Rounding a float number in python

Question

I have a float numer

a = 1.263597

I hope get

b = 1.2635

But when I try

round (a,4)

then result is

1.2636

What should I do?

Why are you expecting "1.2635" when the normal rounding rules should get you "1.2636", since there's a "9" after the "5"? — jjramsey
– jjramsey, Commented Dec 6, 2022 at 16:24

Akshay Sehgal · Accepted Answer · 2022-12-06 16:20:57Z

1

Try math.floor with this small modification -

import math

def floor_rounded(n,d):
    return math.floor(n*10**d)/10**d

n = 1.263597
d = 4

output = floor_rounded(n,d)
print(output)

1.2635

For your example, you can just do math.floor(1.263597 * 10000)/10000

EDIT: Based on the valid comment by @Mark, here is another way of solving this, but this time forcing the custom rounding using string operations.

#EDIT: Alternate approach, based on the comment by Mark Dickinson

def string_rounded(n,d):
    i,j = str(n).split('.')
    return float(i+'.'+j[:d])

n = 8.04
d = 2

output = string_rounded(n,d)
output

8.04

edited Dec 6, 2022 at 16:20

answered Dec 6, 2022 at 8:43

Akshay Sehgal

19.4k3 gold badges26 silver badges57 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Mark Dickinson Over a year ago

Beware corner cases arising from the What You See Is Not What You Get nature of binary floating-point. E.g., floor_rounded(8.04, 2) gives 8.03 on my machine (and floor_rounded(8.03, 2) gives 8.02, so the rounding operation isn't even idempotent).

Akshay Sehgal Over a year ago

Thanks for the feedback! Completely agree with your point above, have added a method using string manipulations which should avoid this.

andthum · Accepted Answer · 2022-12-06 09:00:03Z

1

Plain Python without importing any libraries (even not standard libraries):

def round_down(number, ndigits=None):
    if ndigits is None or ndigits == 0:
        # Return an integer if ndigits is 0
        return int(number)
    else:
        return int(number * 10**ndigits) / 10**ndigits

a = 1.263597
b = round_down(a, 4)
print(b)

1.2635

Note that this function rounds towards zero, i.e. it rounds down positive floats and rounds up negative floats.

edited Dec 6, 2022 at 9:00

answered Dec 6, 2022 at 8:55

andthum

856 bronze badges

Comments

Farid · Accepted Answer · 2022-12-07 08:57:58Z

0

def round_down(number, ndigits=0):
    return round(number-0.5/pow(10, ndigits), ndigits)

Run:

round_down(1.263597, 4)
>> 1.2635

edited Dec 7, 2022 at 8:57

answered Dec 6, 2022 at 8:59

Farid

339 bronze badges

2 Comments

Thierry Lathuille Over a year ago

That's not a good idea. Rounding down, which seems to be what the OP wants, is not the same as rounding a smaller number.

Farid Over a year ago

Right, but there is a trick of adding/subtracting 0.5 to any number when wanting to round up/down that is useful for a fraction of half too

Collectives™ on Stack Overflow

Rounding a float number in python

3 Answers 3

2 Comments

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related