Subtracting Fractions in Python

Rationalise (Canonise) the Fractions before Subtraction

Normally before fractions are subtracted, they are rationalised; i.e., they are put in a form where their denominators become the same. This same denominator is the LCM of the denominators of all the separate fractions.
After this is done, the new numerators can then be subtracted.

Steps for Subtraction of Fractions Python code

The following steps will guide us in writing our code.
Let's illustrate with ⁷/₄ - ²/₅

Step 1:

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 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!

Feel free to try out your own set of numerators and denominators for fractional subtraction.

Python code for SubtractFraction 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}

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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