Problem in FOR loop ... Is it a BUG ?!!

Question

Hello all ,

I discovered that error by chance when I was working on Collections. But I found it in FOR loop in VB.Net.

Have a Look :

Sub Main()
Dim Y As Integer = 10
For x As Integer = 0 To Y
Console.WriteLine(x.ToString())
Y = Y - 1
Next
Console.ReadLine()
End Sub
'and the result was
0
1
2
3
4
5
6
7
8
9
10
'and that is not logically correct !!
'the Correct result should be :
0
1
2
3
4
5
'as Y decreased every step.

In other words : why FOR loop limit is not updated every step!!!! ? Is it a Bug or What ?!


thanks in advance !!

Answer 1

By design.  Try increasing the lower limit within the loop.

Answer 2

The ending condition of VB for loops is always evaluated just once before the looping begins.  This is different from the 'C-style' languages, such as C#, C++, and Java, where the ending condition is constantly re-evaluated.

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Convert between VB, C#, C++, and Java (http://www.tangiblesoftwaresolutions.com)

Answer 3

 Hello ,

@JohnWein: I didn't geet it , what do u mean !?

@David Anton : I got it recently , and i think it is not wise logic , Is it ?! why did they make it like that ?!

Answer 4

I'm not sure, but it's consistent.  The VB 'For' loop is very static - there is nothing executed in the actual 'For' statement after the initial evaluation.  In C#/C++/Java, the initializer is evaluated just once, but the other two statements (usually used for the ending condition and loop incrementer, although you can put any statements there) are executed on every iteration. 

---

Convert between VB, C#, C++, and Java (http://www.tangiblesoftwaresolutions.com)

Answer 5

mmm , I'm not talking about "the actual 'For' statement" ,it was in the code block !! and actually it raises OutOfRange exceptions when u use it like that with collections , or some logic errors just like above snippet !!

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MCP (Developing Web Applications ) MCP (Developing Windows Applications ) MCP (Designing and Implementing Databases with Microsoft® SQL Server™ 2000)

Answer 6

Hi The_knight5000,

As is often said in these forums, VB.Net is NOT C is NOT C# and is NOT C++



Expect different things to happen in VB.Net



However you do raise an interesting point.

You can only affect the START and END values of a FOR NEXT loop in VB.Net
 from outside the loop it seems as I have just tried it.

Dim sb As New System.Text.StringBuilder

Dim LoopStart As Integer = 0

Dim LoopEnd As Integer = 10

For x As Integer = LoopStart To LoopEnd

'You can NOT change LoopStart or LoopEnd within
' a FOR NEXT loop within Vb.Net as I have just tried it.
sb.Append(x.ToString & " ")
Next x

MessageBox.Show(sb.ToString)




Then try this.>>

Option Strict On
Public Class Form1

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

Dim sb As New System.Text.StringBuilder
Dim LoopStart As Integer = 0
Dim LoopEnd As Integer = 10

Do
  For x As Integer = LoopStart To LoopEnd
    'You can NOT change LoopStart or LoopEnd within
    ' a FOR NEXT loop within Vb.Net
    sb.Append(x.ToString & " ")
  Next x
LoopEnd = LoopEnd - 1
Loop Until LoopStart = LoopEnd

MessageBox.Show(sb.ToString)
End Sub
End Class




Regards,

John

---

For links to VB.Net tutorials see here.>> http://social.msdn.microsoft.com/Forums/en-US/vblanguage/thread/29f2179b-997b-4115-a96d-a0834853b835

Answer 7

The_knight5000 said:

mmm , I'm not talking about "the actual 'For' statement" ,it was in the code block !! and actually it raises OutOfRange exceptions when u use it like that with collections , or some logic errors just like above snippet !!

---

MCP (Developing Web Applications ) MCP (Developing Windows Applications ) MCP (Designing and Implementing Databases with Microsoft® SQL Server™ 2000)

Iterating through a collection isn't the same as a for/next loop.

Your 'correct' loop probably arises from your familiarity with C style languages (which VB programmers may consider to be 'incorrect'). If you can answer this question "Which is the correct side of the road to drive a car?" you have answered your above question :)

---

Stephen J Whiteley

Answer 8

Good Evening,

Hey all , I'm not trying to avoid that or asking for alternative way to pass this situation in my code , I just asking why is For loop in VB.Net like that ? , and I'm wondering why do MS Developers Consist on that way also in the newest version ?!!


@John Anthony Oliver: really thanks for ur interest and I know I can't change the limits of the loop inside the loop block :s , and I still asking why ?!
btw , ur snippet does not solve the problem , It just involve code into nested loops !


SJWhiteley :

SJWhiteley said:

Iterating through a collection isn't the same as a for/next loop.

Your 'correct' loop probably arises from your familiarity with C style languages (which VB programmers may consider to be 'incorrect'). If you can answer this question "Which is the correct side of the road to drive a car?" you have answered your above question :)

---

Stephen J Whiteley

Ok man , It may be different  , But we don't care about what we are looping through , I talking here about the FOR Statement itself.

I think loops in Vb or C# or whatever the tool should use the same logic , and Actually i think this VB logic  in implementing for statement is not right .

when I'm talking about collections , I meant some thing like that :

Dim arr As New ArrayList(4)

arr.AddRange(New Integer() {1, 2, 3, 4, 5})

For i As Integer = 0 To arr.Count - 1

Console.WriteLine(arr(i))

arr.RemoveAt(arr.Count - 1)

Next

it should update the end limit (arr.count) every time it loops , but this does not happen , it catch the arr.Count at the first time only , and it will be not updated ..... why ?this is my questing and i think that is incorrect logic !!!!!!!


thanks

---

MHamdy
MCP (Developing Web Applications )
MCP (Developing Windows Applications )
MCP (Designing and Implementing Databases with Microsoft® SQL Server™ 2000)

Answer 9

VB has always performed loops like that. This implies a consistency. And honestly, consistency is a lot more important than correctness (whatever that may be perceived to be), in general.

As for your example, it demonstrates the consistency: once the loop is set, it is set. it does update the count, but in your example, as we have said, the For/Next loop does not re-evaluate its bounds once you have established those bounds.

I suspect, for the foreseeable future, this is how VB loops will always execute. Remember, you have a choice of languages; although they operate on the same framework there are quirks which differentiate them.

Really, asking for a change like this (breaking millions of applications) is not going to happen - until they remove the useless semicolons from C ;)

Seriously, though, this is quite a useful feature - it means you can perform a time consuming/complex operation to calculate your loop bounds (for example, establishing a record count in a dataset), without it having to be recalculated through each iteration.

For i as integer = ComplexOperationLower to ComplexOperationUpper
    ' Do Some More Stuff without reevaluating the ComplexOperationUpper or ComplexOperationLower
next

Or, if that doesn't satisfy you: it is what it is; Like it or lump it; If you can't stand the heat; horses for courses, etc. Personally, I use VB because I can't stand the quirks and inconsistencies of C.

---

Stephen J Whiteley

Answer 10

sjw is correct, and the point about limit being a complicated formula is well taken.  if you need it both ways

Dim aY As Integer = 10

Dim ax As Integer

For ax = 1 To aY 'tell For I want to do something 10 times

Debug.WriteLine(ax.ToString())

aY -= 1

If aY = 5 Then Exit For 'oh, but not really, i meant 5 times

Next

'or

Dim aZ As Integer = 5

ax = 1 : aY = 10

Do While aY <> 0 AndAlso aY <> aZ

Debug.WriteLine(ax.ToString)

ax += 1

aY -= 1

Loop

Answer 11

SJWhiteley said:

Or, if that doesn't satisfy you: it is what it is; Like it or lump it; If you can't stand the heat; horses for courses, etc. Personally, I use VB because I can't stand the quirks and inconsistencies of C.

---

Stephen J Whiteley

Hi again The_knight5000,

Despite what has been previously said, and you having 3 MCP qualifications, I have to agree with Stephen J Whiteley and anyone else who might write; "Like it or lump it"

You can not honestly expect consistencies between ALL programming languages, surely?

If Visual Basic has always been this way since Visual Basic version 1.00 , then why should Microsoft change things in Visual Basic? Besides the BASIC programming language was invented in 1964 according to http://en.wikipedia.org/wiki/Basic_programming_language so that makes it 44 years old ( 1 year older than me!! ).

C was created later in 1972 according to http://en.wikipedia.org/wiki/C_programming_language some 8 years later.

What I am really getting that though is,

if FOR NEXT loops in BASIC have always been like this then why should anyone change the programming language specification and introduce a potential bug - problem?


Regards,

John

---

For links to VB.Net tutorials see here.>> http://social.msdn.microsoft.com/Forums/en-US/vblanguage/thread/29f2179b-997b-4115-a96d-a0834853b835

Answer 12

Hello All

SJWhiteley said:

Or, if that doesn't satisfy you: it is what it is; Like it or lump it

John Anthony Oliver said:

if FOR NEXT loops in BASIC have always been like this then why should anyone change the programming language specification and introduce a potential bug - problem?

Actually I don't know what to say , that is really disappointing !!

Java , PHP , C , C# ... Implements Dynamic for Loop (as I can call) , and it is already stable and consistent !!
and you just should know this Static For loop in VB has a logic error !!

this is the reply of Jonathan Aneja on My Bug Report:

Hi there,

This behavior is by design, as documented in the help topics. VB has several different looping constructs that provide a lot of flexibility here - in this case you could use a "Do While" or "Loop Until" if you want the ability to dynamically change the upper bound.

I'm going to resolve this bug as "by design" for now, but if you have any questions feel free to contact me directly .

Thanks,

Jonathan Aneja
Program Manager, VB Team

thanks all , I just wanted to report about this !

regards,

MHamdy

---

MHamdy
MCP (Developing Web Applications )
MCP (Developing Windows Applications )
MCP (Designing and Implementing Databases with Microsoft® SQL Server™ 2000)

Answer 13

"Actually I don't know what to say , that is really disappointing !!

Java , PHP , C , C# ... Implements Dynamic for Loop (as I can call) , and it is already stable and consistent !!
and you just should know this Static For loop in VB has a logic error !!"

it is well documented (imagine reading the instructions), but so few do.  if you want the code to do what java, c, et al do then write in those languages

imho - for's are used to do something a specific amount of times.  what if the loop was hundred's of lines of code?  imagine debugging that.  the problem is really in your original construct, which probably should have been a do loop.

i got on my soap box once about 12:00 AM PM NOON MIDNIGHT, and even though i was correct, i lost. so my advice is do whatever you need to do to get over your disappointment, and move along.

"Give us grace to accept with serenity the things that cannot be changed, courage to change the things that should be changed, and the wisdom to distinguish the one from the other."

Answer 14

I think everyone has provided a broad, well-rounded explanation and discussion of the topic and there is little more to be squeezed from it.  From my perspective you can be clumped in with the masses who find fault with VB (or down-right hate it) because it does not do the things that their favorite language(s) do.  If you post this on the C# forum you might find an equal chorus willing to agree with you about this.

Answer 15

dbasnett said:

The problem is really in your original construct, which probably should have been a do loop.

Hi ALL,

I agree with this too.

You can achieve the same as the OP The_knight5000 was expecting like this.>>

Option Strict On

Public Class Form1

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

Dim sb As New System.Text.StringBuilder
Dim Y As Integer = 10
Dim x2 As Integer = 0

'The result you are after with a DO LOOP.>>
Do
sb.Append(x2.ToString & Environment.NewLine)
x2 = x2 + 1
Y = Y - 1
Loop Until Y < 5

MessageBox.Show(sb.ToString)


sb = New System.Text.StringBuilder
Dim x3 As Integer = 0
Y = 10

'The result you are after with a WHILE LOOP.>>
While Y > 4
sb.Append(x3.ToString & Environment.NewLine)
x3 = x3 + 1
Y = Y - 1
End While

MessageBox.Show(sb.ToString)

End Sub
End Class

---

For links to VB.Net tutorials see here.>> http://social.msdn.microsoft.com/Forums/en-US/vblanguage/thread/29f2179b-997b-4115-a96d-a0834853b835

Answer 16

i already posted a do loop example, in a lot less space ;)

Answer 17

Thankfully, these kind of 'discussions' pop up a lot less than they used to with VB6/C++.

Historically (and currently?) VBers get very defensive when 'their' language is attacked for being wrong, and C programmers tend to see themselves as better for using an obviously superior language.

Unfortunately, it'll probably always be this way: VB has a lot of baggage associated with it. Some are just quirks, some are actually better and more logical methodologies compared to C, and some should just go away (but won't).

With VB and C# being so comparable, the differences generally fall down to the quirks and taste. Additionally, as has been posted, there is more than one way to do a specific task.

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Stephen J Whiteley