Sortacle

Being confident that your software is correct involves more than just writing code you think is right. That’s why we write tests. As computer scientists, we should therefore think about taking this a step further: to automate testing. How can we do that? We’ll see in this assignment.

There’s another motivation for what we’re going to do, which illustrates a weakness in the nature of testing as you’ve done it until now. When a test fails, the error is either in the implementation of the function (most likely) or in the test itself (usually less likely, but certainly possible). However, there is a third, more subtle reason: both the implementation and test are correct, but the implementation returned a different result than the test expected. The most likely cause is that the problem statement allows a given input to produce multiple different answers.

1 The Oracle

This assignment is not sponsored by any particular large technology company. In fact, if some other company would like to occupy this space, we could always consider changing the assignment’s name....

Given a list of these people, suppose we want to sort them by increasing age. This ordering expressly does not say what should happen to people whose ages are the same; therefore, every permutation of their names is valid. As a result, concrete tests may fail for no good reason.

The problem lies in the fact that we wrote concrete tests at all. Instead, we should have written a function that checks whether the output has the right relationship with the input; then, all the valid outputs for a given input would pass, but no others. This checking function is sometimes called an oracle.

That is what we will do in this assignment: write a testing oracle for a function that sorts lists of persons. Your oracle will be given purported implementations of such a sort function, and it must (try to) determine whether the given function is indeed a valid sorter or not. In addition, you will write a generator of inputs. Combining the two, you arrive at a system that automates the testing of solutions to this problem.

2 Input-Output Specification

Your assignment has three tasks:

• Write a function named generate-input that generates input. It should take an integer length, and return a list of randomly constructed people of that length. Feel free to use the function random.

  generate-input : number -> (listof person)

  Keep in mind that you would not be able to directly test generate-input, as it is returns random lists. However, you could still write tests to check whether the value it produces has reasonable properties.

• Write a function that determines whether the second input is a sorted version of the first.

  valid? : (listof person) (listof person) -> boolean

• Using valid? and generate-input (along with any other edge cases you think to include), write a function that tests whether an algorithm is a valid sorter.

  oracle : ((listof person) -> (listof person)) -> boolean

To do well on this assignment, you will want to spend time considering all the different ways that output could be either invalid or inconsistent with the original problem statement. Be thorough! That’s the name of the game when testing.

3 Testing the Tester

Observe that the test cases for your oracle are programs. That is, to test your oracle, you need good and bad implementations of sorting. To obtain a correct sorting algorithm, either implement one yourself or use the built-in sort function to create one. You can then perturb its output to create incorrect implementations. (Note that you cannot use sort to solve the problem, because that would render the task almost trivial.)

4 Language

Make sure you’re in the Intermediate Student with lambda language. To change it, go to the Language menu and select Choose Language... .

5 Handing In

Using the file navigator, go to your cs019 directory within your home directory. Make a new directory called sortacle. Put your solution, which should be named sortacle.rkt, inside that directory. Open the terminal and paste the following two commands (on separate lines, press enter after each):