week12

common/Stopwatch.java

@@ -0,0 +1,81 @@
+/******************************************************************************
+ *  Compilation:  javac Stopwatch.java
+ *  Execution:    java Stopwatch n
+ *  Dependencies: none
+ *
+ *  A utility class to measure the running time (wall clock) of a program.
+ *
+ *  % java8 Stopwatch 100000000
+ *  6.666667e+11  0.5820 seconds
+ *  6.666667e+11  8.4530 seconds
+ *
+ ******************************************************************************/
+package common;
+
+/**
+ *  The {@code Stopwatch} data type is for measuring
+ *  the time that elapses between the start and end of a
+ *  programming task (wall-clock time).
+ *
+ *  See {@link StopwatchCPU} for a version that measures CPU time.
+ *
+ *  @author Robert Sedgewick
+ *  @author Kevin Wayne
+ */
+
+
+public class Stopwatch {
+
+    private final long start;
+
+    /**
+     * Initializes a new stopwatch.
+     */
+    public Stopwatch() {
+        start = System.currentTimeMillis();
+    }
+
+
+    /**
+     * Returns the elapsed CPU time (in seconds) since the stopwatch was created.
+     *
+     * @return elapsed CPU time (in seconds) since the stopwatch was created
+     */
+    public double elapsedTime() {
+        long now = System.currentTimeMillis();
+        return (now - start) / 1000.0;
+    }
+
+    /**
+     * Unit tests the {@code Stopwatch} data type.
+     * Takes a command-line argument {@code n} and computes the
+     * sum of the square roots of the first {@code n} positive integers,
+     * first using {@code Math.sqrt()}, then using {@code Math.pow()}.
+     * It prints to standard output the sum and the amount of time to
+     * compute the sum. Note that the discrete sum can be approximated by
+     * an integral - the sum should be approximately 2/3 * (n^(3/2) - 1).
+     *
+     * @param args the command-line arguments
+     */
+    public static void main(String[] args) {
+        int n = Integer.parseInt(args[0]);
+
+        // sum of square roots of integers from 1 to n using Math.sqrt(x).
+        Stopwatch timer1 = new Stopwatch();
+        double sum1 = 0.0;
+        for (int i = 1; i <= n; i++) {
+            sum1 += Math.sqrt(i);
+        }
+        double time1 = timer1.elapsedTime();
+        StdOut.printf("%e (%.2f seconds)\n", sum1, time1);
+
+        // sum of square roots of integers from 1 to n using Math.pow(x, 0.5).
+        Stopwatch timer2 = new Stopwatch();
+        double sum2 = 0.0;
+        for (int i = 1; i <= n; i++) {
+            sum2 += Math.pow(i, 0.5);
+        }
+        double time2 = timer2.elapsedTime();
+        StdOut.printf("%e (%.2f seconds)\n", sum2, time2);
+    }
+}

main.typ

@@ -15,7 +15,7 @@ Collection of solutions to programming exercises as part of Introduction to Prog
 )
 
 #{
-  let count = 11;
+  let count = 12;
   for week in range(1, count + 1).filter(it => it != 8) {
     let a = "./week" + str(week) + "/doc.typ"
     include a

week11/Clock.java

@@ -0,0 +1,38 @@
+import common.StdDraw;
+
+public class Clock {
+    public static void main(String[] args) {
+        StdDraw.setScale(-1, 1);
+        StdDraw.enableDoubleBuffering();
+
+        while(true) {
+            var time = System.currentTimeMillis();
+            var ms = (int) (time % 1000);
+            // this could be changed to doubles to have smooth movement
+            var seconds = (int) (time / 1000 % 60);
+            var minutes = (int) (time / 60_000 % 60);
+            var hours = (int) (time / 3_600_000 % 12);
+            var secondsAngle = (double) seconds / 60 * 2*Math.PI;
+            var minutesAngle = (double) minutes / 60 * 2*Math.PI;
+            var hoursAngle = (double) hours / 12 * 2*Math.PI;
+            
+            StdDraw.clear();
+            StdDraw.setPenColor(StdDraw.BLACK);
+            StdDraw.setPenRadius(0.004);
+            StdDraw.line(0, 0, Math.sin(minutesAngle) * 0.8, Math.cos(minutesAngle) * 0.8);
+            StdDraw.line(0, 0, Math.sin(hoursAngle), Math.cos(hoursAngle));
+            
+            StdDraw.setPenColor(StdDraw.RED);
+            StdDraw.setPenRadius(0.002);
+            StdDraw.line(0, 0, Math.sin(secondsAngle), Math.cos(secondsAngle));
+
+            StdDraw.setPenRadius(0.004);
+            StdDraw.setPenColor(StdDraw.BLACK);
+            StdDraw.circle(0, 0, 1);
+            
+            StdDraw.show();
+            StdDraw.pause(1000 - ms);
+        }
+
+    }
+}

week11/RandomSequenceChange.java

@@ -0,0 +1,26 @@
+import common.StdRandom;
+
+public class RandomSequenceChange {
+    public static void main(String[] args) {
+        var arrayLength = 5;
+        var rounds = 100;
+        var data = new int[arrayLength];
+        for(var i = 0; i < data.length; i++)
+            data[i] = i;
+        // number of times any repetition was found after a random shuffle
+        var total = 0;
+        for(var i = 0; i < rounds; i++) {
+            StdRandom.shuffle(data);
+            total += (countSequences(data) > 0) ? 1 : 0;
+        }
+        var chance = 1 - (total / (double) rounds);
+        System.out.printf("The chance of no repetition is %s%%\n", chance * 100);
+    }
+
+    static int countSequences(int[] data) {
+        var output = 0;
+        for(var i = 1; i < data.length; i++)
+            if(data[i] == data[i - 1] + 1) output++;
+        return output;
+    }
+}

week11/doc.typ

@@ -2,7 +2,7 @@
 
 #show: template
 
-= Week 10
+= Week 11
 
 == Exercise 1.3.45
 
@@ -15,7 +15,7 @@ with a small population—say, $x = 0.01$—and study the result of iterating th
 $x = 1 - 1/r$ ? Can you say anything about the population when $r$ is $3.5$? $3.8$? $5$?
 
 #table(
-  columns: 3,
+  columns: 4,
   $3.5$,
   $3.8$,
   $5$,
@@ -26,15 +26,21 @@ $x = 1 - 1/r$ ? Can you say anything about the population when $r$ is $3.5$? $3.
   [Sustained $1-1/r$]
 )
 
+#embedClass(name: "Chaos")
+
 == Exercise 1.4.26
 
 _Music shuffling_. You set your music player to shuffle mode. It plays each of
-the n songs before repeating any. Write a program to estimate the likelihood that
+the $n$ songs before repeating any. Write a program to estimate the likelihood that
 you will not hear any sequential pair of songs (that is, song 3 does not follow song
 2, song 10 does not follow song 9, and so on).
 
+#embedClass(name: "RandomSequenceChange")
+
 == Exercise 1.5.32
 
 _Clock_. Write a program that displays an animation of the second, minute,
 and hour hands of an analog clock. Use the method `StdDraw.pause(1000)` to
 update the display roughly once per second.
+
+#embedClass(name: "Clock")

week12/Dict.java

@@ -1,102 +1,47 @@
 package week12;
 
-public class Dict<K, V> {
+import java.util.Arrays;
+
+public class Dict<K extends Comparable<K>, V> {
     @SuppressWarnings("unchecked")
     private Pair<K, V>[] data = new Pair[4];
-
-    public static void main(String[] args) {
-        System.out.printf("%s %s %s\n", 1, 4, getIndex(1, 4));
-        System.out.printf("%s %s %s\n", 1, 8, getIndex(1, 8));
-        System.out.printf("%s %s %s\n", 1073741824, 4, getIndex(1073741824, 4));
-        System.out.printf("%s %s %s\n", 1073741824, 8, getIndex(1073741824, 8));
-        System.out.printf("%s %s %s\n", Integer.MIN_VALUE, 4, getIndex(Integer.MIN_VALUE, 4));
-        System.out.printf("%s %s %s\n", Integer.MIN_VALUE, 8, getIndex(Integer.MIN_VALUE, 8));
-        System.out.printf("%s %s %s\n", Integer.MAX_VALUE, 4, getIndex(Integer.MAX_VALUE, 4));
-        System.out.printf("%s %s %s\n", Integer.MAX_VALUE, 8, getIndex(Integer.MAX_VALUE, 8));
-    }
+    private int ptr = 0;
 
     public V get(K key) {
-        var hashCode = key.hashCode();
-        var hash = hashCode;
-        // try getting a pair as long as there's a hash collision
-        while(true) {
-            var index = getIndex(hash);
-            var pair = data[index];
-            if(pair == null) return null;
-            if(pair.first.equals(key)) return pair.second;
-            var secondHash = pair.first.hashCode();
-            if(hashCode != secondHash) return null;
-            // this can overflow but should always be in range of the array when used with getIndex
-            hash += 1;
+        var index = binarySearch(key);
+        if(index >= 0) return data[index].second;
+        return null;
+    }
+
+    // adapted from Arrays.binarySearch
+    // edited as Arrays.binarySearch doesn't accept a mapping function
+    private int binarySearch(K key) {
+        int low = 0;
+        int high = data.length - 1;
+
+        while (low <= high) {
+            int mid = (low + high) >>> 1;
+            var midVal = data[mid];
+            int cmp = PairComparator.compare(midVal == null ? null : midVal.first, key);
+            if (cmp < 0)
+                low = mid + 1;
+            else if (cmp > 0)
+                high = mid - 1;
+            else
+                return mid;
         }
-    }
-
-    private static int getIndex(int hashCode, int length) {
-        // using this instead of modulo allows us to simply insert nulls on every odd number when doubling array instead of having to rehash
-        // this changes the range of number from <MIN_VALUE, MAX_VALUE> to <0, data.length>
-        long hash = hashCode;
-        hash -= Integer.MIN_VALUE; // make the number positive, i.e. from 0 to 2*MAX_VALUE+1 (MIN_VALUE = -MAX_VALUE - 1)
-        hash *= length; // this may cause overflow in int
-        hash /= Integer.MAX_VALUE; // this will always put the number back into int range, as array length is at most MAX_VALUE
-        hash /= 2;
-        return (int)hash;
-    }
-
-    private int getIndex(int hashCode) {
-        return getIndex(hashCode, data.length);
+        return -(low + 1); // key not found.
     }
     
     private void increaseBackingSize() {
-        @SuppressWarnings("unchecked")
-        Pair<K, V>[] newData = new Pair[data.length * 2];
-        for(var i = 0; i < data.length; i++) {
-            newData[i * 2] = data[i];
-        }
-        data = newData;
+        data = Arrays.copyOf(data, data.length * 2);
     }
 
     public void put(K key, V value) {
-        var hashCode = key.hashCode();
-        var hash = hashCode;
-        var index = getIndex(hash);
-        var existingPair = data[index];
-        var newPair = new Pair<>(key, value);
-        if(existingPair != null && existingPair.first.equals(key)) {
-            data[index] = existingPair.withSnd(value);
-            return; // some of these returns aren't needed but directly show exit points of the function at a glance
-        } else if(existingPair != null) {
-            var originalHash = existingPair.first.hashCode();
-            if(hash == originalHash) {
-                // full collision of hash, cannot solve so add +1 to hash and try to insert it again
-                // a different possible solution is to use 2d arrays, with the topmost being a mapping of hashcodes to arrays of pairs
-
-                // insert the pair into first empty space after hash collisions. If there's something else though, increase backing size and try again, it should fit right after then.
-                // Simply trying to put again recomputes stuff needlessly but simplifies code.
-                while(true) {
-                    hash += 1;
-                    index = getIndex(hash);
-                    existingPair = data[index];
-                    if(existingPair == null) {
-                        data[index] = newPair;
-                        return;
-                    }
-                    originalHash = existingPair.first.hashCode();
-                    if(hash == originalHash) {
-                        continue;
-                    } else {
-                        increaseBackingSize();
-                        put(key, value);
-                        return;
-                    }
-                }
-            } else {
-                // a better algorithm would be to calculate the required length for the collision to disappear, but this should also work eventually and is simpler
-                increaseBackingSize();
-                put(key, value);
-                return;
-            }
-        } else {
-            data[index] = newPair;
-        }
+        if(data.length >= ptr) increaseBackingSize();
+        data[ptr] = new Pair<>(key, value);
+        ptr += 1;
+        // O(n) when almost sorted, which would be the case here. Allows use of binarySearch
+        Arrays.sort(data, new PairComparator<>());
     }
 }

week12/Multiset.java

@@ -0,0 +1,27 @@
+package week12;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class Multiset<E> {
+    Map<E, Integer> data = new HashMap<>();
+
+    public int count(E e) {
+        var entry = data.get(e);
+        return entry == null ? 0 : entry.intValue();
+    }
+
+    public void add(E e) {
+        data.compute(e, (key, num) -> num == null ? 1 : num+1);
+    }
+    
+    public void remove(E e) {
+        var entry = data.get(e);
+        if(entry == null || entry <= 1) data.remove(e);
+        else data.put(e, entry - 1);
+    }
+
+    public int size() {
+        return data.size();
+    }
+}

week12/Pair.java

@@ -0,0 +1,32 @@
+package week12;
+
+class Pair<A, B> {
+    final A first;
+    final B second;
+
+    Pair(A first, B second) {
+        this.first = first;
+        this.second = second;
+    }
+
+    // these getters doesn't seem to be needed, stdlib doesn't usually use getters for final values and instead just makes them public to be used directly
+    public A getFirst() {
+        return first;
+    }
+
+    public B getSecond() {
+        return second;
+    }
+
+    public Pair<B, A> swap() {
+        return new Pair<B, A>(second, first);
+    }
+
+    public<C> Pair<C, B> withFst(C first) {
+        return new Pair<C, B>(first, second);
+    }
+
+    public<C> Pair<A, C> withSnd(C second) {
+        return new Pair<A, C>(first, second);
+    }
+}

week12/PairComparator.java

@@ -0,0 +1,17 @@
+package week12;
+
+import java.util.Comparator;
+
+public class PairComparator<A extends Comparable<A>, B> implements Comparator<Pair<A, B>> {
+    @Override
+    public int compare(Pair<A, B> o1, Pair<A, B> o2) {
+        return compare(o1 == null ? null : o1.first, o2 == null ? null : o2.first);
+    }
+
+    static <K extends Comparable<K>> int compare(K firstKey, K secondKey) {
+        if(firstKey == secondKey) return 0;
+        if(firstKey == null) return 1;
+        if(secondKey == null) return -1;
+        return firstKey.compareTo(secondKey);
+    }
+}

week12/common

@@ -0,0 +1 @@
+../common

week12/doc.typ

@@ -0,0 +1,37 @@
+#import "./common/common.typ" : *
+
+#show: template
+
+= Week 12
+
+== Exercise 12.07
+
+Write a class `Pair` with two type parameters `A` and `B` to represent an immutable pair of values (i.e. the class should have two `final` fields of type `A` and `B`).
+
+- Add an appropriate constructor and getter methods.
+  - Do not add any setters, as the class should be immutable.
+- Add a method `swap` to the `Pair` class. The `swap` method should return a *new* pair where the first component becomes the second component and vice versa. For example, for the pair `(true, 42)` the method should return `(42, true)`.
+- Add methods `withFst` and `withSnd` to the Pair class. Each method should take a type parameter `C` and return a new pair where the appropriate component has been updated. For example, calling `withFst` with the integer `42` on the pair `(true, "Hello World")` should return `(42, "Hello World")`.
+
+== Exercise 12.08
+
+Write a class `Dict` that takes two type parameters `K` and `V` to represent a dictionary, i.e. a mapping from items of type `K` to items of type `V`. Internally, the dictionary should maintain a single array of pairs of type `Pair<K, V>`. Add the following methods:
+
+- `V get(K key)` returns the value associated with the given key, or null if the key is not found.
+- `void put(K key, V value)` updates the dictionary with a mapping from the key to the value. If the key already exists, its value is updated. Otherwise, a new pair is added.
+
+You may assume the `Dict` can contain at most 100 entries.
+
+== Exercise 12.16
+
+Write a class, which takes one type parameter `E`, to represent a multiset. A multiset is a set that counts how many times it contains each of its elements. Add the following methods:
+
+- `int count(E e)` returns the number of times the element `e` occurs in the multiset.
+- `void add(E e)` adds the element `e` to the multiset. (Adding increments its count.)
+- `void remove(E e)` removes the element `e` from the multiset. (Removing decrements its count.)
+- `int size()` returns the number of different elements in the set (non-duplicate count).
+
+An element can never occur a negative number of times in a multiset.
+
+_Hint: Use an internal map of type `Map<E, Integer>`._
+