[Solved] Is it possible to change the default rounding method in Python?

Question

In one of the programming puzzle in codeabbey, it asks the numbers to be rounding in "Rounding half away from zero".
In short, if the fractional part of x is exactly 0.5, then y = x + 0.5 if x is positive, and y = x − 0.5 if x is negative.
Example 1: 123.5 becomes 124
Example 2: -123.5 becomes -124
https://en.wikipedia.org/wiki/Rounding#Rounding_half_away_from_zero

Python round() method uses the "banker's rounding", which is not the behavior I want. To overcome the issue, I get the fractional part by f = abs(number - int(number)), and then use math module ceil / floor plus some conditional check to make the rounding.

However, the site says it uses the same rounding algorithm in other puzzles unless explicitly specified. It would be a pain to re-implement the above method.

Is there any setting we can change the default rounding behavior in Python?
Or there is a built-in library can achieve the same result?

The puzzle link:
https://www.codeabbey.com/index/task_view/rounding

Answer 1

we cannot change the rounding method of the built-in rounding function...
in the links, they show some examples for modules that allow for different procedures and examples on how to implement them yourself. Not sure, how many of these modules are pre-installed.

https://stackoverflow.com/a/51146310
https://stackabuse.com/rounding-numbers-in-JUMP_LINK__&&__python__&&__JUMP_LINK/
https://realpython.com/python-rounding/
https://medium.com/@akhilnathe/understanding-pythons-round-function-from-basics-to-bankers-b64e7dd73477

Answer 2

Lisa, thanks. It seems "decimal.ROUND_HALF_UP" is what I need.

Answer 3

Bob_Li
This is the method I used in the puzzle.

tmp = a / b
f = abs(tmp - int(tmp))
if tmp < 0:
if f >= 0.5:
res.append(math.floor(tmp))
else:
res.append(math.ceil(tmp))
else:
if f >= 0.5:
res.append(math.ceil(tmp))
else:
res.append(math.floor(tmp))

Your's looks better, and I haven't considered about the use of %0.5

Answer 4

Wilbur Jaywright
I want the number to round up if:
1. It is a positive number;
2. The fractional is exactly 0.5

And round down if it is a negative number (orher condition unchanged).

Maybe this code shows it better.

https://sololearn.com/compiler-playground/cYpMlu2rKybz/?ref=app

Answer 5

Wong Hei Ming you're right. It should have been:

from decimal import localcontext,Decimal,ROUND_HALF_UP

def rhu(a, b):
with localcontext() as ctx:
ctx.rounding=ROUND_HALF_UP
return (Decimal(a)/Decimal(b)).to_integral_value()

print(rhu(12, 8))
print(rhu(11, -3))
print(rhu(400, 5))
#check odd/even unit, <=>±0.5
#odd unit place
print(rhu(5, -4))
print(rhu(6, -4))
print(rhu(7, -4))
print(rhu(5, 4))
print(rhu(6, 4))
print(rhu(7, 4))
#even unit place
print(rhu(9, -4))
print(rhu(10, -4))
print(rhu(11, -4))
print(rhu(9, 4))
print(rhu(10, 4))
print(rhu(11, 4))

Answer 6

Bob_Li
a = Decimal('0.1')
That is what b does, using f-string to create a value (0.05 + 0.05).

Using f-string with Decimal would be handy if the puzzle needs to manipulate the input and the result needs rounding.

Answer 7

Bob_Li
The way you create a Decimal object can lead to unexpected result.

from decimal import Decimal
a = Decimal(0.1)
b = Decimal(f'{0.05 + 0.05}')
print(a + a + a)
# 0.3000000000000000166533453694
print(b + b + b)
# 0.3

Answer 8

Wong Hei Ming
so in your example, it should be
a = Decimal(1)/Decimal(10)
or
a = Decimal('0.1')
print(a+a+a) # 0.3

Answer 9

Not sure what behavior you do want, but try combinations of int and adding or subtracting 0.5?

Answer 10

Wong Hei Ming define your custom round function. %can be used for floating point numbers, so you can use it to detect 0.5, by
n%0.5==0

import math

def round_away(a, b):
n = a/b
if n>=0 and n%0.5==0:
return math.ceil(n)
elif n%0.5==0:
return math.floor(n)
else:
return round(n)

print(round_away(12, 8))
print(round_away(11, -3))
print(round_away(400, 5))
#check odd/even unit, <=>±0.5
#odd unit place
print(round_away(5, -4))
print(round_away(6, -4))
print(round_away(7, -4))
print(round_away(5, 4))
print(round_away(6, 4))
print(round_away(7, 4))
#even unit place
print(round_away(9, -4))
print(round_away(10, -4))
print(round_away(11, -4))
print(round_away(9, 4))
print(round_away(10, 4))
print(round_away(11, 4))

Answer 11

Wong Hei Ming
Lisa 's decimal.ROUND_HALF_UP is nice. I had to do a bit of searching on how to use it, though.

from decimal import localcontext,Decimal,ROUND_HALF_UP

def rhu(a, b):
with localcontext() as ctx:
ctx.rounding=ROUND_HALF_UP
return Decimal(a/b).to_integral_value()

print(rhu(12, 8))
print(rhu(11, -3))
print(rhu(400, 5))
#check odd/even unit, <=>±0.5
#odd unit place
print(rhu(5, -4))
print(rhu(6, -4))
print(rhu(7, -4))
print(rhu(5, 4))
print(rhu(6, 4))
print(rhu(7, 4))
#even unit place
print(rhu(9, -4))
print(rhu(10, -4))
print(rhu(11, -4))
print(rhu(9, 4))
print(rhu(10, 4))
print(rhu(11, 4))

Answer 12

Wong Hei Ming No, it is not possible to directly change the default rounding method in Python's built-in round() function.

If you need a specific rounding behavior, you can implement your own rounding logic using other mathematical functions in Python's math library. For example, you can achieve rounding away from zero by using a combination of abs(), int(), and conditional statements.