Exercise #9 Solution

In this exercise, we will be working with inheritance, and thinking about how we can set up our inheritance hierarchy to make our lives easier. There is an easy way and a di cult way to complete this exercise. If you plan appropriately, you'll nd you won't have to write very much code at all.




Shapes




Consider the following 4 shapes:




Parallelogram: a four sided gure with parallel pairs of sides. A Parallelogram is de ned by the lengths of its two pairs of sides (labelled base and side in the picture below) and the interior angle (in degrees) between adjacent sides (labelled theta in the picture below).































Rectangle: a parallelogram with four right angles. A Rectangle is de ned by the lengths of its two pairs of sides (labelled base and side in the picture below).































Rhombus: a parallelogram with four equal sides. A Rhombus is de ned by the length of its sides (labelled base in the picture below) and the interior angle (in degrees) between adjacent sides (labelled theta in the picture below).


































Square: a parallelogram with four equal sides and four right angles; both a Rectangle and a Rhombus. A Square is de ned by the length of its sides (labelled base in the picture below).


































1
Your rst task is to gure out the IS-A hierarchy of these shapes. Draw it out in a piece of paper. Are there any instances of multiple parents? 1




Your Task




You must write four classes: Parallelogram, Rectangle, Rhombus and Square, ensuring that the class hierarchy follows your diagram from the previous step. You may create additional classes if you wish (are there any good reasons why you might want to do this?). The parameters of init methods should always be input in the following order: (base, side, theta) (theta being given in degrees), though of course, not every class' init will take all three. So for example, a Rectangle will only take (base, side).







Objects of these classes must have the following methods:




area() - returns the area of the shape




{ Note: The area of a parallelogram is computed by base side sin(theta)




{ Warning: function math.sin in Python expects its argument to be an angle given in radians - take a look at the function math.radians to convert between degrees and radians.




bst() - returns a list of three oats: [base, side, theta]. Even if a shape doesn't need one of the parameters for its input, it should still be able to return it. (e.g., a 10 x 10 square would return: [10.0, 10.0, 90.0]).




When printed, each shape should return a string with text in the following format \I am a shape with area area". Four example, a 10 x 10 square would return the string: \I am a Square with area 100"2.




Be Lazy




One way to solve this would be to write four completely independent classes, and have each class completely implement all of their own functions. This would be a bad idea (why?). If you use inheritance correctly, you should nd the exercise much simpler. Remember, you should never calculate something when you can just get another method to do the work for you. Hint: It's possible to set more than one parent for your class. Are there any shapes here for which that would be a sensible thing to do?




What to Submit




All of your code should be submitted to MarkUs in a le called ex9.py that doesn't import anything other than math, and does not use print or input. As usual your test cases should be in a le called ex9.test.


























































In Python, you can inherit from multiple parents. Just add them separated by commas: class MyClass(ParentA, ParentB)



2\I am a Square with area 100.0" would also be acceptable, you don't need to do any rounding




2