Initial data commit

Task/Dutch-national-flag-problem/Python/dutch-national-flag-problem-1.py

@@ -0,0 +1,35 @@
+import random
+
+colours_in_order = 'Red White Blue'.split()
+
+def dutch_flag_sort(items, order=colours_in_order):
+    'return sort of items using the given order'
+    reverse_index = dict((x,i) for i,x in enumerate(order))
+    return sorted(items, key=lambda x: reverse_index[x])
+
+def dutch_flag_check(items, order=colours_in_order):
+    'Return True if each item of items is in the given order'
+    reverse_index = dict((x,i) for i,x in enumerate(order))
+    order_of_items = [reverse_index[item] for item in items]
+    return all(x <= y for x, y in zip(order_of_items, order_of_items[1:]))
+
+def random_balls(mx=5):
+    'Select from 1 to mx balls of each colour, randomly'
+    balls = sum([[colour] * random.randint(1, mx)
+                 for colour in colours_in_order], [])
+    random.shuffle(balls)
+    return balls
+
+def main():
+    # Ensure we start unsorted
+    while True:
+        balls = random_balls()
+        if not dutch_flag_check(balls):
+            break
+    print("Original Ball order:", balls)
+    sorted_balls = dutch_flag_sort(balls)
+    print("Sorted Ball Order:", sorted_balls)
+    assert dutch_flag_check(sorted_balls), 'Whoops. Not sorted!'
+
+if __name__ == '__main__':
+    main()

Task/Dutch-national-flag-problem/Python/dutch-national-flag-problem-2.py

@@ -0,0 +1,5 @@
+from itertools import chain
+def dutch_flag_sort2(items, order=colours_in_order):
+    'return summed filter of items using the given order'
+    return list(chain.from_iterable(filter(lambda c: c==colour, items)
+                                    for colour in order))

Task/Dutch-national-flag-problem/Python/dutch-national-flag-problem-3.py

@@ -0,0 +1,3 @@
+def dutch_flag_sort2(items, order=colours_in_order):
+    'return summed filter of items using the given order'
+    return [c for colour in order for c in items if c==colour]

Task/Dutch-national-flag-problem/Python/dutch-national-flag-problem-4.py

@@ -0,0 +1,3 @@
+def dutch_flag_sort3(items, order=colours_in_order):
+    'counts each colour to construct flag'
+    return sum([[colour] * items.count(colour) for colour in order], [])

Task/Dutch-national-flag-problem/Python/dutch-national-flag-problem-5.py

@@ -0,0 +1,50 @@
+import random
+
+colours_in_order = 'Red White Blue'.split()
+
+def dutch_flag_sort(items):
+    '''\
+    In-place sort of list items using the given order.
+    Python idiom is to return None when argument is modified in-place
+
+    O(n)? Algorithm from Go language implementation of
+    http://www.csse.monash.edu.au/~lloyd/tildeAlgDS/Sort/Flag/'''
+
+    lo, mid, hi = 0, 0, len(items)-1
+    while mid <= hi:
+        colour = items[mid]
+        if colour == 'Red':
+            items[lo], items[mid] = items[mid], items[lo]
+            lo += 1
+            mid += 1
+        elif colour == 'White':
+            mid += 1
+        else:
+            items[mid], items[hi] = items[hi], items[mid]
+            hi -= 1
+
+def dutch_flag_check(items, order=colours_in_order):
+    'Return True if each item of items is in the given order'
+    order_of_items = [order.index(item) for item in items]
+    return all(x <= y for x, y in zip(order_of_items, order_of_items[1:]))
+
+def random_balls(mx=5):
+    'Select from 1 to mx balls of each colour, randomly'
+    balls = sum(([[colour] * random.randint(1, mx)
+                 for colour in colours_in_order]), [])
+    random.shuffle(balls)
+    return balls
+
+def main():
+    # Ensure we start unsorted
+    while 1:
+        balls = random_balls()
+        if not dutch_flag_check(balls):
+            break
+    print("Original Ball order:", balls)
+    dutch_flag_sort(balls)
+    print("Sorted Ball Order:", balls)
+    assert dutch_flag_check(balls), 'Whoops. Not sorted!'
+
+if __name__ == '__main__':
+    main()