Default Boolean Value

Question

I am using a boolean embedded in a for next loop. I thought the value would ALWAYS default to false inside this loop, but it seems to hold the value of true inside the loop, even when moving to another value of i. What's up with this?

For i as integer = 0 to 5
   Dim b as boolean
   .... use boolean value later...
Next i

Answer 1

b is a local variable like other variables outside the loop. b is just not visible outside the loop.

The following two methods are functionally absolutely equal:

 

   Sub test1()

      For i = 0 To 5
         Dim var As Boolean
         var = True
      Next

      For i = 0 To 5
         Dim var As Integer
         var = 17
      Next

   End Sub

   Sub test2()

      Dim var1 As Boolean
      Dim var2 As Integer

      For i = 0 To 5
         var1 = True
      Next

      For i = 0 To 5
         var2 = 17
      Next

   End Sub

---

Armin

Answer 2

I am using a boolean embedded in a for next loop. I thought the value would ALWAYS default to false inside this loop, but it seems to hold the value of true inside the loop, even when moving to another value of i. What's up with this?

For i as integer = 0 to 5
   Dim b as boolean
   .... use boolean value later...
Next i

Hi Derek,

The value of b as a Boolean can be whatever you decide to make it inside your loop.

Try this code with one Button on a Form and note the results please.

The variable b will only be accessible within your loop.

 

 

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        For i As Integer = 0 To 5
            Dim b As Boolean

            If i Mod 2 = 0 Then
                b = True
            Else
                b = False
            End If

            MessageBox.Show("i = " & i.ToString & "     b = " & b.ToString)

        Next i

    End Sub

End Class

 

---

---

Regards,

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Answer 3

I am using a boolean embedded in a for next loop. I thought the value would ALWAYS default to false inside this loop, but it seems to hold the value of true inside the loop, even when moving to another value of i. What's up with this?

For i as integer = 0 to 5
   Dim b as boolean
   .... use boolean value later...
Next i

Hi Derek,

That will only happen if you do not change it from FALSE to TRUE as in this code.>>

 

Public Class Form1

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        For i As Integer = 0 To 5
            Dim b As Boolean

            MessageBox.Show("i = " & i.ToString & "     b = " & b.ToString)

        Next i

    End Sub

End Class

---

---

Regards,

---

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---

Answer 4

Fascinating. I completely thought that because it Re-Dim'd the variable, it would reset it to the default. Can't believe I missed that one. It really is doing a Redim Preserve.

Answer 5

Well, ReDim out of an array context doesn't make much sense. :-) But I guess I know what you mean.
You just have to be aware of where the variable is located: There's only the stack or the heap. Locals are on the stack. Consequently they don't lose their value until the function returns.

EDIT:
A declaration is not an executable statement. It's just there to make the compiler generate the code to reserve space for locals at function entry. You see it if you single-step through the code: The declaration line is not executed.
I'm already hearing someone saying that, for example, "Dim x(5) as integer" is an executable statment. Yes, but that's just a short for declaration + initilization in one line (actually Dim x() As Integer <CR> Redim x(5) ). Executable is only the initilization.

---

Armin

Answer 6

Hi Armin,

        'Dim x(5) As Integer
        'Could be doing:>>

        Dim x() As Integer = CType(Array.CreateInstance(GetType(Integer), 6), Integer())

        'Not neccessarily:>
        ' Dim x() As Integer
        ' ReDim x(5)

---

---

Regards,

---

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Answer 7

Fascinating. I completely thought that because it Re-Dim'd the variable, it would reset it to the default. Can't believe I missed that one. It really is doing a Redim Preserve.

Hi Derek,

It seems that on setting a breakpoint with

Dim b As Boolean

is setting up the variable 'b' once within a loop.

However if you do this:>>

 

Dim b As Boolean = New Boolean

it forces b to be declared again in each iteration of the loop.

I hope this helps.  :)

Try out this example which creates 6 Buttons but I only have

Dim btn As Button = New Button

in the code once.

 

Option Strict On
Option Explicit On
Option Infer Off

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        Button1.Location = New Point(5, 5)

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        For i As Integer = 0 To 5

            Dim b As Boolean = New Boolean
            Dim btn As Button = New Button

            btn.Size = New Size(100, 20)

            With btn
                btn.Text = i.ToString
                btn.Location = New Point(150, i * (btn.Height + 10))
            End With
            Me.Controls.Add(btn)

        Next

    End Sub

End Class

 

---

---

Regards,

---

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Answer 8

You are showing another way of creating an array. I understand, but did I miss the point?

---

Armin

Answer 9

Well, ReDim out of an array context doesn't make much sense. :-) But I guess I know what you mean.
You just have to be aware of where the variable is located: There's only the stack or the heap. Locals are on the stack. Consequently they don't lose their value until the function returns.

EDIT:
A declaration is not an executable statement. It's just there to make the compiler generate the code to reserve space for locals at function entry. You see it if you single-step through the code: The declaration line is not executed.
I'm already hearing someone saying that, for example, "Dim x(5) as integer" is an executable statment. Yes, but that's just a short for declaration + initilization in one line (actually Dim x() As Integer <CR> Redim x(5) ). Executable is only the initilization.

---

Armin

Hi Armin,

That depends on whether the declaration contains the keyword NEW or not.

If the declaration contains an = sign it looks like it is executed too, as in this loop:>>

        For i As Integer = 0 To 5

            Dim b As Boolean = False

        Next

Try the above with a breakpoint on the loop and then remove the

= False

Please try my code in the previous post.  :)

---

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Regards,

---

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Answer 10

    "That depends on whether the declaration contains the keyword NEW or not."

If it contains a new keyword, the logical line contains an initlization. The declaration itself is still not executable, only the initilization.

     dim bla as new form

is just a short for

    dim bla as form

    bla = new form

If you execute the latter, you see that the declartion is (still) not executable.

 

     "If the declaration contains an = sign it looks like it is executed too, as in this loop:"

Yes, but executable is the assignment, not the declaration.

 

 

---

Armin

Answer 11

Hi Armin,

Agreed.  :-)

Yes but something still happens in a declaration like memory reserved or whatever.

( The memory pointer is set to NOTHING. ) Otherwise you would not be able to execute the assignment.  :)

 

Only after the assignment is the pointer set to wherever the value type or reference type is in memory.

---

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Regards,

---

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Answer 12

Hi John,

Yes but something still happens in a declaration like memory reserved or whatever.

( The memory pointer is set to NOTHING. ) Otherwise you would not be able to execute the assignment.  :)

Sure, the declaration makes something happen, i.e. makes the compiler generate the code for reserving stack space (as I wrote in my first message), though the declaration line is not an executable statement (as you see if you single-step).

For example, you can put the declaration later in the code:

1: Dim x as integer
2: x = 47
3: Dim y as integer
4: y = 12

Stack space is always allocated at function entry. This is true for both variable x and y. If "dim y as integer" was an executable statement, the order of execution would be line 1, 3, 2, 4, but that does not happen.

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Armin