Android Question Why does this byte loop fail?

RB Smissaert · Jun 8, 2020

Could somebody tell why this fails?

B4X:

Sub TestByteLoop
 
 Dim i As Byte
 Dim arrBytes(128) As Byte
 
 For i = 1 To 127
  Log(i)
  arrBytes(i) = i '<< how can i go past 127 here?
  Log(arrBytes(i))
  'If i = 127 Then Exit 'this fixes it, but why is it needed?
 Next
 
End Sub

RBS

LucaMs · Jun 8, 2020

RB Smissaert said:

Could somebody tell why this fails?

B4X:

Sub TestByteLoop

Dim i As Byte
Dim arrBytes(128) As Byte

For i = 1 To 127
  Log(i)
  arrBytes(i) = i '<< how can i go past 127 here?
  Log(arrBytes(i))
  'If i = 127 Then Exit 'this fixes it, but why is it needed?
Next

End Sub

RBS

I don't understand the problem. If you declare your array that way (128) it will have positions (index) between 0 and 127.

LucaMs · Jun 8, 2020

LucaMs said:
I don't understand the problem.

Understood; this is worse:

B4X:

Sub aaaaa
    Dim i As Byte
    For i = 0 To 255
        If i > 127 And i < 131 Then
            Log(i)
        End If
    Next
End Sub

LucaMs · Jun 8, 2020

It seems that despite this documentation:

For Next loops only work with Int or higher (long)

Thabo Maqelepo · Jun 8, 2020

RB Smissaert said:

Could somebody tell why this fails?

B4X:

Sub TestByteLoop

Dim i As Byte
Dim arrBytes(128) As Byte

For i = 1 To 127
  Log(i)
  arrBytes(i) = i '<< how can i go past 127 here?
  Log(arrBytes(i))
  'If i = 127 Then Exit 'this fixes it, but why is it needed?
Next

End Sub

RBS

Your array has a has a static width (128) if you want more, initialize it with a larger value 256, 512 etc etc

LucaMs · Jun 8, 2020

B4XBytesBuilder

RB Smissaert · Jun 8, 2020

LucaMs said:
B4XBytesBuilder

I am using that and it is very useful, but not sure how that answers the question. I understand that byte is poor choice for a loop iterator, but how does i become -128 if the loop limit is set at 127?

RBS

agraham · Jun 8, 2020

Bytes are signed values in Java/B4X so have a maximum positive value of 127 and a minimum negative value of -128. The loop exits when the loop counter is incremented past the limit. If you increment a byte value of 127 it overflows to -128.

RB Smissaert · Jun 8, 2020

agraham said:
Bytes are signed values in Java/B4X so have a maximum positive value of 127 and a minimum negative value of -128. The loop exits when the loop counter is incremented past the limit. If you increment a byte value of 127 it overflows to -128.

Yes, I understand that, but why does it enter one more iteration if the iterator has already reached 127?
You can see this if you step through the code in debug mode.

RBS

agraham · Jun 8, 2020

agraham said:
The loop exits when the loop counter is incremented past the limit.

The test for exiting the loop is greater (or less) than, not equality. The counter is incremented (or decremented) at the end of the loop and the test then made whether to continue the loop or not.

RB Smissaert · Jun 8, 2020

agraham said:
The test for exiting the loop is greater (or less) than, not equality. The counter is incremented (or decremented) at the end of the loop and the test then made whether to continue the loop or not.

Still don't get this. If the counter is -128 it is outside the specified range and it still continues.

RBS

agraham · Jun 8, 2020

In an incrementing loop the test is greater than (in a decrementing loop it is less than). -128 is not greater than 127 so the loop continues.

RB Smissaert · Jun 8, 2020

agraham said:
In an incrementing loop the test is greater than (in a decrementing loop it is less than). -128 is not greater than 127 so the loop continues.

OK, thanks, I thought it would consider the range.
All cleared up then.

RBS

Sagenut · Jun 8, 2020

Can the problem be that i is declared as Byte and then used as the For variable?
Maybe just use a different variable in For Cycle.

techknight · Jun 8, 2020

Ya all bytes in Java are signed. Unlike C where you can declare a uint8

Also your index needs to start at 0, not 1. so 0 to 127.

RB Smissaert · Jun 8, 2020

techknight said:
Ya all bytes in Java are signed. Unlike C where you can declare a uint8

Also your index needs to start at 0, not 1. so 0 to 127.

> Also your index needs to start at 0, not 1

Why?

RBS

KMatle · Jun 8, 2020

The loop ittself is ok but you can't set a byte to more than 127

RB Smissaert said:

Could somebody tell why this fails?

B4X:

Sub TestByteLoop

Dim i As Byte
Dim arrBytes(128) As Byte

For i = 1 To 127
  Log(i)
  arrBytes(i) = i '<< how can i go past 127 here?
  Log(arrBytes(i))
  'If i = 127 Then Exit 'this fixes it, but why is it needed?
Next

End Sub

RBS

The loop is ok but you can't store a value >127 in a byte. So when you need to fill all 255 possible values make a loop from 0 to 254 and subtract 127 from i. Vice versa add 127 to the value to get unsigned bytes.

You can create a byte array as long as you want but the VALUES in it can be from -127 to +127. 127+127+0=255 different values unsigned.

techknight · Jun 8, 2020

RB Smissaert said:
> Also your index needs to start at 0, not 1

Why?

RBS

because arrBytes(0) would be unused otherwise. Unless this is your intent.

Without the context, I have no idea what youre trying to achieve.

RB Smissaert · Jun 8, 2020

techknight said:
because arrBytes(0) would be unused otherwise. Unless this is your intent.

Without the context, I have no idea what youre trying to achieve.

I copied it from a bit of real code where array element 0 needed to be untouched.
The Sub as posted doesn't do anything and all the array elements remain unused.

RBS

techknight · Jun 8, 2020

Android Question Why does this byte loop fail?

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