Python Round Function

Last Updated on February 28, 2023 by Prepbytes

Assume you go to a store and purchase many goods, and the total bill has two decimal places. Have you ever wondered how such a large bill gets rounded off? The round function is used for rounding off those values. In Python, the round function is used to round integers up to a specified number of digits after the decimal point

In this article, we will explore the syntax and parameters of the round() function, and provide some examples of how to use it in different contexts.

Python Round() Function

In Python, the round() function is used to round a number to a specified number of decimal places or to the nearest multiple of a specified value. It can be used with both positive and negative numbers. The round() function is often used in mathematical and financial applications where precision is important. Since it’s a built-in Python function, it does not require the import of any modules.

Syntax of Python Round Function

The syntax for the round() function is as follows:

round(number, ndigits)

Parameters of Python Round Function

The round() function takes two parameters:

• number (required) – The number that you want to round. This can be an integer, a floating-point number, or a complex number.
• ndigits (optional) – The number of decimal places to round to. If not specified, it defaults to 0.

Return Value of Python Round Function

The Python round function returns a floating-point number that represents the rounded value of the input number.

Examples of Python Round Function

Here are some examples of how to use the Python round function in different contexts:

Example 1: Basic usage of round()
In its simplest form, the round() function can be used to round a number to the nearest integer. For example

Code:

num = 4.6
result = round(num)
print(result)

Output:

5

Explanation:
In this example, we have a floating-point number num with the value 4.6. We call the round() function on num and store the result in the variable result. The round() function rounds the number num to the nearest integer, which in this case is 5. The result variable now contains the rounded value.

Example 2: Rounding to a specified number of decimal places
The round() function can also be used to round a number to a specified number of decimal places. For example

Code:

num = 3.14159
result = round(num, 2)
print(result)

Output:

3.14

Explanation:
In this example, we have a floating-point number num with the value 3.14159. We call the round() function on num and pass in 2 as the second argument, which specifies that we want to round num to 2 decimal places. The round() function rounds num to 3.14, which is the closest number to num with 2 decimal places. The result variable now contains the rounded value.

Example 3: Rounding to the nearest multiple
The round() function can also be used to round a number to the nearest multiple of a specified value. For example

Code:

num = 17
result = round(num, -1)
print(result) 

Output:

20

Explanation:
In this example, we have an integer num with the value `17’.We call the round() function on num and pass in -1 as the second argument, which specifies that we want to round num to the nearest multiple of 10. The round() function rounds num to 20, which is the closest multiple of 10 to 17. The result variable now contains the rounded value.

Example 4: Rounding negative numbers
The round() function can also be used to round negative numbers. However, the behavior of the round() function with negative numbers can be a bit unexpected. For example

Code:

num = -4.5
result = round(num)
print(result) # Output: -4

Output:

-4

Explanation:
In this example, we have a negative floating-point number num with the value -4.5. We call the round() function on num and store the result in the variable result. The round() function rounds the number num to the nearest integer, which in this case is -4. The result variable now contains the rounded value.

If we had instead rounded num to 1 decimal place:

Code:

num = -4.5
result = round(num, 1)
print(result)

Output:

4.5

Explanation:
In this example, we call the round() function on num and pass in 1 as the second argument, which specifies that we want to round num to 1 decimal place. The round() function rounds num to -4.5, which is the closest number to num with 1 decimal place. The result variable now contains the rounded value.

It’s worth noting that the behavior of the round() function with negative numbers can be different on different platforms and versions of Python. If you need precise control over how negative numbers are rounded, it’s best to use a third-party library like NumPy.

Precision Loss

One important thing to keep in mind while using the round function in Python is that it can cause a precision loss in floating-point numbers. This is because floating point numbers are represented in a binary format and not all decimal numbers can be represented exactly in binary. This can lead to rounding errors in calculations involving floating point numbers.

For example, let’s say we want to add the following two numbers:

Code:

a = 0.1
b = 0.2
c = a + b
print(c)

Output:

0.30000000000000004

Explanation:
As you can see, the result is not exactly 0.3 due to precision loss.

To avoid precision loss, it is recommended to use the decimal module in Python. This module provides a Decimal class that allows us to perform calculations with a specified level of precision.

Conclusion
In conclusion, the round function in Python is a very useful built-in function that allows us to round off a number to a specified number of digits. It can be used in many different ways to perform mathematical operations or display output in a desired format. It is important to keep in mind that the round function can cause a precision loss in floating point numbers, and in such cases, the decimal module should be used instead to ensure accurate results.

Frequently Asked Questions (FAQs)

Here are some Frequently Asked Questions on Python Round Function:

Ques 1. What happens if the ndigits parameter is negative?
Ans. If the ndigits parameter is negative, the number is rounded off to the left of the decimal point. For example, if ndigits is -2, the number is rounded off to the nearest hundred.

Ques 2. How can I specify the rounding mode to use with the round function?
Ans. By default, the round function uses the round half up rounding mode. However, you can specify a different rounding mode by using the decimal module in Python.

Ques 3. Can the round function be used with complex numbers?
Ans. No, the round function cannot be used with complex numbers. It only works with real numbers.

Ques 4. Does the round function modify the original number?
Ans. No, the round function does not modify the original number. It returns a new rounded number.

Ques 5. Can the round function be used with strings?
Ans. No, the round function only works with numeric values. If you want to round off a string, you need to convert it to a numeric value first.

Ques 6. What happens if the ndigits parameter is not specified?
Ans. If the ndigits parameter is not specified, the number is rounded off to the nearest integer.

Ques 7. What is the default rounding mode used by the round function?
Ans. The default rounding mode used by the round function is round half up.

Ques 8. Can the round function cause a precision loss in floating point numbers?
Ans. Yes, the round function can cause a precision loss in floating point numbers. This is because floating point numbers are represented in a binary format and not all decimal numbers can be represented exactly in binary. This can lead to rounding errors in calculations involving floating point numbers.

Ques 9. How can I avoid precision loss when using the round function with floating point numbers?
Ans. To avoid precision loss, it is recommended to use the decimal module in Python. This module provides a Decimal class that allows us to perform calculations with a specified level of precision.