Simple arithmetic question

[ebolisa]

Hi,

I'd like to round the number 12.13 to 12 and 12.45 to 13.
None of the following functions return the format I'm looking for. I guess I could use if then function to get around it but, I'm sure there's an easy way to solve this.

TIA

import math

a = 12.13
b = 12.45

print(round(a), round(b))  # incorrect
print(math.ceil(a), math.ceil(b))  # incorrect
print(math.floor(a), math.floor(b))  # incorrect
print(math.trunc(a), math.trunc(b))  # incorrect

[deanhystad]

What is the logic of your rounding? 12.45 should round to 12.

[ebolisa]

What is the logic of your rounding? 12.45 should round to 12.

Because 12.45 should be 12.5 and therefore, 13.
12.13 should be 12.1 and therefore, 12.

More info: here

[Gribouillis]

Because 12.45 should be 12.5 and therefore, 13.

I have never seen this strange way of rounding numbers. Here you are rounding to the nearest dime, then to the nearest dollar. This is not the method described by the page that you linked above. In this page they say that one can round to the nearest dollar or to the nearest cent but they don't suggest to use one after the other.

Note that the results returned by round() are not what you could expect in limit cases

>>> round(11.5)
12
>>> round(12.5)
12
>>> round(13.5)
14
>>> round(14.5)
14
>>> round(15.5)
16
>>> round(16.5)
16

You could replace this by

from math import floor
def myround(x):
    return floor(x + 0.5)

>>> myround(11.5)
12
>>> myround(12.5)
13
>>> myround(13.5)
14
>>> myround(14.5)
15
>>> myround(15.5)
16

[bowlofred]

That page appears to not agree with your logic:

Round down for a dollar amount that has 0 to 49 cents. For example, $89.27 rounds down to $89.

So 12.45 should round down to 12 if you're rounding to whole dollars according to that page because 45 is between 0 and 49.

[deanhystad]

In Python 12.5 doesn't round to 13 either. Oops, I see Gribouillis included the odd case of 12.5 in his post.

Numerically your idea of cascaded rounding doesn't make any sense. Rounding should return the closest value and 12.45 is closer to 12 (0.45 difference) than it is 13 (0.55 difference). Your rounding has a positive bias of 0.05. I want my bank to use your rounding when calculating interest on my savings, but I don't want it used to calculate my taxes.