Data commit

Task/Forward-difference/Python/forward-difference-2.py

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+'''Forward difference'''
+
+
+from itertools import islice
+from operator import sub
+
+
+# forwardDifference :: Num a => [a] -> [a]
+def forwardDifference(xs):
+    '''1st order forward difference of xs.
+    '''
+    return [sub(*x) for x in zip(xs[1:], xs)]
+
+
+# nthForwardDifference :: Num a => [a] -> Int -> [a]
+def nthForwardDifference(xs):
+    '''Nth order forward difference of xs.
+    '''
+    return index(iterate(forwardDifference)(xs))
+
+
+# ------------------------- TEST --------------------------
+# main :: IO ()
+def main():
+    '''Nth order forward difference.'''
+
+    xs = [90, 47, 58, 29, 22, 32, 55, 5, 55, 73]
+
+    print('9th order forward difference of:')
+    print(xs)
+    print('')
+    print(
+        ' -> ' + repr(nthForwardDifference(xs)(9))
+    )
+
+    print('\nSuccessive orders of forward difference:')
+    print(unlines([
+        str(i) + ' -> ' + repr(x) for i, x in
+        enumerate(take(10)(
+            iterate(forwardDifference)(xs)
+        ))
+    ]))
+
+
+# ------------------- GENERIC FUNCTIONS -------------------
+
+# index (!!) :: [a] -> Int -> a
+def index(xs):
+    '''Item at given (zero-based) index.'''
+    return lambda n: None if 0 > n else (
+        xs[n] if (
+            hasattr(xs, "__getitem__")
+        ) else next(islice(xs, n, None))
+    )
+
+
+# iterate :: (a -> a) -> a -> Gen [a]
+def iterate(f):
+    '''An infinite list of repeated
+       applications of f to x.
+    '''
+    def go(x):
+        v = x
+        while True:
+            yield v
+            v = f(v)
+    return go
+
+
+# take :: Int -> [a] -> [a]
+# take :: Int -> String -> String
+def take(n):
+    '''The prefix of xs of length n,
+       or xs itself if n > length xs.
+    '''
+    return lambda xs: (
+        xs[0:n]
+        if isinstance(xs, (list, tuple))
+        else list(islice(xs, n))
+    )
+
+
+# unlines :: [String] -> String
+def unlines(xs):
+    '''A single string formed by the intercalation
+       of a list of strings with the newline character.
+    '''
+    return '\n'.join(xs)
+
+
+# MAIN ---
+if __name__ == '__main__':
+    main()