The round() Function with a Pandas Series

Let's find out how the round operator works in pandas.

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Try it yourself

Try executing the code below to see the result.

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import pandas as pd
s = pd.Series([-2.5, -1.5, -0.5, 0.5, 1.5, 2.5])
print(s.round())

Explanation

Rounding is easy most of the time. For example, round(1.1) evaluates to 1 and round(1.8) evaluates to 2.

The question is, how do we round the .5.5 numbers? Should we round up or round down? Turns out, there are a lot of ways to do it.

Python 3 uses bankers’ rounding. In this type of rounding, odd numbers are rounded up, and even numbers are rounded down.

Bankers' Rounding

Number

Rounded Number

3.5

4.0

2.5

2.0

4.2

4.0

-2.0

-2.0

-3.7

-4.0

2.6

3.0

The reasoning behind this method is that if we round a list of numbers, the errors will cancel each other out assuming there are roughly the same number of odd and even numbers.

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