two's complement bytes

[Adam W.]

I'm dabbling with AVR's for a project I have and that means I have to
use C (ageist my will). Because my AVR will be tethered to my laptop,
I am writing most of my logic in python, in the hopes of using at
little C as possible.

In my quest I came across a need to pass a pair of sign extended two's
complement bytes. After painfully reading the wikipedia article on
what two's complement was, I then thought of how I would handle this
in python. I don't really recall ever having to work in binary with
python, so I really am clueless on what to do.

I can feed python either two hex bytes or binary, but how do I convert
it into an int, and more importantly how do I make sure it handles the
sign properly?

 

[castironpi]

I'm dabbling with AVR's for a project I have and that means I have to
use C (ageist my will).  Because my AVR will be tethered to my laptop,
I am writing most of my logic in python, in the hopes of using at
little C as possible.

In my quest I came across a need to pass a pair of sign extended two's
complement bytes.  After painfully reading the wikipedia article on
what two's complement was, I then thought of how I would handle this
in python.  I don't really recall ever having to work in binary with
python, so I really am clueless on what to do.

I can feed python either two hex bytes or binary, but how do I convert
it into an int, and more importantly how do I make sure it handles the
sign properly?

Try this out. Does it come close to what you want?

import struct
struct.pack( 'i', ~10 )
~struct.unpack( 'i', _ )[ 0 ]

import struct
struct.pack( 'i', ~10 ) '\xf5\xff\xff\xff'
~struct.unpack( 'i', _ )[ 0 ] 10

 

[Adam W.]

Try this out.  Does it come close to what you want?

import struct
struct.pack( 'i', ~10 )
~struct.unpack( 'i', _ )[ 0 ]

import struct
struct.pack( 'i', ~10 ) '\xf5\xff\xff\xff'
~struct.unpack( 'i', _ )[ 0 ]

10- Hide quoted text -

- Show quoted text -

Humm, so how do you use it Let me give you some examples and then
you can run it through:

0b1111110010010000 or 0xFC90 Should equal -880
0b0000011111010000 or 0x07D0 Should equal +2000

 

[castironpi]

Try this out.  Does it come close to what you want?

import struct
struct.pack( 'i', ~10 )
~struct.unpack( 'i', _ )[ 0 ]

import struct
struct.pack( 'i', ~10 ) '\xf5\xff\xff\xff'
~struct.unpack( 'i', _ )[ 0 ]

10- Hide quoted text -

- Show quoted text -

Humm, so how do you use it  Let me give you some examples and then
you can run it through:

0b1111110010010000 or 0xFC90  Should equal -880
0b0000011111010000 or 0x07D0  Should equal +2000

In this case I look at:

struct.unpack( '>h', '\xfc\x90' )[0] -880
struct.unpack( '>h', '\x07\xd0' )[0]

2000

 

[Adam W.]

Try this out.  Does it come close to what you want?
import struct
struct.pack( 'i', ~10 )
~struct.unpack( 'i', _ )[ 0 ]
import struct
struct.pack( 'i', ~10 )
'\xf5\xff\xff\xff'
~struct.unpack( 'i', _ )[ 0 ]
10- Hide quoted text -
- Show quoted text -

Humm, so how do you use it  Let me give you some examples and then
you can run it through:

0b1111110010010000 or 0xFC90  Should equal -880
0b0000011111010000 or 0x07D0  Should equal +2000

In this case I look at:

struct.unpack( '>h', '\xfc\x90' )[0] -880
struct.unpack( '>h', '\x07\xd0' )[0]

2000- Hide quoted text -

- Show quoted text -

Perfect, thank you! I will have to read up on struct to see how you
did that.