June 2018 Update

Task/Dutch-national-flag-problem/00DESCRIPTION

@@ -1,4 +1,4 @@
-[[File:Dutch_flag_3.jpg|200px||right]]
+[[File:Dutch_flag_3.jpg|350px||right]]
 
 The Dutch national flag is composed of three coloured bands in the order red then white and lastly blue. The problem posed by [[wp:Edsger Dijkstra|Edsger Dijkstra]] is:
 :Given a number of red, blue and white balls in random order, arrange them in the order of the colours Dutch national flag.

Task/Dutch-national-flag-problem/AWK/dutch-national-flag-problem-1.awk

@@ -0,0 +1,47 @@
+BEGIN {
+    weight[1] = "red"; weight[2] = "white"; weight[3] = "blue";
+    # ballnr must be >= 3. Using very high numbers here may make your computer
+    # run out of RAM. (10 millions balls ~= 2.5GiB RAM on x86_64)
+    ballnr = 10
+
+    srand()
+    # Generating a random pool of balls. This python-like loop is actually
+    # a prettyfied one-liner
+    do
+        for (i = 1; i <= ballnr; i++)
+            do
+                balls[i] = int(3 * rand() + 1)
+            # These conditions ensure the 3 first balls contains
+            # a white, blue and red ball. Removing 'i < 4' would
+            # hit performance a lot.
+            while ( (i < 4 && i > 1 && balls[i] == balls[i - 1]) ||
+                    (i < 4 && i > 2 && balls[i] == balls[i - 2]) )
+    while (is_dnf(balls, ballnr))
+
+    printf("BEFORE: ")
+    print_balls(balls, ballnr, weight)
+
+    # Using gawk default quicksort. Using variants of PROCINFO["sorted_in"]
+    # wasn't faster than a simple call to asort().
+    asort(balls)
+
+    printf("\n\nAFTER : ")
+    print_balls(balls, ballnr, weight)
+
+    sorting = is_dnf(balls, ballnr) ? "valid" : "invalid"
+    print("\n\nSorting is " sorting ".")
+}
+
+function print_balls(balls, ballnr, weight      ,i) {
+    for (i = 1; i <= ballnr; i++)
+        printf("%-7s", weight[balls[i]])
+}
+
+function is_dnf(balls, ballnr) {
+    # Checking if the balls are sorted in the Dutch national flag order,
+    # using a simple scan with weight comparison
+    for (i = 2; i <= ballnr; i++)
+        if (balls[i - 1] > balls[i])
+            return 0
+    return 1
+}

Task/Dutch-national-flag-problem/AWK/dutch-national-flag-problem-2.awk

@@ -0,0 +1,5 @@
+BEFORE: blue   red    white  red    white  blue   red    white  blue   white
+
+AFTER : red    red    red    white  white  white  white  blue   blue   blue
+
+Sorting is valid.

Task/Dutch-national-flag-problem/Java/dutch-national-flag-problem.java

@@ -6,41 +6,25 @@ public class DutchNationalFlag {
         RED, WHITE, BLUE
     }
 
-    public static void main(String[] args) {
+    public static void main(String[] args){
         DutchColors[] balls = new DutchColors[12];
-
         DutchColors[] values = DutchColors.values();
         Random rand = new Random();
-        for (int i = 0; i < balls.length; i++)
-            balls[i] = values[rand.nextInt(values.length)];
 
+        for (int i = 0; i < balls.length; i++)
+            balls[i]=values[rand.nextInt(values.length)];
         System.out.println("Before: " + Arrays.toString(balls));
 
-        dutchNationalFlagSort(balls);
+        Arrays.sort(balls);
+        System.out.println("After:  " + Arrays.toString(balls));
 
-        System.out.println("After : " + Arrays.toString(balls));
-    }
-
-    private static void dutchNationalFlagSort(DutchColors[] items) {
-        int lo = 0, mid = 0, hi = items.length - 1;
-
-        while (mid <= hi)
-            switch (items[mid]) {
-                case RED:
-                    swap(items, lo++, mid++);
-                    break;
-                case WHITE:
-                    mid++;
-                    break;
-                case BLUE:
-                    swap(items, mid, hi--);
-                    break;
+        boolean sorted = true;
+        for (int i = 1; i < balls.length; i++ ){
+            if (balls[i-1].compareTo(balls[i]) > 0){
+                sorted=false;
+                break;
             }
-    }
-
-    private static void swap(DutchColors[] arr, int a, int b) {
-        DutchColors tmp = arr[a];
-        arr[a] = arr[b];
-        arr[b] = tmp;
+        }
+        System.out.println("Correctly sorted: " + sorted);
     }
 }

Task/Dutch-national-flag-problem/Ring/dutch-national-flag-problem.ring

@@ -0,0 +1,25 @@
+# Project : Dutch national flag problem
+# Date    : 2017/11/23
+# Author : Gal Zsolt (~ CalmoSoft ~)
+# Email   : <calmosoft@gmail.com>
+
+flag = ["Red","White","Blue"]
+balls = list(10)
+
+see "Random: |"
+for i = 1 to 10
+     color = random(2) + 1
+     balls[i] = flag[color]
+     see  balls[i] + " |"
+next
+see nl
+
+see "Sorted: |"
+for i = 1 to 3
+     color = flag[i]
+     for j = 1 to 10
+          if balls[j] = color
+             see balls[j] + " |"
+          ok
+     next
+next

Task/Dutch-national-flag-problem/Ruby/dutch-national-flag-problem.rb

@@ -0,0 +1,25 @@
+class Ball
+  FLAG = {red: 1, white: 2, blue: 3}
+
+  def initialize
+    @color = FLAG.keys.sample
+  end
+
+  def color
+    @color
+  end
+
+  def <=>(other)  # needed for sort, results in -1 for <, 0 for == and 1 for >.
+    FLAG[self.color] <=> FLAG[other.color]
+  end
+
+  def inspect
+    @color
+  end
+end
+
+balls = []
+balls = Array.new(8){Ball.new} while balls == balls.sort
+
+puts "Random: #{balls}"
+puts "Sorted: #{balls.sort}"