Rounding Decimals in Python

Rounding Decimals in Python

Introduction

Whether you're working with financial data, scientific calculations, or any other type of data that requires precise decimal arithmetic, knowing how to round decimal numbers accurately can make all the difference. In Python, there are various methods for rounding digits, each with its unique pros and cons.

In this article, we'll take a look at the different methods of rounding decimals in Python, and offer tips and best practices to help you get a better understanding of rounding decimals in this programming language. We'll discuss round() and format() functions, as well as the decimal module.

Using the round() Function

The first approach for rounding decimals in Python we'll discuss in this article is using the round() function. It is a built-in Python function that allows you to round numbers to a specified number of decimal places. It takes two arguments - number and ndigits:

round(number, ndigits=None)

The number argument is the decimal number that you want to round, and the ndigits argument (optional) specifies the number of decimal places to round to.

Note: If ndigits is not specified, round() will round the number to the nearest integer.

Now, let's take a look at a simple example. Say we have the following decimal number:

x = 3.14159

And say we want to round x to two decimal places. We'll use the round() function with ndigits set to 2:

rounded_x = round(x, 2)

The rounded_x variable would now hold the value 3.14, as expected.

Rounding Halfway Cases

That's all fine, but what if we want to round any of the halfway cases (numbers that end in .5). In that case, the round() function uses a "round half to even" algorithm. This means that the function rounds to the nearest even number. For example, round(2.5) will round down to 2, andround(3.5) will round up to 4.

Note: It's worth noting that this behavior can lead to unexpected results when rounding large sets of numbers. If you need to round a large set of numbers and want to avoid bias towards the even numbers, you may want to consider using another rounding method.

Rounding Floating Point Numbers

By default, most of the decimal numbers in Python are internally stored as a float data type. That means decimal numbers are actually stored as floating point numbers. That method of storing decimal numbers is known for approximating actual numbers that you want to store due to the limitations of the discrete nature of computers in general. Meaning that the infinite number of decimal numbers has to be somehow stored as an array of definite numbers of bits (zeros and ones).

To illustrate that, let's take a look at the following decimal number:

x = 3.175

It's fair to assume that if we round this number to 2 decimal places, the resulting number would be 3.18, right?

rounded_x = round(x, 2)
print(rounded_x)

But, as we can see form the output, that's actually not the case:

3.17

This shows the difficulties we face when working with floating point numbers in general - not all numbers are accurately stored. In this example, the number 3.175 was actually stored as 3.17499999999999982236431605997495353221893310546875 which explains why it was rounded down to 3.17 instead of 3.18, which we expected.

All-in-all, the round() function is certainly the most common method for rounding digits in Python, and it's usually suitable for most use cases. However, if you need more control over the rounding method, you may want to consider using the decimal module or the format() method instead.

Using the format() Method

The format() method is another built-in Python function that we can use to round decimals. It works by formatting a given number as a string, and then manipulating the string to display the desired number of decimal places.

Note: Obviously, this is not ideal if you want to actually work with numbers, but can be great approach if you need to display your rounded number in a specific way.

Now, let's take a look at the syntax of the format() method:

"{:.nf}".format(number)
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The n section specifies the number of decimal places we want to round to. The number argument is the decimal number that you want to round.

Consider the same decimal number we used in the previous section:

x = 3.14159

Let's use the format() to round x to two decimal places:

rounded_x = "{:.2f}".format(x)

The rounded_x variable would now hold the value 3.14.

Examples

Take a look at a few more examples of using the format() method to round decimals:

# Round to the nearest integer
x = 3.14159
rounded_x = "{:.0f}".format(x)
# rounded_x is 3

# Round to one decimal place
x = 3.14159
rounded_x = "{:.1f}".format(x)
# rounded_x is 3.1

# Round to two decimal places
x = 3.14159
rounded_x = "{:.2f}".format(x)
# rounded_x is 3.14

One advantage of using the format() method over the round() function is that you can control the formatting of the rounded number more precisely. You can use the format() method to add leading zeros, commas for thousands separators, and other formatting options.

On the other hand, the format() method has some limitations. For example, it may not always produce the expected result when rounding halfway cases, and it may not be suitable for large sets of numbers. In these cases, you should probably consider using the decimal module instead.

Using the decimal Module

The decimal module is a Python module that provides support for working with decimal numbers. It offers a way to perform accurate decimal arithmetic, which floating point numbers can't perform.

To use the decimal module for rounding decimals, you first need to create a Decimal object that represents the decimal number you want to round. Then, to round the number, you can then use the quantize() method of the Decimal object:

Decimal(number).quantize(Decimal('.nf'))

Here, n is the number of decimal places to round to, and number is the decimal number that you want to round.

Let's take a look at the example number we've used in previous sections - 3.14159 and round it to two decimal places using the decimal module:

from decimal import Decimal

x = Decimal(3.14159)
rounded_x = x.quantize(Decimal('.01'))

The rounded_x variable would now hold the value 3.14.

A Few More Examples

Here are a few more examples of using the decimal module to round decimals:

# Round to the nearest integer
x = 3.14159
rounded_x = Decimal(x).quantize(Decimal('1'))
# rounded_x is 3

# Round to one decimal place
x = 3.14159
rounded_x = Decimal(x).quantize(Decimal('.1'))
# rounded_x is 3.1

# Round to two decimal places
x = 3.14159
rounded_x = Decimal(x).quantize(Decimal('.01'))
# rounded_x is 3.14

As you can see, the decimal module allows for precise rounding of decimal numbers, and it can be a good choice when accuracy is important. However, it can be slower than the other rounding methods, and it may require more code to use effectively. It's also worth noting that the decimal module may not always produce the expected result when rounding halfway cases, so you may want to test your code carefully to ensure that it behaves as expected.

Best Practices for Rounding Decimals in Python

  1. Know your requirements: Before choosing a method to round decimals, it's important to understand your requirements. Do you need exact decimal arithmetic, or is a rough approximation sufficient? Do you need to display the rounded number as a string, or do you need to perform further calculations with it? Answering these questions can help you choose the right method for your needs.
  2. Use the built-in round() function for simple cases: The built-in round() function is the simplest way to round decimal numbers in Python, and it works well for most simple cases. Use it when you don't need precise decimal arithmetic, and when you don't need to perform further calculations with the rounded number.
  3. Use the format() method for more control: If you need more control over the formatting of the rounded number, use the format() method. It allows you to specify the number of decimal places to round to, and to control other aspects of the formatting as well.
  4. Use the decimal module for exact decimal arithmetic: If you need to perform exact decimal arithmetic, use the decimal module. It allows you to control the precision of the decimal calculations, and it provides a way to round decimal numbers to the desired number of decimal places.
  5. Be aware of rounding errors: Rounding decimal numbers can introduce rounding errors, especially when working with very large or very small numbers. Be aware of these errors, and test your code carefully to ensure that it produces the expected results.
  6. Avoid rounding halfway cases: Halfway cases occur when the number being rounded is exactly halfway between two possible rounded values. Rounding halfway cases can produce unexpected results, so it's generally better to avoid them whenever possible.
  7. Document your code: When rounding decimals in Python, it's important to document your code clearly. Explain why you are rounding the number, what method you are using, and any assumptions or limitations that apply. This can help ensure that your code is clear, correct, and maintainable over time.

Conclusion

In this article, we've covered the basics of rounding decimals in Python, exploring a few different methods for achieving the desired result. These include using the built-in round() function, the format() method, and finally the decimal module.

By following the best practices in this article, you can avoid rounding errors, choose the right method for your needs, and document your code for better clarity and maintainability.

All-in-all, rounding decimals may seem like a small detail, but it's definitely an important part of many Python applications.

Last Updated: March 28th, 2023
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