Rounding error in Python with non
I'm beginner in Python, and I have one question.
Why does rounding a number like 5.5, 7.5, (anything).5 with odd integer part applying round(num) work correctly (rule 5/4), but rounding number like (anything).5 with non-odd integer part by the same function returns just an integer part? (But if we add a little number like 0.000000001 to that decimal number it works correctly)
I mean the next:
round(9.5)
returns 10, and it's correct. But
round(8.5)
returns 8, and it isn't correct. And
round(8.5 + 0.0000000000001)
returns 9.
Why it works incorrect?
I use Python 3.2.2 at Windows.
Python 3.x, in contrast to Python 2.x, uses Banker's rounding for the round() function.
This is the documented behaviour:
[I]f two multiples are equally close, rounding is done toward the even choice (so, for example, both round(0.5) and round(-0.5) are 0, and round(1.5) is 2).
Since floating point numbers by their very nature are only approximations, it shouldn't matter too much how "exact" half-integers are treated – there could always be rounding errors in the preceding calculations anyway.
Edit : To get the old rounding behaviour, you could use
def my_round(x):
return int(x + math.copysign(0.5, x))
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