Conversion from hex to signed int not working

user_5549711 0
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I'm trying to convert this hex string F1 into a signed integer.
So I should get as result -15

I have this Python function right here which works good, but somehow doesn't work for this string F1

Code: 

def get_signed_value(value):
    return -(value & 0x8000) | (value & 0x7fff)

If I pass for example FF5F as value, then I get -161 as response which is correct.
But if I pass F1 I get 241 as response which is not the correct value.

hex python-3.x python
1 Answers

your function is designed for a 16-bit number with a value range of āˆ’32.768 (0x8000) to 32.767 (0x7fff), while 0xf1 is only a 8-bit number with a value range of -128 (0x80) to 127 (0x7f). Thus, the return statement in the function doesn't apply correctly.

You have to change the function according to what is the number of bits in your hex number. For 8-bit:

def get_8bit_signed_value(value):
    return -(value & 0x80) | (value & 0x7f)

Then

get_8bit_signed_value(int('0xf1', 16))

returns your desired -15 for 0xf1. You can automate this function for bitnumbers with nbits digits by calculating the value range limits by 1 << (nbits-1)

user_103093420
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