Natural ternary logic?

Question

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How can I find the missing value more concisely?

Is there a nice way of expressing the commutative operator T on the alphabet a b c using the language of Python, where

a T b == c
b T c == a
c T a == b

My best attempt is to hardcode things:

def T(first, second):
    if first is 'a' and second is 'b':
        return 'c'
    if first is 'a' and second is 'c':
        return 'c'
    if first is 'b' and second is 'c':
        return 'a'
    if first is 'b' and second is 'a':
        return 'c'
    if first is 'c' and second is 'a':
        return 'b'
    if first is 'c' and second is 'b':
        return 'a'

Please define the requirements for "commutative ternary operator T". What's it do? — Steven Rumbalski, Feb 15 '12 at 15:23
commutative: `i T j == j T i` for any `i, j`. ternary: "on the alphabet `a, b, c`" — Randomblue, Feb 15 '12 at 15:25
http://stackoverflow.com/questions/8792440/how-can-i-find-the-missing-value-more-concisely/ is this what you want? — ChessMaster, Feb 15 '12 at 15:26
what values do you want "a T a", "b T b", and "c T c" to have? — Edward Loper, Feb 15 '12 at 15:32
@Randomblue: Because it does not work, at least not reliably. See http://stackoverflow.com/questions/2988017/string-comparison-in-python-is-vs (and note that the same is true for integer, small ones may be cached but in general you can have multiple integer objects with equal value). — , Feb 15 '12 at 15:35

score 10 · Accepted Answer · edited Feb 15 '12 at 15:28

10

How about this:

alphabet = set(['a', 'b', 'c'])
def T(x, y):
    return (alphabet - set([x, y])).pop()

Using it like this:

T('a', 'b')
> 'c'

edited Feb 15 '12 at 15:28

eumiro

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answered Feb 15 '12 at 15:26

Óscar López

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Your defined `T('a','c')` produces `'b'` and not the `'c'` that the OP defined in his example? Is that just a mistake in his example??? – the wolf Feb 15 '12 at 18:22
1

@carrot-top I believe that's a mistake in the example – Óscar López Feb 15 '12 at 18:58

score 3 · Answer 2 · answered Feb 15 '12 at 15:23

3

l = ['a', 'b', 'c']
return list(set(l) - set((first, second)))[0]

answered Feb 15 '12 at 15:23

eumiro

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PaulMcG · Answer 3 · 2012-02-15T17:08:58.280

Here is a Python class that defines operator '|' such that you can write 'a' |T| 'b' and get your resulting 'c':

class Ternary(object):
    def __init__(self, *items):
        if len(items) != 3:
            raise ValueError("must initialize with exactly 3 items")

        self.items = set(items)
        self.left = None

    def __ror__(self, other):
        ret = Ternary(*list(self.items))
        ret.left = other
        return ret

    def __or__(self, other):
        if self.left is not None:
            ret = (self.items-set([self.left,other])).pop()
            return ret
        else:
            raise ValueError("cannot process right side without left side")

T = Ternary('a', 'b', 'c')
for test in """'a' |T| 'c'
               'a' |T| 'b'
               'c' |T| 'b'""".splitlines():
    test = test.strip()
    print test, '->', eval(test)

Prints:

'a' |T| 'c' -> b
'a' |T| 'b' -> c
'c' |T| 'b' -> a

score 1 · Answer 4 · answered Feb 15 '12 at 15:35

1

>>> def T(first, second):
...     s = ord('a') + ord('b') + ord('c')
...     return chr(s - ord(first) - ord(second))
... 
>>> T('a', 'b')
'c'

answered Feb 15 '12 at 15:35

Mariy

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score 1 · Answer 5 · answered Feb 15 '12 at 18:13

1

How about a look-up table:

def T(first, second):
   d={'ab':'c',
      'ac':'c',
      'bc':'a',
      'ba':'c',
      'ca':'b',
      'cb':'a'}

      st=''.join([first,second])
      if d[st]: return d[st]
      else: return None

answered Feb 15 '12 at 18:13

the wolf

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Natural ternary logic?

5 Answers5