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Effortlessly Round Numbers Up in Python


How to Round Numbers in Python

By David Amos

With Python’s powerful data science ecosystem becoming increasingly popular for data analysis, it is essential to understand how to avoid introducing bias into datasets. One aspect of this is understanding rounding bias and how it can impact your data. In this tutorial, we will cover various rounding strategies in Python and how they can be implemented. We will also explore how rounding affects data and which rounding strategy minimizes the impact. Additionally, we will discuss rounding in NumPy arrays and pandas DataFrames, and when to apply different rounding strategies.

Python’s Built-in round() Function

Python provides a built-in round() function that allows you to round numbers. The function takes two arguments: n and ndigits, and returns n rounded to ndigits. If ndigits is not provided, the number is rounded to an integer. Let’s take a closer look at how the round() function works.

Rounding Algorithm

The traditional method of rounding a number involves shifting the decimal point and considering the first decimal digit. If the digit is less than 5, the number is rounded down; otherwise, it is rounded up. To use this algorithm with Python’s round() function, you multiply the number by 10^p (10 raised to the power of p), round the resulting number to the nearest integer, and then divide it by 10^p to shift the decimal point back. Let’s see an example:

>>> round(2.5)

In this case, 2.5 is rounded to the nearest whole number, which is 3. Similarly, if we want to round 1.64 to one decimal place, we can do the following:

>>> round(1.64, 1)

The value 1.64 is rounded to one decimal place, resulting in 1.6.

While this algorithm works for simple rounding, there are other strategies available that may better suit specific situations.

Better Rounding Strategies in Python

Python offers various rounding strategies to handle different scenarios. Let’s explore some of these strategies and how to implement them in Python.

Rounding Half Up

The “round half up” strategy involves rounding the number up when the digit in the first decimal place is five or greater. For example:

>>> round(2.5)
>>> round(2.56)

In this case, 2.5 is rounded up to 3. Similarly, 2.56 is also rounded up to 3.

Rounding Half Down

The “round half down” strategy is the opposite of “round half up.” It rounds the number down when the digit in the first decimal place is five or greater.

>>> round(2.5)
>>> round(2.56)

Here, 2.5 is rounded down to 2, and 2.56 is also rounded down to 2.

Rounding Half Away From Zero

The “round half away from zero” strategy rounds the number away from zero when the digit in the first decimal place is five or greater.

>>> round(2.5)
>>> round(2.56)
>>> round(-2.5)
>>> round(-2.56)

In this case, 2.5 and 2.56 are both rounded up to 3. However, -2.5 and -2.56 are rounded down to -3.

Rounding Half to Even

The “round half to even” strategy, also known as “banker’s rounding,” rounds the number to the nearest even number when the digit in the first decimal place is five. This strategy aims to minimize the bias introduced by rounding.

>>> round(2.5)
>>> round(3.5)

Here, 2.5 is rounded down to 2, while 3.5 is rounded up to 4.

These rounding strategies can be useful in different scenarios, depending on your requirements and the type of data you’re working with.

Rounding NumPy Arrays and pandas Series

In addition to rounding individual numbers, it is often necessary to round entire arrays or series of numbers. Python libraries like NumPy and pandas provide convenient functions to achieve this.

Rounding NumPy Arrays

To round NumPy arrays, you can use the numpy.round() function. This function allows you to specify the number of decimals to round to.

import numpy as np
arr = np.array([1.234, 2.567, 3.891])
rounded_arr = np.round(arr, 1)

The output of this code will be:

[1.2 2.6 3.9]

The values in the array are rounded to one decimal place.

Rounding pandas Series and DataFrame

For rounding pandas Series and DataFrame objects, you can use the Series.round() and DataFrame.round() methods, respectively. These methods allow you to specify the number of decimals to round to.

import pandas as pd
data = {
'A': [1.234, 2.567, 3.891],
'B': [4.321, 5.654, 6.987]
df = pd.DataFrame(data)
rounded_df = df.round(1)

The output of this code will be:

0 1.2 4.3
1 2.6 5.7
2 3.9 7.0

The values in both the Series and DataFrame are rounded to one decimal place.


Understanding how to round numbers in Python is crucial for avoiding bias in datasets. By implementing different rounding strategies, you can minimize the impact of rounding on your data. Python’s built-in round() function, as well as libraries like NumPy and pandas, provide convenient tools for rounding numbers. By applying the appropriate rounding strategy, you can ensure the accuracy and integrity of your data analysis.

Additional Resources

For more information on Python rounding and related topics, please refer to the following resources:

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