Can someone explain to me why is numpy round acting strange with this exact number rounding:
df = pd.DataFrame({'c': [121921117.714999988675115, 445, 22]})
df = np.round(df['c'], 8)
Result:
121921117.71499997
445.0
22.0
Expected:
121921117.71499999
445.0
22.0
It’s obvious that the first number is not rounded well, any ideas?
EDIT: Since I’m focused here on the precision, not so much on the performance, I’ve used python round function to solve this problem:
df.applymap(round, ndigits=8)
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Answer
Check the small print 2 in the documentation of round aka around.
The short answer is that round “uses a fast but sometimes inexact algorithm” and to use format_float_positional if you want to see
the correct result.
>>> import pandas as pd
>>> df = pd.DataFrame({'c': [121921117.714999988675115, 445, 22]})
>>> df["c"][0]
121921117.71499999
>>> round(df["c"][0],8)
121921117.71499997
>>> np.format_float_positional(df["c"][0],8)
'121921117.71499999'