function round() doesn't work as I expect

Question

I am trying to use round function here. Sometimes it round down from .5 sometimes round up. So what's the problem?

Source file:

print("rounding up 0.5 is",round(.5))
print("rounding up 1.5 is",round(1.5))
print("rounding up 2.5 is",round(2.5))
print("rounding up 3.5 is",round(3.5))

Output:

rounding up 0.5 is 0
rounding up 1.5 is 2
rounding up 2.5 is 2
rounding up 3.5 is 4

This is python3 I suppose? Because it works fine for python2 — Willem Van Onsem, May 12 '15 at 19:46
Round-.5-to-even-digit is standard in many fields of math, especially statistics, so it's often the standard in many programming languages as well. — Lee Daniel Crocker, May 12 '15 at 19:47
duplicate: http://stackoverflow.com/questions/10825926/python-3-x-rounding-behavior — jojo, May 12 '15 at 19:50

score 5 · Answer 1 · answered May 12 '15 at 19:46

5

From the docs:

if two multiples are equally close, rounding is done toward the even choice

So when you say rounding up, it's not necessarily rounding up. It's just rounding.

answered May 12 '15 at 19:46

TigerhawkT3

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*"It's just rounding"* This specific technique (rounding to even integers when at *.5) is called ["banker's rounding"](https://en.wikipedia.org/wiki/Rounding#Round_half_to_even) – Cory Kramer May 12 '15 at 19:49

function round() doesn't work as I expect

1 Answers1