I need to take an integer (for example, -200) and change it into a hexadecimal (FF 38), then into an integer 0-255 to send via serial. The example I was given was:
-200 = hex FF38 = [hex FF] [hex 38] = [255] [56]
I've tried using struct.pack('>h', -200) but that returned the incorrect value. I also tried hex() but that returned a negative hex value that was incorrect as well.
I don't know what else I should be trying.
>>> hex(struct.unpack('>H', struct.pack('>h', -200))[0])
'0xff38'
First, you pack the -200 as a signed 2-byte short int. Then you unpack that data as an unsigned 2-byte short int. This gives you 65336, of which hex is ff38
If you need to send 0-255 integers, this might work better:
>>> struct.unpack('>BB', struct.pack('>h', -200))
(255, 56)
>>> struct.unpack('>BB', struct.pack('>h', 200))
(0, 200)
Where you're unpacking into two unsigned chars
Requisite documentation on struct
struct.error: integer out of range for 'H' format code
struct.pack('>h', n) converts an integer assert n.bit_length() <= 16 into two-byte sequence: '\xff\x38'. No other calls are necessary unless OP wants a decimal representation for some reason then map(str, bytearray(b'\xff\x38')) == ['255', '56']Two's complement arithmetic and some string slicing:
hex(200 - (1 << 16)) #2 bytes -> 16 bits
Out[26]: '-0xff38'
int(hex(200 - (1 << 16))[-4:-2], 16)
Out[28]: 255
int(hex(200 - (1 << 16))[-2:], 16)
Out[29]: 56
Note that you'll have to take care to correctly handle non-negative numbers (i.e. don't take the two's complement) and when the length of the string returned by hex is of varying length.
b'255'(three bytes) or just a as a byte:b'\xff'(single byte)?