% CSC 211 Assignment 4 % Quicksort

Quicksort is an elegant, recursive sorting algorithm. Like any sorting algorithm, it can be applied to more than just numbers.

In this assignment, you'll implement quicksort such that it can sort instances of your DNA class. What makes this interesting is that there are multiple ways in which you might compare two DNA objects. For example, is one sequence shorter or longer than another? Or, if we want to arrange DNA objects in a catalog of some sort, we might want to order them alphabetically by header¹. In this case, we might ask whether one header should appear before another header.

Some new features and concepts you'll need for this assignment are reference types, templated functions, and recursion.

• Get into your Docker development environment
• Download the assignment framework with git clone https://github.com/csc211/a4
• Look in the resulting directory a4. You'll see a working dna.cpp and its associated header file, as well as a sort.cpp (which isn't yet complete), and a sort.h.
• You'll need to write (or copy from your lab 6) swap.h.
• Note: ./compile will not work yet.
  • You need to write a main.cpp containing a main(). Its design is up to you!
  • You need to fill in the function definitions in sort.cpp
• Once you can compile, you'll produce an executable binary called sortdna, so you can run it as ./sortdna (with whatever arguments you implement.)

Ultimately, you must implement quicksort on DNA sequences.

To do this, you will need to write two comparator functions: seqLenLessThan and headerLessThan, as described in the comments in sort.h

You will also need to write quicksort (which is a recursive function) and partition (you may use either the Lomuto or Hoare algorithm).

I have given you an implementation of sort() itself. This function is not recursive; it calls your quicksort() function. Please make sure you understand what it's doing, and ask the course staff if you do not.

1. Write a trivial main() and fill in enough of each function definition in sort.cppthat you can compile the project (you may temporarily want to void some argument variables).

2. Write swap(), and make sure it works!

   • In lab 6, you wrote a templated swap(). You can reuse it for this assignment.

3. Write partition() next. Modify your main() to test partition() on some small examples.

   • Once partition() is done, you are almost finished.

4. Write the recursive quicksort() and modify main() to test it.

For your own sake, write your own tests and run them before you try submitting to gradescope!

If you are iterating on Gradescope before you have passed some sensible tests of your own, you are wasting time. Gradescope takes minutes to give you an answer, and it's not always informative, and you can't debug it. Your own test cases take milliseconds to give you an answer, and you can debug it!

Now, this is the first time you have used function references, or passed a function as a parameter to another function. So, here is a hint for your main.cpp. When you call sort(), you need to pass it the function you wish to use for the sorting comparator. Suppose that function were called sortByFitness() and it magically gave you most fit DNA sequence (which would be very handy if you were running a simulation of Evolution). Then, if all your DNA objects were in a vector called vec, you might do: sort(vec, &sortByFitness). Note that there is a & before the name of sortByFitness, and also that there are no function-call parentheses after the name sortByFitness. You are passing a reference to the function, rather than calling the function.

The compiler is your friend, not your enemy. Compile after you write a little bit of code; it's far easier to find a problem if you know it's in the last few lines of code you wrote.

For this assignment, correctly passing all tests on Gradescope is worth 70% of your grade. Another 10% is based on using a templated swap() inside your partition() function². The remaining 20% is based on reasonable commenting habits, and the structure and organization of your code.

If you circumvent the purpose of this assignment by calling std::sort() you will receive zero credit.

You will submit both sort.cpp and swap.h via Gradescope, where its functional correctness will be graded automatically. You can submit as many times as you like; only the last submission will count towards your grade.