I'm working with Python and I would like to simulate the effect of a C/C++ cast on an integer value.
For example, if I have the unsigned number 234 on 8 bits, I would like a formula to convert it to -22 (signed cast), and another function to convert -22 to 234 (unsigned cast).
I know numpy already has functions to do this, but I would like to avoid it.
You could use bitwise operations based on a 2^(n-1) mask value (for the sign bit):
size = 8
sign = 1 << size-1
number = 234
signed = (number & sign-1) - (number & sign)
unsigned = (signed & sign-1) | (signed & sign)
print(signed) # -22
print(unsigned) # 234
You can easily create such a function yourself:
def toInt8(value):
valueUint8 = value & 255
if valueUint8 & 128:
return valueUint8 - 256
return valueUint8
>>> toInt8(234)
-22
You can make a version that accepts the number of bits as a parameter, rather than it being hardcoded to 8:
def toSignedInt(value, bits):
valueUint8 = value & (2**bits - 1)
if valueUint8 & 2**(bits-1):
return valueUint8 - 2**bits
return valueUint8
>>> toSignedInt(234, 8)
-22
