Data commit

Task/Quickselect-algorithm/Python/quickselect-algorithm-2.py

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+'''Quick select'''
+
+from functools import reduce
+
+
+# quickselect :: Ord a => Int -> [a] -> a
+def quickSelect(k):
+    '''The kth smallest element
+       in the unordered list xs.'''
+    def go(k, xs):
+        x = xs[0]
+
+        def ltx(y):
+            return y < x
+        ys, zs = partition(ltx)(xs[1:])
+        n = len(ys)
+        return go(k, ys) if k < n else (
+            go(k - n - 1, zs) if k > n else x
+        )
+    return lambda xs: go(k, xs) if xs else None
+
+
+# TEST ----------------------------------------------------
+# main :: IO ()
+def main():
+    '''Test'''
+
+    v = [9, 8, 7, 6, 5, 0, 1, 2, 3, 4]
+    print(list(map(
+        flip(quickSelect)(v),
+        range(0, len(v))
+    )))
+
+
+# GENERIC -------------------------------------------------
+
+
+# flip :: (a -> b -> c) -> b -> a -> c
+def flip(f):
+    '''The (curried) function f with its
+       arguments reversed.'''
+    return lambda a: lambda b: f(b)(a)
+
+
+# partition :: (a -> Bool) -> [a] -> ([a], [a])
+def partition(p):
+    '''The pair of lists of those elements in xs
+       which respectively do, and don't
+       satisfy the predicate p.'''
+    def go(a, x):
+        ts, fs = a
+        return (ts + [x], fs) if p(x) else (ts, fs + [x])
+    return lambda xs: reduce(go, xs, ([], []))
+
+
+# MAIN ---
+if __name__ == '__main__':
+    main()