Quick.java

Below is the syntax highlighted version of Quick.java from §2.3 Quicksort.

/******************************************************************************
 *  Compilation:  javac Quick.java
 *  Execution:    java Quick < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   https://algs4.cs.princeton.edu/23quicksort/tiny.txt
 *                https://algs4.cs.princeton.edu/23quicksort/words3.txt
 *
 *  Sorts a sequence of strings from standard input using quicksort.
 *
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java Quick < tiny.txt
 *  A E E L M O P R S T X                 [ one string per line ]
 *
 *  % more words3.txt
 *  bed bug dad yes zoo ... all bad yet
 *
 *  % java Quick < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 *
 *  Remark: For a type-safe version that uses static generics, see
 *
 *    https://algs4.cs.princeton.edu/23quicksort/QuickPedantic.java
 *
 ******************************************************************************/

/**
 *  The {@code Quick} class provides static methods for sorting an
 *  array and selecting the ith smallest element in an array using quicksort.
 *  <p>
 *  For additional documentation, see
 *  <a href="https://algs4.cs.princeton.edu/23quicksort">Section 2.3</a>
 *  of <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class Quick {

    // This class should not be instantiated.
    private Quick() { }

    /**
     * Rearranges the array in ascending order, using the natural order.
     * @param a the array to be sorted
     */
    public static void sort(Comparable[] a) {
        StdRandom.shuffle(a);
        sort(a, 0, a.length - 1);
        assert isSorted(a);
    }

    // quicksort the subarray from a[lo] to a[hi]
    private static void sort(Comparable[] a, int lo, int hi) {
        if (hi <= lo) return;
        int j = partition(a, lo, hi);
        sort(a, lo, j-1);
        sort(a, j+1, hi);
        assert isSorted(a, lo, hi);
    }

    // partition the subarray a[lo..hi] so that a[lo..j-1] <= a[j] <= a[j+1..hi]
    // and return the index j.
    private static int partition(Comparable[] a, int lo, int hi) {
        int i = lo;
        int j = hi + 1;
        Comparable v = a[lo];
        while (true) {

            // find item on lo to swap
            while (less(a[++i], v)) {
                if (i == hi) break;
            }

            // find item on hi to swap
            while (less(v, a[--j])) {
                if (j == lo) break;      // redundant since a[lo] acts as sentinel
            }

            // check if pointers cross
            if (i >= j) break;

            exch(a, i, j);
        }

        // put partitioning item v at a[j]
        exch(a, lo, j);

        // now, a[lo .. j-1] <= a[j] <= a[j+1 .. hi]
        return j;
    }

    /**
     * Rearranges the array so that {@code a[k]} contains the kth smallest key;
     * {@code a[0]} through {@code a[k-1]} are less than (or equal to) {@code a[k]}; and
     * {@code a[k+1]} through {@code a[n-1]} are greater than (or equal to) {@code a[k]}.
     *
     * @param  a the array
     * @param  k the rank of the key
     * @return the key of rank {@code k}
     * @throws IllegalArgumentException unless {@code 0 <= k < a.length}
     */
    public static Comparable select(Comparable[] a, int k) {
        if (k < 0 || k >= a.length) {
            throw new IllegalArgumentException("index is not between 0 and " + a.length + ": " + k);
        }
        StdRandom.shuffle(a);
        int lo = 0, hi = a.length - 1;
        while (hi > lo) {
            int i = partition(a, lo, hi);
            if      (i > k) hi = i - 1;
            else if (i < k) lo = i + 1;
            else return a[i];
        }
        return a[lo];
    }



   /***************************************************************************
    *  Helper sorting functions.
    ***************************************************************************/

    // is v < w ?
    private static boolean less(Comparable v, Comparable w) {
        if (v == w) return false;   // optimization when reference equals
        return v.compareTo(w) < 0;
    }

    // exchange a[i] and a[j]
    private static void exch(Object[] a, int i, int j) {
        Object swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }


   /***************************************************************************
    *  Check if array is sorted - useful for debugging.
    ***************************************************************************/
    private static boolean isSorted(Comparable[] a) {
        return isSorted(a, 0, a.length - 1);
    }

    private static boolean isSorted(Comparable[] a, int lo, int hi) {
        for (int i = lo + 1; i <= hi; i++)
            if (less(a[i], a[i-1])) return false;
        return true;
    }


    // print array to standard output
    private static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++) {
            StdOut.println(a[i]);
        }
    }

    /**
     * Reads in a sequence of strings from standard input; quicksorts them;
     * and prints them to standard output in ascending order.
     * Shuffles the array and then prints the strings again to
     * standard output, but this time, using the select method.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        Quick.sort(a);
        show(a);
        assert isSorted(a);

        // shuffle
        StdRandom.shuffle(a);

        // display results again using select
        StdOut.println();
        for (int i = 0; i < a.length; i++) {
            String ith = (String) Quick.select(a, i);
            StdOut.println(ith);
        }
    }

}