Tie breaking of round with numpy

Question

Standard numpy round tie breaking is following IEEE 754 convention, to round half towards the nearest even number. Is there a way to specify different rounding behavior, e.g. round towards zero or towards -inf? I'm not talking about ceil or floor, I just need different tie breaking.

Answer 1

NumPy doesn't give any control over the internal rounding mode. Here's two alternatives:

1. Use gmpy2, as outlined in this answer. This gives you full control over the rounding mode, but using gmpy2 for simple float math is likely to be slower than NumPy.

2. Use fesetround via ctypes to manually set the rounding mode. This is system-specific because the constants may vary by platform; check fenv.h for the constant values on your platform. On my machine (Mac OS X):

   import numpy as np
   import ctypes
   FE_TONEAREST = 0x0000
   FE_DOWNWARD = 0x0400
   FE_UPWARD = 0x0800
   FE_TOWARDZERO = 0x0c00
   libc = ctypes.CDLL('libc.dylib')
   
   v = 1. / (1<<23)
   print repr(np.float32(1+v) - np.float32(v/2)) # prints 1.0
   libc.fesetround(FE_UPWARD)
   print repr(np.float32(1+v) - np.float32(v/2)) # prints 1.0000002