Android Question Wrong value in Long-variables?

[W. Graf]

Hi all,

at first: there's no hurry for my question!! I have a workaround. I only want to know, why this happens.

My question was also described by another user in 2013 too:
https://www.b4x.com/android/forum/threads/wrong-value-of-variable.31840/#post-185747

Another similar thread was there (Dec 2015):
https://www.b4x.com/android/forum/threads/variable-defined-as-long-acts-link-int.61817/#post-390078

But both threads couldn't answer my question (I didn't understand the answers).


I have the following sub:

Sub Test As Long
   Dim i64Result As Long
   Dim i32TempUpper As Int
   Dim i32TempLower As Int


   i32TempUpper = 2482035392
   i32TempLower = 136775


   'INCORRECT! -> Result = -7786483237554874368
   i64Result = (i32TempUpper * 4294967296) + i32TempLower


   'Correct Result = -7786483237554874809
   i64Result = (i32TempUpper * 4294967296)
   i64Result  = i64Result + i32TempLower

   Return i64Result
End Sub

In the old threads, the answer was, that I have to explicitly use long-variables in the formula to get the correct value. If I don't, then the compiler works with integers (32-bit), right?
So I think: because my variables are 32-Bit-integers, the compiler convert every result in 32-bit before doing the next math operation (the addition in my case).
Ok, I understand this.

But If I have a look on the binary bits of my results, the upper 32-bits are correct! Only the lower 32-bits are wrong. It seems, that the multiplication was correct and the addition fails. But if it is a convert-problem (long<->int), then the upper 32-bits (multiplication) should be wrong, not?

Bits of WRONG result (-7786483237554874368):
1001 0011 1111 0000 1101 1010 1100 0000
0000 0000 0000 0010 0001 1000 0000 0000

Bits of correct result (-7786483237554874809):
1001 0011 1111 0000 1101 1010 1100 0000
0000 0000 0000 0010 0001 0110 0100 0111

Again: I can work with my workaround. But I would be happy, if I could understand it

Thank you and have a great day!
Wolfgang

 

[Erel]

I've tested this code in both debug and release modes and the result was the same (the correct value):

Sub Activity_Create(FirstTime As Boolean)
   Dim i64Result As Long
   Dim i32TempUpper As Int
   Dim i32TempLower As Int
   i32TempUpper = 2482035392
   i32TempLower = 136775
   i64Result = (i32TempUpper * 4294967296) + i32TempLower
   Log(i64Result)
   'Correct Result = -7786483237554874809
   i64Result = (i32TempUpper * 4294967296)
   i64Result  = i64Result + i32TempLower
   Log(i64Result)
End Sub

 

[W. Graf]

Hi Erel,

in Release-Mode, I couldn't reproduce it. In Debug-Mode, you can see it. But it is not so easy to reproduce this. I'm very confused about the phenomenas.
It seems, that there is a cache-problem too.
I used a real (physical) device.
Please try this:

Sub Activity_Create(FirstTime As Boolean)
   Dim i64Result As Long
   Dim i32TempUpper As Int
   Dim i32TempLower As Int
   i32TempUpper = 2482035392
   i32TempLower = 136775
   i64Result = (i32TempUpper * 4294967296) + i32TempLower
   Log(i64Result)
   'Correct Result = -7786483237554874809
   i64Result = (i32TempUpper * 4294967296)
   i64Result  = i64Result + i32TempLower
   Log(i64Result)
End Sub

--> Start it, stop it and then start it again. Now you should get the same results (ending xxxx809).

So please change the code now:

Sub Activity_Create(FirstTime As Boolean)
   Dim i64Result As Long
   Dim i32TempUpper As Int
   Dim i32TempLower As Int
   i32TempUpper = 2482035392
   i32TempLower = 136775
   i64Result = (i32TempUpper * 4294967296) + i32TempLower
   Log(i64Result)
   'Correct Result = -7786483237554874809
   i64Result = (i32TempUpper * 4294967296)
   i64Result  = i64Result + i32TempLower
   Log(i64Result)
  
   Dim x As String
   x = "BlaBla" 
End Sub

--> Now start again, you will get two different results (ending xxxxx368 and xxxxx809).

Note: sometimes it was vice versa: the first test returns two different values, and the second test two identical values -> I have no idea?!

Thank you!
Wolfgang

 

[Erel]

This is a debugging issue, you should use long variables and it will work properly.

 

[W. Graf]

Thank you for your analysis and information.

I will follow your advice.

Have a great day!