Different ways to round numbers in Python

There are several ways to round numbers in Python. We can use the built-in round function to handle simple use cases, the Math module, or the NumPy library to handle anything more sophisticated.

Using Python's round function

Python's round function uses the "round half to even" strategy. That means if the number being rounded is precisely halfway between two rounded numbers, the rounding is done to the nearest even number.

round(2.5) #would result in 3
round(3.5) #would result in 4

Here are some examples using the round() function

1. Rounding to a specific number of decimal places

num = 3.141592653589793 
rounded_num = round(num, 2)
print(rounded_num) # Output: 3.14

2. Rounding to the nearest number

num = 3.7 
rounded_num = round(num) 
print(rounded_num) # Output: 4

num = 3.3 
rounded_num = round(num) 
print(rounded_num) # Output: 3

3. Rounding to the nearest multiple of a given value

base = 5
num = 17
print(base * round(num/base)) # Output: 15

Rounding with Python's Math module

Python's math module offers several functions for rounding that offer more flexibility over the round function.

1. Rounding up with the ceil() function

import math
num = 3.1
rounded_num = math.ceil(num)
print(rounded_num)  # Output: 4

2. Rounding down with the floor() function

import math
num = 3.9
rounded_num = math.floor(num)
print(rounded_num)  # Output: 3

3. Rounding to the nearest integer with the trunc() function

The trunc() function truncates the decimal points of the number

import math
num = 3.7
rounded_num = math.trunc(num)
print(rounded_num)  # Output: 3

4. Rounding to the nearest multiple of a given value using the ceil() and floor() functions

import math
num = 17
base = 5
rounded_num = math.floor(num / base) * base  # round down to nearest base
print(rounded_num)  # Output: 15

rounded_num = math.ceil(num / base) * base  # round up to nearest base
print(rounded_num)  # Output: 20

Rounding with Python's Numpy library

The numpy library offers several methods to round numbers. The numpy doesn't come with the default Python installation. I used the below command to install it.

pip3 install numpy

1. Rounding to a specific number of decimal places using the round() function

import numpy as np
num = 3.141592653589793
rounded_num = np.round(num, 3)  # round to 3 decimal places
print(rounded_num)  # Output: 3.142

2. Rounding to the nearest integer using the rint() function

import numpy as np
num = 3.7
rounded_num = np.rint(num)
print(rounded_num)  # Output: 4.0

num = 3.5
rounded_num = np.rint(num)
print(rounded_num)  # Output: 4.0

num = 3.4
rounded_num = np.rint(num)
print(rounded_num)  # Output: 3.0

Note that the rint() function uses the "half-to-even" rounding approach.

3. Rounding up to the nearest integer using the ceil() function

import numpy as np
num = 3.01
rounded_num = np.ceil(num)
print(rounded_num)  # Output: 4.0

4. Rounding down to the nearest integer using the floor() function

import numpy as np
num = 3.99
rounded_num = np.floor(num)
print(rounded_num)  # Output: 3.0

5. Rounding to the nearest multiple of a given value using the around() function

import numpy as np
num = 17
base = 5
rounded_num = np.around(num / base) * base
print(rounded_num)  # Output: 15.0

num = 28
base = 5
rounded_num = np.around(num / base) * base
print(rounded_num)  # Output: 30.0