QuickPedantic.java

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

/******************************************************************************
 *  Compilation:  javac QuickPedantic.java
 *  Execution:    java QuickPedantic < 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.
 *  This version uses static generics for type safety.
 *
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java QuickPedantic < 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 QuickPedantic < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 ******************************************************************************/

public class QuickPedantic {

    // quicksort the array
    public static <Key extends Comparable<Key>> void sort(Key[] a) {
        StdRandom.shuffle(a);
        sort(a, 0, a.length - 1);
    }

    // quicksort the subarray from a[lo] to a[hi]
    private static <Key extends Comparable<Key>> void sort(Key[] 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);
    }

    // partition the subarray a[lo .. hi] by returning an index j
    // so that a[lo .. j-1] <= a[j] <= a[j+1 .. hi]
    private static <Key extends Comparable<Key>> int partition(Key[] a, int lo, int hi) {
        int i = lo;
        int j = hi + 1;
        Key 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 v = a[j] into position
        exch(a, lo, j);

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

   /***************************************************************************
    *  Rearranges the elements in a so that a[k] is the kth smallest element,
    *  and a[0] through a[k-1] are less than or equal to a[k], and
    *  a[k+1] through a[n-1] are greater than or equal to a[k].
    ***************************************************************************/
    public static <Key extends Comparable<Key>> Key select(Key[] a, int k) {
        if (k < 0 || k >= a.length) {
            throw new IndexOutOfBoundsException("Selected element out of bounds");
        }
        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 <Key extends Comparable<Key>> boolean less(Key v, Key w) {
        return v.compareTo(w) < 0;
    }

    // exchange a[i] and a[j]
    private static <Key extends Comparable<Key>> void exch(Key[] a, int i, int j) {
        Key swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }



   /***************************************************************************
    *  Check if array is sorted - useful for debugging.
    ***************************************************************************/
    private static <Key extends Comparable<Key>> boolean isSorted(Key[] a) {
        for (int i = 1; i < a.length; 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]);
        }
    }

    // Read strings from standard input, sort them, and print.
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        QuickPedantic.sort(a);
        show(a);
        assert isSorted(a);

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