. how could it be ? i define the parameter of end looping base on arraylist.count -1
The main is that you are using a For loop from the first index to the last index but you don't account for the change of index when you remove a value. If there might be multiple values then you should start and the end rather than the beginning. In that case, removing an item won't affect the indexes of the items you are yet to test. If there can only be one match then you should be exiting the loop when you find one.
Either way, while you don't have to, I would suggest using a For Each loop on the outside. If you want to perform an action for each item in a list then that's exactly what a For Each loop is for. Only use a For loop if you need to use the loop counter for something other than accessing each item in turn.
For multiple matches:
For Each dateDead As Date In dateListDead
For i = dateListNotMinggu.Count - 1 To 0 Step -1
If CDate(dateListNotMinggu(i)) = dateDead Then
dateListNotMinggu.RemoveAt(i)
End If
Next
Next
For a single match:
For Each dateDead As Date In dateListDead
For i = 0 To dateListNotMinggu.Count - 1
If CDate(dateListNotMinggu(i)) = dateDead Then
dateListNotMinggu.RemoveAt(i)
Exit For
End If
Next
Next
Note that I have also cast the Date values as that type for comparison, which is required with Option Strict On. Option Strict is Off by default but you should always turn it On because it will help you write better code by focusing on data types.
Also, the code above would work with a List(Of Date) as well as an ArrayList but the casts would not be required with a List(Of Date). That's one of the advantages of using a generic List(Of T) over an ArrayList, which paces no restrictions on what it can contain.
If you really must use a For loop because that's what your homework assignment says then it would look like this:
For i = 0 To dateListDead.Count - 1
For j = dateListNotMinggu.Count - 1 To 0 Step -1
If CDate(dateListNotMinggu(j)) = CDate(dateListDead(i)) Then
dateListNotMinggu.RemoveAt(j)
End If
Next
Next
and this:
For i = 0 To dateListDead.Count - 1
For j = 0 To dateListNotMinggu.Count - 1
If CDate(dateListNotMinggu(j)) = CDate(dateListDead(i)) Then
dateListNotMinggu.RemoveAt(j)
Exit For
End If
Next
Next
Note that it is convention to use i as a first option for a loop counter, then j for the first nested loop, then k for the second nested loop. You should only use something else if you have good reason to do so. Remember that the loop counter doesn't represent the value in the list but rather its index. That's why you use i for index and not d for date or the like.
EDIT:
As per Jimi's comment below, the way this would usually be tackled is with a simple LINQ query. If you were using LINQ then you definitely wouldn't be using an ArrayList but rather a List(Of Date). In that case, the code would look like this:
dateListNotMinggu = dateListNotMinggu.Except(dateListDead).ToList()
If you were completely insane and wanted to use LINQ and ArrayLists then this would work:
dateListNotMinggu = New ArrayList(dateListNotMinggu.Cast(Of Date)().
Except(dateListDead.Cast(Of Date)()).
ToArray())
Take note that, as I replied in the comments, using LINQ will generate a new list, rather than changing the existing one.
I am trying to use a collection to set variables equal to 0 each time before they run through a sub. I have a bunch of metrics all as separate sub routines, and for each sub, I declare the variables as long and then set all of them equal to 0 and then have them pull from whatever data they need to. I'd like to be able to not have to set them equal to 0 every time, but rather set them equal at the beginning or something so it doesn't have to be put in in each subroutine, but I am unclear as to how to do that. I tried this code but I have no idea if it is even remotely right or where to put it:
Dim myVars As Collection
Set myVars = New Collection
Dim j As Variant
myVars.Add a
myVars.Add b
myVars.Add c
myVars.Add d
myVars.Add e
For j = 1 To myVars.Count
myVars(j) = 0
Next j
If you declare your variables as numeric types (eg. Long,Integer,Byte,Double,Single) then they're automatically assigned a value of 0.
You'd be better off with strongly typed variables in any case, and it would avoid the need for using a collection or making the variables global in scope.
Can someone please explain why the following code would generate an overflow error in VBA when the recipient of the operation c is an Integer?
Dim a As byte, b As Byte
Dim c As Integer
a = 3: b = 100
c = a * b
or does it mean that every operation involving 'Byte` variables would have to yield only a result be between 0 and 255 regardless of the recipient variable type?
or does it mean that every operation involving byte variables would have to yield only a result be between 0 and 255 regardless of the recipient variable type
Yes, because Bytes only hold values from 0 to 255, multiplying 3 x 100, you are passing (overflowing) its capacity, even though afterwards you are passing the result into an integer.
Because you are multiplying two Bytes together, VBA assumes the result to be a Byte too. It is only after the calculation that the result is then cast to an integer.
To get around this, you must cast at least one of the variables. This lets VBA know that it must make room for a larger result:
Dim a As Byte, b As Byte
Dim c As Integer
a = 3
b = 100
c = a * CInt(b) ' <-- Cast b as Integer to prevent Overflow error
In the do while loop structure usually there's a part where you declare a variable equal to a number (in this case i) and then in a second part you make a increment (i+1). I've made this example in vba, but the structure could be repeated in several different programming languages like the for in php when you're getting data from a database. Now, what I would like to understand better is the relation between the previous mentioned declarations, that is i = some number and i = i + 1 . Wouldn't this generate a problem of interpretation since you're declaring a variable to something and then assigning a different value right after it? Is the second declaration of the variable value, i = i + 1, a new variable calling the previous one or both i's are the same? This is the general orientation I intend with this question. I think explaining the scoop of both variables would help understanding. Thanks!
Sub DoWhile()
Dim x, i, sum
x = 10
i = 1
sum = 0
Do While i < x
sum = sum + i
i = i + 1
Loop
MsgBox “Sum = ” & sum
End Sub
A variable is really just a location in memory. That location can have any value. By setting i=i+1, you're really saying "take the value at position i, add 1 to it, and store it at position i". No new variable is created. There's no problem with the computer interpreting this- what it cares about is the location of i, which isn't changing. It still knows where to find i, regardless of how many times you change the value there.
Since you have created the variable i as a global variable, any reference or modification to i in the sub will be on the same variable. That being said:
Dim i as int
i = 1
Do while i < 11
MsgBox("The value of i is: " & i)
i = i + 1
Loop
would display 10 messageboxes showing the value of i being between 1 and 10.
When the program encounters i = i + 1, the computer 'sees' this as take the value of i, add one to it, and store the result in the variable i.
Hope that helps.
Is that possible for me to do nested loop in VB with same counter
The code is somehow like this
For a As Integer = 1 to Console.ReadLine
For a = 1 to a
Console.WriteLine("*")
Next
Console.WriteLine()
Next
The program is designed for drawing a triangle of * with just a single variable at all
VB just disallow me to use a in nested loop again
Error: ...Variable 'a' is alreay used by a independent loop.
I have my own usage, can only use 1 variable.
What changing the second FOR loop to a WHILE loop?
For a As Integer = 1 to Console.ReadLine
Do While a <=5
Console.WriteLine("Line: " & a)
Exit Do
Loop
Next
Here's a different idea. You may consider splitting your integer variable into 2 parts 16-bit parts, keep user's input in the upper 16-bits, and current iteration value in the lower 16-bits (you'll need to use WHILE instead of FOR).
In fact, what you need is to start your inner counter by the value of a, if I've understand. And what you are doing is create another loop inside starting by 1.
For a As Integer = 1 to Console.ReadLine
For b As Integer = a to 5
Console.WriteLine("Line: " & a)
Next
Next
You cannot declare another variable with the same name in the same scope. But the inner loop is in the same scope as the outer loop. That's why you get that compiler error.
You can use a different name:
Dim i As Int32
If Int32.TryParse(Console.ReadLine, i) AndAlso i > 0 Then
For a As Integer = 1 To i
For aa = 1 To i
Console.WriteLine("Line: {0} {1}", a, aa)
Next
Next
End If
To draw a triangle, as you describe, you need two variables, like this:
For a As Integer = 1 to Console.ReadLine
For b As Integer = 1 to a
Console.Write("*")
Next
Console.WriteLine()
Next
If you used the same variable in the inner loop, the inner loop would change the value of the variable, which in most cases would not be what you want, and in all cases would be incredibly confusing. For that reason, VB forces you to use a different iterator in each nested For loop.