.. index:: PF4 recursion SP1 history sorting .. _sorting: Sorting Algorithms =================================== Sorting algorithms represent foundational knowledge that every computer scientist and IT professional should at least know at a basic level. And it turns out to be a great way of learning about why arrays are important. In this section, we're going to take a look at a number of well-known sorting algorithms with the hope of sensitizing you to the notion of *performance*--a topic that is covered in greater detail in courses such as algorithms and data structures. This is not intended to be a comprehensive reference at all. The idea is to learn how these classic algorithms are coded in the teaching language for this course, C#, and to understand the essentials of analyzing their performance, both theoretically and experimentally. For a full theoretical treatment, we recommend the outstanding textbook by Niklaus Wirth [WirthADP]_, who invented the Pascal language. (We have also adapted some examples from Thomas W. Christopher's [TCSortingJava]_ animated sorting algorithms page. The sorts and supporting functions are all in :repsrc:`sorting/sorting.cs`, but we start one bit at a time: .. index:: exchanging array elements array; exchange elements Exchanging Array Elements ----------------------------- We'll begin by introducing you to a simple method, whose only purpose in life is to swap two data values at positions ``m`` and ``n`` in a given integer array: .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-exchange-begin :end-before: chunk :linenos: :dedent: 6 For example if we have an array nums, shown with indices: .. code-block:: none nums: -1 8 11 22 9 -5 2 index: 0 1 2 3 4 5 6 Then after ``Exchange(nums, 2, 5)`` the array would look like .. code-block:: none nums: -1 8 -5 22 9 11 2 index: 0 1 2 3 4 5 6 In general, swapping two values in an array is no different than swapping any two integers. Suppose we have the following integers ``a`` and ``b``:: int a, b; int t; a = 25; b = 35; t = a; a = b; b = t; After this code does its job, the value of ``a`` would be 35 and the value of ``b`` would be 25. So in the ``Exchange()`` function above, if we have two different array elements at positions ``m`` and ``n``, we are basically getting each value at these positions, e.g. ``data[m]`` and ``data[n]`` and treating them as if they were ``a`` and ``b`` in the above code. You might find it helpful at this time to verify that the above code does what we're saying it does, and a good way is to type it directly into the C# interpreter (csharp) so you can see it for yourself. The ``Exchange()`` function is vital to all of the sorting algorithms in the following way. It is used whenever two items are found to be out of order. When this occurs, they will be *swapped*. This doesn't mean that the item comes to its final resting place in the array. It just means that for the moment, the items have been reordered so we'll get closer to having a sorted array. Let's now take a look at the various sorting algorithms. .. index:: sorting; bubble sort algorithms; bubble sort array; nested loop bubble sort Bubble Sort ----------- The Bubble Sort algorithm works by repeatedly scanning through the array exchanging adjacent elements that are out of order. Watching this work with a strategically-placed ``Console.WriteLine()`` in the outer loop, you will see that the sorted array grows right to left. Each sweep picks up the largest remaining element and moves to the right as far as it can go. It is therefore not necessary to scan through the entire array each sweep, but only to the beginning of the sorted portion. We define the number of *inversions* as the number of element pairs that are out of order. They needn't be adjacent. If ``data[7] > data[16]``, that's an inversion. Every time an inversion is required, we also say that there is corresponding data *movement*. If you look at the ``Exchange()`` code, you'll observe that a swap requires three movements to take place, which happens very quickly on most processors but still amounts to a significant cost. There can be at most :math:`N \cdot \frac{N-1}{2}` inversions in the array of length :math:`N`. The maximum number of inversions occurs when the array is sorted in reverse order and has no equal elements. Bubble Sort exchanges only adjacent elements. Each such exchange removes precisely one inversion; therefore, Bubble Sort requires :math:`O(N^2)` exchanges. .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-bubblesort-begin :end-before: chunk :linenos: :dedent: 6 .. index:: sorting; selection sort algorithms; selection sort nested loop selection sort Selection Sort -------------- The Selection Sort algorithm works to minimize the amount of data movement, hence the number of ``Exchange()`` calls. .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-selectionsort-begin :end-before: chunk :linenos: :dedent: 6 It's a remarkably simple algorithm to explain. As shown in the code, the actual sorting is done by a function, ``IntArraySelectionSort()``, which takes an array of data as its only parameter, like Bubble sort. The way Selection Sort works is as follows: #. An outer loop visits each item in the array to find out whether it is the minimum of all the elements after it. If it is not the minimum, it is going to be swapped with whatever item in the rest of the array is the minimum. #. We use a helper function, ``IntArrayMin()`` to find the position of the minimum value in the rest of the array. This function has a parameter, ``start`` to indicate where we wish to begin the search. So as you can see from the loop in ``IntArraySelectionSort()``, when we start with position ``i``, and we compare to the later elements from position ``i + 1`` to the end of the array, updating the position of the smallest element so far. An illustration to accompany the discussion: .. code-block:: none data: 12 8 -5 22 9 2 index: 0 1 2 3 4 5 i k The first time through the loop, ``i`` is 0, and ``k`` gets the value 2, since data[2] is -5, the smallest element. Those two positions get swapped. .. code-block:: none data: -5 8 12 22 9 2 index: 0 1 2 3 4 5 The next time through the loop i is 1 and k becomes 5: .. code-block:: none data: -5 8 12 22 9 2 index: 0 1 2 3 4 5 i k After the swap: .. code-block:: none data: -5 2 12 22 9 8 index: 0 1 2 3 4 5 and so on. Here is the data after each of the last three swaps: .. code-block:: none -5 2 8 22 9 12 -5 2 8 9 22 12 -5 2 8 9 12 22 Consider the first call to ``IntArrayMin``. .. code-block:: none data: 12 8 -5 22 9 2 index: 0 1 2 3 4 5 Initially ``minPos`` is 0. Here are the changes for each value of ``pos``: .. code-block:: none pos=1: 8 = data[1] < data[0] = 12, so minPos becomes 1 pos=2: -5 = data[2] < data[1] = 8, so minPos becomes 2 pos=3: 22 = data[3] is not < data[2] = -5, so minPos still 2 pos=4: 9 = data[4] is not < data[2] = -5, so minPos still 2 pos=5: 2 = data[5] is not < data[2] = -5, so minPos still 2 and 2 gets returned, and we ge the swap .. code-block:: none data: -5 8 12 22 9 2 index: 0 1 2 3 4 5 The next call to ``IntArrayMin`` has ``start`` as 1, so ``minPos`` is initially 1, and we compare to the elements of data at index 2, 3, 4, and 5.... We won't do the full algorithmic analysis here. Selection Sort is interesting because it does most of its work through *comparisons*, with the same number of them no matter how the data are ordered, exactly :math:`N \cdot \frac{N-1}{2}`, which is :math:`O(N^2)` The number of *exchanges* is O(N). The comparisons are a non-trivial cost, however, and do show in our own performance experiments with randomly-generated data. .. index:: Shuffle exercise exercise; Shuffle Shuffle Exercise ~~~~~~~~~~~~~~~~~ Complete the ``Shuffle`` function and add a ``Main`` method to test it:: /// Shuffle the elements of an array into random positions, /// changing the array. An array containing /// 2, 5, 7, 7, 7, 9 *might* end up in the order /// 7, 7, 2, 9, 7, 5. static void Shuffle(int[] a) Use a Random and do something close to a reverse of selection sort, using ``Exchange`` with a random position. .. index:: sorting; insertion sort algorithms; insertion sort nested loop insertion sort Insertion Sort -------------- In the Insertion Sort algorithm, we build up a longer and longer sorted list from the bottom of the array. We repeatedly insert the next element into the sorted part of the array by sliding it down (using our familiar ``Exchange()`` method) to its proper position. .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-insertionsort-begin :end-before: chunk :linenos: :dedent: 6 Consider the earlier example array as we illustrate some of the steps. I use the symbol '-' for an element that we know to be in the sorted list at the beginning of the array, and '@' over the next one we are trying to insert *at* the right position. We start with a one-element sorted list and try to position the second element: .. code-block:: none - @ data: 12 8 -5 22 9 2 index: 0 1 2 3 4 5 After each outer loop in sequence we end up with: .. code-block:: none - - @ data: 8 12 -5 22 9 2 index: 0 1 2 3 4 5 - - - @ data: -5 8 12 22 9 2 index: 0 1 2 3 4 5 - - - - @ data: -5 8 12 22 9 2 index: 0 1 2 3 4 5 - - - - - @ data: -5 8 9 12 22 2 index: 0 1 2 3 4 5 - - - - - - data: -5 2 8 9 12 22 index: 0 1 2 3 4 5 Let us illustrate several times just through the inner loop, the first time, when the 8 is moved into position from index ``j`` = 1, so ``i`` starts at 1. We show the letter i over the data at index ``i``, and show the comparison test to be done with a `>` with a '?' over it. .. code-block:: none ? i data: 12 > 8 -5 22 9 2 1>0 and 12>8: true, so swap, loop index: 0 1 2 3 4 5 i data: 8 12 -5 22 9 2 0>0 false, skip comparison of data, end loop index: 0 1 2 3 4 5 Let us also illustrate at a later time through the inner loop, when the 9 is moved into position from index ``j`` = 4, so ``i`` starts at 4. .. code-block:: none ? i data: -5 8 12 22 > 9 2 4>0 and 22>9: true, so swap, loop index: 0 1 2 3 4 5 ? i data: -5 8 12 > 9 22 2 3>0 and 12>9: true, so swap, loop index: 0 1 2 3 4 5 ? i data: -5 8 > 9 12 22 2 2>0 and 8>9: false, end inner loop index: 0 1 2 3 4 5 The 9 started at index ``j`` = 4, and now the list is sorted up through index 4. This will require as many exchanges as Bubble Sort, since only one inversion is removed per exchange. So Insertion Sort also requires :math:`O(N^2)` exchanges. On average Insertion Sort requires only half as many comparisons as Bubble Sort, since the average distance an element must move for random input is one-half the length of the sorted portion. .. index:: sorting; Shell sort nested loop Shell sort algorithms; Shell sort Shell Sort ---------- Shell Sort is basically a trick to make Insertion Sort run faster. If you take a quick glance at the code and look beyond the presence of two additional *outer loops*, you'll notice that the code looks very similar. Since Insertion Sort removes one inversion per exchange, it cannot run faster than the number of inversions in the data, which in worst case is :math:`O(N^2)`. Of course, it can't run faster than N, either, because it must look at each element, whether or not the element is out of position. We can't do any thing about the lower bound O(N), but we can do something about the number of steps to remove inversions. The trick in Shell Sort is to start off swapping elements that are further apart. While this may remove only one inversion sometimes, often many more inversions are removed with intervening elements. Shell Sort considers the subsequences of elements spaced k elements apart. There are k such sequences starting at positions 0 through k-1 in the array. In these sorts, elements k positions apart are exchanged, removing between 1 and 2(k-1)+1 inversions. Swapping elements far apart is not sufficient, generally, so a Shell Sort will do several passes with decreasing values of k, ending with k=1. The following examples experiment with different series of values of k. In this first example, we sort all subsequences of elements 8 apart, then 4, 2, and 1. Please note that these intervals are to show how the method works--not how the method works *best*. .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-shellsort-begin :end-before: chunk :linenos: :dedent: 6 .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-shellsort-naive-begin :end-before: chunk :linenos: :dedent: 6 In general, shell sort with sequences of jump sizes that are powers of one another doesn't do as well as one where most jump sizes are not multiples of others, mixing up the data more. In addition, the number of intervals must be increased as the size of the array to be sorted increases, which explains why we allow an *arbitrary* array of intervals to be specified. Without too much explanation, we show how you can choose the intervals differently in an *improved* shell sort, where the intervals have been chosen so as not to be multiples of one another. Donald Knuth has suggested a couple of methods for computing the intervals: .. math:: h_0 = 1 h_{k+1} = 3 h_k + 1 t = \lfloor log_3 n \rfloor - 1 Here we are using notation for the *floor* function :math:`\lfloor x \rfloor` means the largest integer :math:`\le x`. This results in a sequence 1, 4, 13, 40, 121.... You stop computing values in the sequence when :math:`t = log_3 n - 1`. (So for n=50,000, you should have about 9-10 intervals.) For completeness, we note that :math:`log_3 n` must be sufficiently large (and > 2) for this method to work. Our code ensures this by taking the *maximum* of :math:`log_3 n` and 1. Knuth also suggests: .. math:: h_0 = 1 h_{k+1} = 2 h_k + 1 t = \lfloor log_2 n \rfloor - 1 This results in a sequence 1, 3, 7, 15, 31.... Here is the improvement to our naive method that dynamically calculates the intervals based on the first suggestion of Knuth: .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-shellsort-better-begin :end-before: chunk :linenos: :dedent: 6 Shell sort is a complex sorting algorithm to make "work well", which is why it is not seen often in practice. It is, however, making a bit of a comeback in embedded systems. We nevertheless think it is a very cool algorithm to have heard of as a computer science student and think it has promise in a number of situations, especially in systems where there are limits on available memory (e.g. embedded systems). .. index:: sorting; Quicksort algorithms; Quicksort recursion; Quicksort Quicksort Quicksort a.k.a. Partition Sort ---------------------------------- This sort is a more advanced example that uses *recursion*. We list it because it is one of the best sorts for *random* data, having an *average* time behavior of :math:`O(N \log N)`. Quicksort is a rather interesting case. While it has an excellent average behavior, it has a worst case performance of :math:`O(N ^2)`. .. later We'll have a bit more to say about this algorithm in our discussion of recursion. .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-quicksort-begin :end-before: chunk :linenos: :dedent: 6 Though the initial call is to sort an entire array, this is accomplished by dealing with sections, so the main work is done in the version with the two extra parameters, giving the lowest and highest index considered. It picks an arbitrary element as *pivot*, and then swaps elements with values above and below the pivot until the part of the array being processed is in three sections: #. elements <= pivot; #. possibly an element equal to pivot #. elements >= pivot. Though sections 1 and 3 are not sorted, there are no inversions *between* any two separate sections, so only the smaller sections 1 and 3 need to be sorted *separately*, and only then if they have at least two elements. They can be sorted by calling the *same* function, but with a smaller range of indices to deal with in each case. These *recursive* calls stop when a part is reduced to one element. .. index:: loop; invariant Another optional glimpse at an advanced topic: The outer ``while`` loop in lines 11-20 has fairly complicated logic. To prove it is correct overall, you can state and prove the simpler *loop invariant* expressed in the comments above the loop, lines 7-10. This allows the conclusion after the loop in comment lines 21-24. .. index:: Random seed regenerate random numbers Random Data Generation ------------------------ Now it is time to talk about how we are going to check the performance in a real-world situation. We're going to start by modeling the situation when the data are in random order. The following code generates a random array: .. literalinclude:: ../../examples/introcs/sorting/sorting.cs :start-after: chunk-random-begin :end-before: chunk :linenos: :dedent: 6 There are a few things to note in this code: #. We use the random number generator option to include a *seed*. Random numbers aren't truly random. The particular sequence is just determined by a seed. The simplest way to create a Random object uses a seed taken from the system clock. #. In order to regenerate a particular example, we actually need the random sequence to be consistent, so we know that each of the sorting algorithms is being tested using the same random data. #. Because the sorting algorithms *modify* the data that are passed to it, we need to have a way of regenerating the sequence for the next test. Hence we specify the same seed each time. (We could also copy the data, but it is kind of a waste of memory.) This completes the discussion of the ``Sorting`` class in :repsrc:`sorting/sorting.cs`. .. index:: Stopwatch performance - Stopwatch and TimeSpan timing Timing ------- Separate from the basic sorts is the idea of checking their performance. The rest of the code is in the driver class in :repsrc:`sorting/sorting_demo.cs`. We need the ability to time the various sorting algorithms. Luckily, the .Net framework/library gives us a way of doing so through its ``Stopwatch`` class. This class supports methods that you would expect if you've ever used a stopwatch (the kind found in sports): - Reset: Resets the elapsed time to zero. We need this so we can use the same Stopwatch for each sorting algorithm. - Start: Starts the stopwatch. Will keep recording time until stopped. - Stop: Stops the stopwatch. - ElapsedMilliseconds: Not really a method but a property (like a variable). We'll use this to get the total time that has elapsed between pairs of Start/Stop events in milliseconds. So let's take a look at the ``Main()`` method's code to see how we compare the sorting algorithms. The ``Main()`` method should be thought of as an *experiment* that tests the performance of each of the sorting algorithms. As all of the tests follow the same pattern, we're going to look at the basic variable setup first, and then one test. .. literalinclude:: ../../examples/introcs/sorting/sorting_demo.cs :start-after: chunk-drivervars-begin :end-before: chunk :dedent: 6 The variables declared here are to set up the apparatus: - ``arraySize``: The size of the array where we wish to test the performance. We will use this to create an array with ``arraySize`` random values. - ``randomSeed``: This allows the user to vary the seed that is used to create the random array. We often want to do this to determine whether our performance results are stable when run a large number of times with different distributions. We won't go into too much detail here but consider it an important part of building good performance benchmarks. - ``watch``: The stopwatch object we're using to do the timings of all experiments. - ``data``: The array to be sorted. .. literalinclude:: ../../examples/introcs/sorting/sorting_demo.cs :start-after: chunk-driverparameters-begin :end-before: chunk This code is designed so we can accept the parameters ``arraySize`` and ``randomSeed`` from the command line or by prompting the user. When programmers design benchmarks, they often try to make it possible to run them with minimal user interaction. For the purposes of teaching, we wanted to make it possible to run it with or without command-line parameters. .. literalinclude:: ../../examples/introcs/sorting/sorting_demo.cs :start-after: chunk-driverapparatus-begin :end-before: chunk :dedent: 6 This code fragment is one of the tests in ``Main``. All of the tests set up for timing first, run the desired sorting algorithm (bubble sort in this case), and report the timing results. Since the timing setup and reporting is always done the same way, they actions are placed in helping methods: .. literalinclude:: ../../examples/introcs/sorting/sorting_demo.cs :start-after: chunk timing :end-before: chunk :dedent: 6 ``TimeSetup``: #. Creates the 'random' array of ``data``, based on ``randomSeed``. #. Resets the ``Stopwatch`` object to zero. #. Starts the ``Stopwatch``. After the sort, ``TimeResult``: #. Stops the ``Stopwatch`` and get the elapsed time (``watch.ElapsedMilliseconds``). The value is an integer (long) number of milliseconds (thousandths of a second). #. Prints the performance results in seconds. You can see all of the tests in :repsrc:`sorting/sorting_demo.cs`. Running the Code ---------------------- Here's the output of a trial run on one of our computers. The results will vary depending on your computer's CPU, among other factors. .. code-block:: none bin/Debug$ mono Sorting.exe 1000 12 Quick Sort: 0.000 Naive Shell Sort: 0.000 Better Shell Sort: 0.000 Insertion Sort: 0.001 Selection Sort: 0.002 Bubble Sort: 0.003 bin/Debug$ mono Sorting.exe 1000 55 Quick Sort: 0.000 Naive Shell Sort: 0.000 Better Shell Sort: 0.000 Insertion Sort: 0.001 Selection Sort: 0.002 Bubble Sort: 0.003 bin/Debug$ mono Sorting.exe 10000 2 Quick Sort: 0.001 Naive Shell Sort: 0.019 Better Shell Sort: 0.002 Insertion Sort: 0.134 Selection Sort: 0.174 Bubble Sort: 0.321 bin/Debug$ mono Sorting.exe 50000 2 Quick Sort: 0.006 Naive Shell Sort: 0.441 Better Shell Sort: 0.015 Insertion Sort: 3.239 Selection Sort: 4.172 Bubble Sort: 8.028 bin/Debug$ mono Sorting.exe 100000 2 Quick Sort: 0.014 Naive Shell Sort: 1.794 Better Shell Sort: 0.034 Insertion Sort: 13.158 Selection Sort: 16.736 Bubble Sort: 31.334 At least based on randomly-generated arrays, the performance can be summarized as follows: - Bubble Sort is rather unimpressive as expected. In fact, this algorithm is never used in practice but is of historical interest. Like the brute-force style of searching, it does way too much work to come up with the right answer! - Selection Sort and Insertion Sort are also rather unimpressive on their own. Even though Selection Sort can in theory do a lot less data movement, it must make a large number of comparisons to find the minimum value to be moved. Again it is way too much work. Insertion Sort, while unimpressive, fares a bit better and turns out to be a nice building block (if modified) for the Shell Sort. Varying the interval size drastically reduces the amount of data movement (and the distance it has to move). - Shell Sort does rather well, especially when we pick the right intervals. In practice, the intervals also need to be adjusted based on the size of the array, which is what we do as larger array sizes are considered. This is no trivial task but a great deal of work has already been done in the past to determine functions that generate good intervals. - The Quicksort is generally fastest on random data. It is by far the most commonly used sorting algorithm. Yet there are signs that Shell sort is making a comeback in embedded systems, because it concise to code and is still quite fast. See [WikipediaShellSort]_, where it is mentioned that the [uClibc]_ library makes use of Shell sort in its ``qsort()`` implementation, rather than implementing the library sort with the more common quicksort. .. [WirthADP] Niklaus Wirth, Algorithms + Data Structures = Programs, Prentice Hall, 1976. .. [WikipediaShellSort] http://en.wikipedia.org/wiki/Shellsort .. [uClibc] http://en.wikipedia.org/wiki/UClibc .. [TCSortingJava] http://tools-of-computing.com/tc/CS/Sorts/SortAlgorithms.htm