Dividing large number by 10 in python gives the wrong result

Question

Say I want to remove the last digit of a number n. For that I use the code int(n/10).

Sadly this gives the wrong result for large numbers. For example n = 4474630975855204960 divided by 10 gives 447463097585520512.

What's the reason for this behavior? How can I fix it?

Try to multiply by 0.1, for large numbers perhaps is better to use some libraries that are more accurate. — Sachihiro, Dec 23 '20 at 01:00
The `/` operator returns a float value which is imprecise. Use `//` instead. — Michael Butscher, Dec 23 '20 at 01:01
https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html — Mitch Wheat, Dec 23 '20 at 01:01
@JohnGordon @Julien Integer division with `//` instead of `/` does not work for this. — Nico G., Dec 23 '20 at 01:07
@JohnGordon Ok I tried `print("%.1f" % (n//10))` and it didn't work. `print(n//10)` does work. — Nico G., Dec 23 '20 at 01:12
You could use the `decimal` module. `decimal.Decimal("4474630975855204960").rotate(-1)` — tdelaney, Dec 23 '20 at 01:29
`%.1f` is floating point format, so you have the same original problem. — John Gordon, Dec 23 '20 at 15:55

score 7 · Accepted Answer · answered Dec 23 '20 at 01:08

7

For some math operations, the Python interpreter will handle long integers for you and you don't have to think about it.

Division is different and converts integers to floats, which aren't handled well in Python.

You can get around this by directly using integer division - two // rather than just one /.

Input

4474630975855204960//10

Output

447463097585520496

answered Dec 23 '20 at 01:08

Ryan Ward

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10

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No language handles floats any better; they are a fixed-precision approximation of real numbers. – chepner Dec 23 '20 at 01:12

score 3 · Answer 2 · answered Dec 23 '20 at 01:08

This syntax varies across python versions, use // to get integer division

$ python2 -c 'print("%d" % (4474630975855204960/10))'  # Integer division
447463097585520496
$ python3 -c 'print("%d" % (4474630975855204960/10))'  # Float division
447463097585520512
$ python3 -c 'print("%d" % (4474630975855204960//10))'  # True division
447463097585520496

score 1 · Answer 3 · answered Dec 23 '20 at 01:02

1

You can convert from int to str, remove the last character, and convert it back to an int:

n = 4474630975855204960

print(int(str(n)[:-1]))

Output:

447463097585520496

answered Dec 23 '20 at 01:02

Red

26,798
7
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That's a good solution to my problem, but it does not answer why the division is wrong. – Nico G. Dec 23 '20 at 01:08
@NicoG. Oh, sorry. I'm sure that the link below your post covers it though :) – Red Dec 23 '20 at 01:10
2

This is 10x slower than simply dividing by 10 and harder to read. – chepner Dec 23 '20 at 01:18

Dividing large number by 10 in python gives the wrong result

3 Answers3