Added example solutions to several chapters. Feel free to create a pull request with your answers. Also for the chapters that have no solutions yet :)

CH_05_functional_programming/exercise_01/solution_00.py

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+# Implement the quicksort algorithm.
+import random
+
+# one-liner approach
+qs = lambda xs: xs if len(xs) <= 1 else qs(
+    [x for x in xs[1:] if x < xs[0]]) + [xs[0]] + qs(
+    [x for x in xs[1:] if x >= xs[0]])
+
+
+# more verbose approach
+def quicksort(xs):
+    if len(xs) <= 1:
+        return xs
+    else:
+        left = quicksort([x for x in xs[1:] if x < xs[0]])
+        right = quicksort([x for x in xs[1:] if x >= xs[0]])
+        middle = [xs[0]]
+        return left + middle + right
+
+
+def main():
+    # test
+    xs = random.sample(range(1000), 100)
+    assert quicksort(xs) == sorted(xs)
+    assert qs(xs) == sorted(xs)
+
+
+if __name__ == '__main__':
+    main()

CH_05_functional_programming/exercise_02/solution_00.py

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+# Write a groupby function that isn’t affected by sorting.
+import collections
+
+
+def groupby(func, seq):
+    groups = collections.defaultdict(list)
+    for item in seq:
+        groups[func(item)].append(item)
+    return groups
+
+
+def main():
+    # Explicitly defined test data for clarity.
+    xs = [0, 1, 2, 3, 4, 5, 6, 7]
+
+    assert groupby(lambda x: x % 2, xs) == {
+        0: [0, 2, 4, 6],
+        1: [1, 3, 5, 7],
+    }
+
+    assert groupby(
+        lambda x: 'even' if x % 2 == 0 else 'odd',
+        xs,
+    ) == {'even': [0, 2, 4, 6], 'odd': [1, 3, 5, 7]}
+
+    assert groupby(lambda x: x > 5, xs) == {
+        False: [0, 1, 2, 3, 4, 5],
+        True: [6, 7],
+    }
+
+
+if __name__ == '__main__':
+    main()

CH_05_functional_programming/exercise_03/solution_00.py

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+# Write a groupby function that returns lists of results instead of 
+# generators.
+
+import pprint
+
+
+def groupby(iterable, key=None):
+    '''
+    Return a dictionary of lists of items grouped by the key function.
+
+    Note that as opposed to the itertools.groupby function, this function
+    does not require the iterable to be sorted.
+    '''
+    if key is None:
+        key = lambda x: x
+    groups = {}
+    for item in iterable:
+        groups.setdefault(key(item), []).append(item)
+    return groups
+
+
+def main():
+    # Demo data from the itertools docs
+    pprint.pprint(groupby('AAAABBBCCDAABBB'))
+    pprint.pprint(groupby('AAAABBBCCD'))
+
+
+if __name__ == '__main__':
+    main()