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Storing a negative SmallInterger in a ByteArray

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Mispunt

Storing a negative SmallInterger in a ByteArray

Hi people,

I am a bit confused...
I am working on an implementation to communicate with the Lego
Mindstorms NXT. Al works fine, except I have to store some negative
numbers in the ByteArray that should be send to the NXT. Everytime I
try, I get an improper store into indexable object.

Is it possible to store negative values in a ByteArray?

Mispunt
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Lukas Renggli

Re: Storing a negative SmallInterger in a ByteArray

> I am a bit confused...
> I am working on an implementation to communicate with the Lego
> Mindstorms NXT. Al works fine, except I have to store some negative
> numbers in the ByteArray that should be send to the NXT. Everytime I
> try, I get an improper store into indexable object.

The elements of a ByteArray are unsigned bytes, a value between 0 and 255.

> Is it possible to store negative values in a ByteArray?

There are several helper methods implemented in ByteArray. Have a look
at the code I sent you, the NxtConnection>>#send: primitive makes use
of these methods:

doubleAt: index put: value bigEndian: bool
longAt: index put: value bigEndian: aBool
shortAt: index put: value bigEndian: aBool
unsignedLongAt: index put: value bigEndian: aBool
unsignedShortAt: index put: value bigEndian: aBool

Cheers,
Lukas

--
Lukas Renggli
http://www.lukas-renggli.ch
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Bert Freudenberg

Re: Storing a negative SmallInterger in a ByteArray

In reply to this post by Mispunt

On Mar 5, 2007, at 11:01 , Mispunt wrote:

> Hi people,
>
> I am a bit confused...

Maybe you just missed the classes where they talked about bits,
bytes, words, and how they relate to numbers? ;)

> I am working on an implementation to communicate with the Lego
> Mindstorms NXT. Al works fine, except I have to store some negative
> numbers in the ByteArray that should be send to the NXT. Everytime I
> try, I get an improper store into indexable object.
>
> Is it possible to store negative values in a ByteArray?

Sure. It's just bits after all. There are no "negative" bits, a bit
is 0 or 1. It depends on the interpretation of the bit pattern you
are sending, the receiving side might interpret some bit patterns as
negative values and others as positive values.

The most common convention is to use "two's complement", see

http://en.wikipedia.org/wiki/Two's_complement

E.g., in 8-bit two's complement ("signed char" in C), the bit pattern
"11111111" stands for -1, whereas as an 8-bit "unsigned char" with
the same "11111111" pattern would be interpreted as 255.

You just have to find out how negative values are represented in the
NXT format, like signed chars, signed shorts, signed int or whatever.

Then use the appropriate accessor to put the right bit pattern for
the value into the byte array:

byteArray signedByteAt: byteOffset put: value "8 bits"
byteArray signedShortAt: byteOffset put: value "16 bits"
byteArray signedLongAt: byteOffset put: value "32 bits"

Note these methods use the FFI plugin because they are designed to
work with C code.

- Bert -

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Mispunt

Re: Storing a negative SmallInterger in a ByteArray

Thanks Bert it workt with the Two's complement. It is possible to do:
-50 + 256 to get the correct number.

Lukas, you probably need to do the same thing because you can't move
the robot backwards.

Mispunt.

On 3/5/07, Bert Freudenberg <[hidden email]> wrote:

> On Mar 5, 2007, at 11:01 , Mispunt wrote:
>
> > Hi people,
> >
> > I am a bit confused...
>
> Maybe you just missed the classes where they talked about bits,
> bytes, words, and how they relate to numbers? ;)
>
> > I am working on an implementation to communicate with the Lego
> > Mindstorms NXT. Al works fine, except I have to store some negative
> > numbers in the ByteArray that should be send to the NXT. Everytime I
> > try, I get an improper store into indexable object.
> >
> > Is it possible to store negative values in a ByteArray?
>
> Sure. It's just bits after all. There are no "negative" bits, a bit
> is 0 or 1. It depends on the interpretation of the bit pattern you
> are sending, the receiving side might interpret some bit patterns as
> negative values and others as positive values.
>
> The most common convention is to use "two's complement", see
>
> http://en.wikipedia.org/wiki/Two's_complement
>
> E.g., in 8-bit two's complement ("signed char" in C), the bit pattern
> "11111111" stands for -1, whereas as an 8-bit "unsigned char" with
> the same "11111111" pattern would be interpreted as 255.
>
> You just have to find out how negative values are represented in the
> NXT format, like signed chars, signed shorts, signed int or whatever.
>
> Then use the appropriate accessor to put the right bit pattern for
> the value into the byte array:
>
> byteArray signedByteAt: byteOffset put: value "8 bits"
> byteArray signedShortAt: byteOffset put: value "16 bits"
> byteArray signedLongAt: byteOffset put: value "32 bits"
>
> Note these methods use the FFI plugin because they are designed to
> work with C code.
>
> - Bert -
>
>
> _______________________________________________
> Beginners mailing list
> [hidden email]
> http://lists.squeakfoundation.org/mailman/listinfo/beginners
>

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Lukas Renggli

Re: Storing a negative SmallInterger in a ByteArray

> Lukas, you probably need to do the same thing because you can't move
> the robot backwards.

Hehe, yes. I actually never tried that code with an actual robot,
that's probably why it is broken ;-)

Cheers,
Lukas

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Lukas Renggli
http://www.lukas-renggli.ch
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