86 lines
2.2 KiB
Python
86 lines
2.2 KiB
Python
'''Middle 3 digits'''
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# mid3digits :: Int -> Either String String
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def mid3digits(n):
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'''Either the middle three digits,
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or an explanatory string.'''
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m = abs(n)
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s = str(m)
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return Left('Less than 3 digits') if (100 > m) else (
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Left('Even digit count') if even(len(s)) else Right(
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s[(len(s) - 3) // 2:][0:3]
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)
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)
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# TEST ----------------------------------------------------
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def main():
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'''Test'''
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def bracketed(x):
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return '(' + str(x) + ')'
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print(
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tabulated('Middle three digits, where defined:\n')(str)(
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either(bracketed)(str)
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)(mid3digits)([
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123, 12345, 1234567, 987654321, 10001, -10001, -123,
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-100, 100, -12345, 1, 2, -1, -10, 2002, -2002, 0
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])
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)
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# GENERIC -------------------------------------------------
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# Left :: a -> Either a b
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def Left(x):
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'''Constructor for an empty Either (option type) value
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with an associated string.'''
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return {'type': 'Either', 'Right': None, 'Left': x}
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# Right :: b -> Either a b
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def Right(x):
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'''Constructor for a populated Either (option type) value'''
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return {'type': 'Either', 'Left': None, 'Right': x}
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# compose (<<<) :: (b -> c) -> (a -> b) -> a -> c
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def compose(g):
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'''Function composition.'''
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return lambda f: lambda x: g(f(x))
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# either :: (a -> c) -> (b -> c) -> Either a b -> c
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def either(fl):
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'''The application of fl to e if e is a Left value,
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or the application of fr to e if e is a Right value.'''
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return lambda fr: lambda e: fl(e['Left']) if (
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None is e['Right']
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) else fr(e['Right'])
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# even :: Int -> Bool
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def even(x):
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'''Is x even ?'''
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return 0 == x % 2
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# tabulated :: String -> (b -> String) -> (a -> b) -> [a] -> String
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def tabulated(s):
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'''Heading -> x display function -> fx display function ->
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f -> value list -> tabular string.'''
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def go(xShow, fxShow, f, xs):
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w = max(map(compose(len)(str), xs))
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return s + '\n' + '\n'.join(
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xShow(x).rjust(w, ' ') + ' -> ' + fxShow(f(x)) for x in xs
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)
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return lambda xShow: lambda fxShow: lambda f: lambda xs: go(
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xShow, fxShow, f, xs
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)
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if __name__ == '__main__':
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main()
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