Python round() — A Simple Guide with Video

Python’s built-in round() function takes two input arguments: a number and an optional precision in decimal digits. It rounds the number to the given precision and returns the result. The return value has the same type as the input number—or integer if the precision argument is omitted. Per default, the precision is set to 0 digits, so round(3.14) results in 3.

Usage

Learn by example! Here are some examples of how to use the round() built-in function.

>>> round(3.14)
3
>>> round(3.14, ndigits=1)
3.1
>>> round(3.13, ndigits=-1)
0.0
>>> round(4458.1242, ndigits=-1)
4460.0
>>> round(3.14159, ndigits=3)
3.142

Syntax round()

You can use the round() method as follows:

Syntax:
round(number, precision)    # rounds the number to the given precision and returns the result 
ArgumentnumberNumerical value—in most cases a float.
Argumentndigits(Optional) Precision in number of decimal digits. Default 0.
Return ValuenumberNumerical value rounded to the given precision.

Video round()

Interactive Shell Exercise: Understanding round()

Consider the following interactive code:

Exercise: Guess the output before running the code.


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How to Round All Values in a List in Python?

Problem Formulation: Given a list of numerical values. How to round all values in the list to a given precision?

To round all values of a list, use list comprehension [round(x, ndigits=precision) for x in list] to apply the round() function to all values in the list and place the resulting numerical values into a new list:

lst = [3.14, 3.44444, 4.424242]
rounded = [round(x, ndigits=2) for x in lst]
print(rounded)
# [3.14, 3.44, 4.42]

Python round() vs NumPy round()

Python’s round() function rounds a single numerical value to a single rounded value, given a certain precision. NumPy‘s np.round() function can do the same but more: if you pass an array or list of numerical values, it’ll round all values in the passed array-like data structure. In addition to that, in contrast to Python’s built-in round() function that returns an integer if no precision argument is given, the function np.round() always returns a float.

>>> import numpy as np
>>> lst = [3.14, 3.44444, 4.424242]
>>> round(lst[0])
3
>>> np.round(lst[0])
3.0
>>> np.round(lst)
array([3., 3., 4.])

Summary

Python’s built-in round() function takes two input arguments:

  • a number, and
  • an optional precision in decimal digits.

It rounds the number to the given precision and returns the result.

>>> round(3.14, ndigit=1)
3.1
>>> round(3.14)
3

The return value has the same type as the input number—or integer if the precision argument is omitted. Per default, the precision is set to 0 digits, so round(3.14) results in 3.

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