How to Round number in Python

In Python, numbers can be rounded using the built-in round() function and other methods, depending on the desired precision and rounding behavior. Here’s a detailed explanation:

1. Using the `round()` Function

The round() function rounds a number to the nearest integer or to a specified number of decimal places.

Syntax:

round(number, ndigits)

number: the number to be rounded.
ndigits : An optional parameter, an integer number of decimal places to round to. If not specified, rounds to the nearest integer.

Examples:

# Rounding to the nearest integer
print(round(4.6))  # Output: 5
print(round(4.4))  # Output: 4

# Rounding to a specific number of decimal places
print(round(3.14159, 2))  # Output: 3.14
print(round(2.71828, 3))  # Output: 2.718

# Rounding negative numbers
print(round(-2.5))  # Output: -2
print(round(-3.5))  # Output: -4

2. The `math.floor()` and `math.ceil()` Functions

If you want to always round down or up, use the math.floor() or math.ceil() functions of the math module.

math.floor(): Rounds down to the nearest integer.
math.ceil(): Rounds up to the nearest integer.

Examples:

import math

# Rounding down
print(math.floor(4.8))  # Output: 4
print(math.floor(-4.8)) # Output: -5

# Rounding up
print(math.ceil(4.2))   # Output: 5
print(math.ceil(-4.2))  # Output: -4

3. Rounding Toward Zero with `int()`

If you need to truncate the decimal part without rounding, converting to an integer will remove the decimal portion.

Examples:

print(int(4.8))   # Output: 4
print(int(-4.8))  # Output: -4

4. Using `decimal.Decimal` for Custom Rounding

For precise rounding in financial or scientific applications, you can use the decimal module. It offers control over rounding modes.

Example:

from decimal import Decimal, ROUND_HALF_UP, ROUND_DOWN

# ROUND_HALF_UP (traditional rounding)
num = Decimal('2.675')
print(num.quantize(Decimal('0.01'), rounding=ROUND_HALF_UP))  # Output: 2.68

# ROUND_DOWN (always rounds down)
num = Decimal('2.678')
print(num.quantize(Decimal('0.01'), rounding=ROUND_DOWN))    # Output: 2.67

5. Rounding in NumPy

If you’re working with arrays, the numpy library provides rounding functions like numpy.round().

Example:

import numpy as np

arr = np.array([1.234, 2.456, 3.678])
print(np.round(arr, 2))  # Output: [1.23 2.46 3.68]

Rounding Behavior

Ties (e.g., 2.5, 3.5): Python’s round() rounds to the nearest even number to reduce bias in large datasets (Banker’s Rounding).

print(round(2.5))  # Output: 2
print(round(3.5))  # Output: 4

2. Control over Rounding Modes: Use decimal for strict control over rounding behavior.

Choosing the Appropriate Function

For general purpose rounding use round().
For rounding in one direction use math.floor() or math.ceil().
For very precise or controlled rounding use decimal.Decimal.
Use numpy for array rounding.