4. Arithmetic Methods¶

Now we’re ready to add another method to our Fraction class. In particular, we will implement an arithmetic method. Let’s begin by considering what it means to add two fractions together. Remember that you can only add fractions if they have the same denominator. The easiest way to find a common denominator is to multiply the two individual denominators together. Anything we do to the denominator needs to the done to the numerator. This gives us the following equation for fraction addition:

a/b + c/d = (ad + cb)/bd

Our add method will take a Fraction as a parameter. It will return a new Fraction representing the sum. We will use the equation shown above to compute the new numerator and the new denominator. Since this equation will not give us lowest terms, we will utilize a similar technique as was used in the simplify method to find the greatest common divisor and then divide each part of the new fraction.

def add(self, fraction2):

    new_num = self.num * fraction2.den + self.den * fraction2.num
    new_den = self.den * fraction2.den

    common = find_gcd(new_num,new_den)

    return Fraction(new_num//common,new_den//common)

You can try the addition method and then modify the fractions and retry.

def find_gcd(numerator, denominator):
    while numerator % denominator != 0:
        old_num = numerator
        old_den = denominator

numerator = old_den
        denominator = old_num % old_den

return denominator

class Fraction:

def __init__(self, top, bottom):

self.num = top        # the numerator is on top
        self.den = bottom     # the denominator is on the bottom

def __repr__(self):
        return str(self.num) + "/" + str(self.den)

def simplify(self):
        common = find_gcd(self.num, self.den)

self.num = self.num // common
        self.den = self.den // common

def add(self,fraction2):

new_num = self.num * fraction2.den + self.den * fraction2.num
        new_den = self.den * fraction2.den

common = find_gcd(new_num, new_den)

return Fraction(new_num // common, new_den // common)

def main():
    f1 = Fraction(1, 2)
    f2 = Fraction(1, 4)

f3 = f1.add(f2)
    print(f3)

if __name__ == "__main__":
    main()

One final modification to this method will be quite useful. Instead of invoking the add method, we can use the addition operator +. This requires that we implement another special method, this time called __add__, in order to override the default implementation of +. The details of the method are the same as above.

def __add__(self, fraction2):

    new_num = self.num * fraction2.den + self.den * fraction2.num
    new_den = self.den * fraction2.den

    common = find_gcd(new_num, new_den)

    return Fraction(new_num // common, new_den // common)

Now we can perform addition in the same manner that we are used to with other numeric data.

f1 = Fraction(1, 2)
f2 = Fraction(1, 4)

f3 = f1 + f2    # calls the __add__ method of f1
print(f3)