Wrong answer from math.log(python 3)

Question

Today, I used math.log() function to get the logarithm of 4913 to the given base 17. The answer is 3, but when I ran the code below, I got 2.9999999999999996.

1) Is it because math.log(x, b)'s calculation is log(x) / log(b)?

2) Is there any solution to get the correct answer 3?

import math
print(math.log(4913,17))

Your conjecture (#1) is quite likely. As for #2, there are many methods. Do you know a priori that the answer will be an integer? — President James K. Polk, Mar 18 '19 at 13:28
Better duplicate: [Log precision in python](https://stackoverflow.com/questions/48042460/log-precision-in-python) — nwellnhof, Mar 18 '19 at 13:30
@nwellnhof i reopened the question as the OP also wants to know how to get an integer solution. — hiro protagonist, Mar 18 '19 at 13:32
Possible duplicate of [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Peter O., Mar 20 '19 at 07:43

hiro protagonist · Answer 1 · 2019-03-18T15:58:51.050

3

you could use the gmpy2 library:

import gmpy2

print(gmpy2.iroot(4913, 3))
# (mpz(17), True)

print(gmpy2.iroot(4913 + 1, 3))
# (mpz(17), False)

which tells you the result and whether or not it is exact.

also have a look at Log precision in python and Is floating point math broken?.

edited Mar 18 '19 at 15:58

answered Mar 18 '19 at 13:31

hiro protagonist

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Thank you so much for the answer! – Saisiot Mar 18 '19 at 22:34

score 2 · Accepted Answer · answered Mar 18 '19 at 13:39

2

Yes, the documentation says so explicitly.

Another solution is to use the Decimal class, from the "decimal" library:

import math
from decimal import Decimal, getcontext
getcontext().prec = 6
Decimal(math.log(4913))/Decimal(math.log(17))

answered Mar 18 '19 at 13:39

Itamar Mushkin

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or `Decimal(4913).ln() / Decimal(17).ln()` – Gabriel Mar 18 '19 at 13:46

Wrong answer from math.log(python 3)

2 Answers2