Subtract Fractions in Python | Step-by-Step Guide with Code Examples

subtracting fractions with Different Denominators | Rationalising Fractions in Python

Learning how to subtract fractions in Python is an important step for junior secondary students who are beginning to combine mathematics with coding. In this tutorial, we'll walk through the process of subtracting fractions with different denominators, explain how to use the LCM (Lowest Common Multiple) method, and show you how to rationalise or canonise fractions before subtraction. By the end, you'll be able to write a simple Python program to subtract fractions step by step, giving you both a solid maths foundation and practical coding skills to implement libraries like the built-in Python fractions module.

Like with Adding Fractions in Python, fractional numbers are also rationalised before subtraction. This means they are put in a form where their denominators become the same. This identical denominator is the LCM of the previous denominators of all the separate fractions.
After this is done, the new numerators can then be subtracted.

Steps for Subtraction of Fractions with Different Denominators - Python Algorithm

The following steps will guide us in writing our Python code for subtracting fractions.
Let's illustrate these steps with the example fractional expression ⁷/₄ - ²/₅

Step 1:

Using the Find LCM in Python class from the Primary Category, find the LCM of the denominators.
⇒ LCM of 4 & 5 = 20

Step 2:

In a turn by turn fashion, divide the found LCM from Step 1 by each denominator, multiplying the quotient by the corresponding numerator.
⇒ ^{((7 x 5) - (2 x 4))}/₂₀
= ^{(35 - 8)}/₂₀

Step 3:

Go ahead and subtract the numerators.
⇒ ²⁷/₂₀

Create a new Python module file; call it SubtractFraction.py
Type out the adjoining Python code for subtracting fractions.

Note: You can comment out the AddFraction Python object code in the main class from the previous lesson or simply continue from where it stopped.

So! Python Fun Practice Exercise - Add Fractions

As a fun practice exercise, feel free to try out your own fractions with different numerators and denominators, and see how the Python code divides those fractions.

Python Code for Subtracting Fractions - Module File

from AddFraction import PlusFraction

# A class with a parent
class MinusFraction(PlusFraction):

    # A constructor
    def __init__(self, fractions):
        super().__init__(fractions)

    # Returns a dictionary of the new fraction
    def doSubtract(self):
        # STEPS 1, 2:
        self.canoniseFraction()

        # STEP 3:
        # subtract all transformed numerators
        self.answer = self.new_numerators[0]
        for count in range(1, len(self.new_numerators)): self.answer -= self.new_numerators[count]

        return {'numerator':self.answer, 'denominator':self.lcm}

Python Code for Subtracting Fractions - Main Class

#!/usr/bin/python
from SubtractFraction import MinusFraction

##
# Subtracting fractions
##
numerators   = [9, 3, 5, 7]
denominators = [2, 4, 12, 18]
fractions = {'numerators':numerators, 'denominators':denominators}

print('\n    Solving:\n')
# Print as fraction
for numerator in fractions['numerators']: print('{:13d}'.format(numerator), end='')
print('{}{:>12}'.format('\n', ' '), end='')
for wasted in range(len(numerators)-1): print('{}'.format('-     +      '), end='')
print('{:>1}'.format('-'))
for denominator in fractions['denominators']: print('{:13d}'.format(denominator), end='')
print('\n\n')

# use the SubtractFraction class
sub_fract = MinusFraction(fractions)
fraction = sub_fract.doSubtract()

print('{:25d}'.format(fraction['numerator']))
print('{:>25}'.format('Answer   =   -'))
print('{:25d}'.format(fraction['denominator']))

print('\n\n')

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