Decimal Fixed Point and Floating Point Arithmetic in Python

This article will explain how decimal fixed-point and floating-point arithmetic work in Python, which is useful for performing accurate calculations in various applications.

Numbers in Python can be stored in two ways: floating-point and decimal fixed-point. Floating-point numbers are fast but can sometimes be incorrect because of how computers store them.

On the other hand, Decimal fixed-point numbers are more accurate and useful when working with precise calculations. 

By the end of this article, you will understand how both types of numbers work, when to use them, and how to write programs using them.

Floating-Point Arithmetic

The following example will use the float type to perform floating-point arithmetic in Python. Here we will add 0.1 and 0.2, which are floating-point numbers. And the output will show the result of this addition.

x = 0.1 + 0.2  
print(x)

Output

This will create the following outcome -

0.30000000000000004

The above output is not what we expect because of how floating-point numbers are stored in computers. The result is slightly off due to the limitations of binary representation.

Decimal Fixed-Point Arithmetic

This is another way to check if a number is stored as a decimal fixed-point number. The Decimal class from the decimal module helps us to perform arithmetic operations accurately.

The output of this example will show the correct result of adding 0.1 and 0.2 using Decimal.

# Import the Decimal class from the decimal module
from decimal import Decimal

# Create Decimal objects and perform addition
x = Decimal("0.1") + Decimal("0.2")  

# Print the result
print(x) 

Output

This will generate the following result -

0.3

Currency Calculation Using Decimal

The following example shows how to use the Decimal class for currency calculations. It sets a precision level and performs a multiplication operation to calculate the total price of items.

The output will show the total price calculated using Decimal.

# Import decimal and getcontext from decimal module
from decimal import Decimal, getcontext

# Set precision
getcontext().prec = 4  

# Define price and quantity as Decimal objects
price = Decimal("19.99")
qty = Decimal("3")

# Calculate total and print it
total = price * qty

# Print the total
print(total)  

Output

This will produce the following result -

59.97

Difference between Floating-Point and Decimal

In this example, we compare floating-point and Decimal arithmetic in Python. The output shows that floating-point addition may have small precision errors, while Decimal addition provides an exact result.

# Import Decimal class from decimal module
from decimal import Decimal

# Floating-point addition
float_result = 0.1 + 0.2

# Decimal addition
decimal_result = Decimal("0.1") + Decimal("0.2")

# Print both results
print("Floating-point result:", float_result)
print("Decimal result:", decimal_result)

Output

This will lead to the following outcome -

Floating-point result: 0.30000000000000004
Decimal result: 0.3

Floating-Point vs Decimal Fixed-Point

Here are the differences between floating-point and decimal fixed-point in tabular form -