Cast a 10 bit unsigned integer to a signed integer in python

Question

I have a list of numbers in the range 0-1023. I would like to convert them to integers such that 1023 maps to -1, 1022 maps to -2 etc.. while 0, 1, 2, ....511 remain unchanged.

I came up with a simple:

def convert(x):
 return (x - 2**9) % 2**10 - 2**9

is there a better way?

• ashastral: In some programming languages, the % operator yields a negative (or zero) value for a negative left operand, so the code could perhaps be made clearer by changing (x - 2**9) to (x + 2**9). Aside from that, this seems like a reasonable implementation.

Answer 1

Naivest possible solution:

def convert(x):
 if x >= 512:
 x -= 1024
 return x

• John1024: Alternatively, reducing three lines to one: return x if x < 512 else x - 1024
• Brave Sir Robin: or return x - 1024 * (x >= 512)
• ashastral: @rmartinjak, multiplying by a boolean requires Python to do an extra cast, and as a result that solution is a bit slower than the other ones. Your original solution and John1024's seem to be roughly equivalent. I haven't done any rigorous testing yet, though.

Answer 2

def convert(x):
 return x - (x >> 9) * 1024

Using ctypes:

from ctypes import c_short
def convert(x):
 return c_short(x << 6).value >> 6

• yota: perfect answer, I may add some explanations

Answer 3

 max_uint_value = 512
 def unassigned_to_int(uint):
 return uint - max_uint_value if uint >= max_uint_value else uint