3 state, 3 input function with bitwise operators does not produce expected result

Question

This is some code that I wrote up to generate a set of numbers;

def _tnry(x,y,z):
    a = None

    if((y == 0 & z == 0) | (x == 0 & y == 0) | (x == 0 & z == 0)):
        a = 1

    if((y == 1 & z == 1) | (x == 1 & y == 1) | (x == 1 & z == 1)):
        a = 2

    if((y == 2 & z == 2) | (x == 2 & y == 2) | (x == 2 & z == 2)):
        a = 0

    print(x,y,z,'ternary =',a )

I am having some problems with the output when you give the input of the following:

_tnry(0,1,2)
_tnry(0,2,1)
_tnry(1,0,2)
_tnry(1,2,0)
_tnry(2,0,1)
_tnry(2,1,0)

As far as I can see in my code a should not ever come out as being equal to 0, 1 or 2. I want to force it to always be None in the examples given.

All other output from the script is how I want it to come out.

Answer 1

Bitwise operators & have higher precedence over other operators, so

y == 0 & z == 0

is actually interpreted as

y == (0 & z) == 0

Which ends up becoming

y == 0 == 0

Which is different from what you want. This applies to all your conditions.

You'll want to fix this by using another layer of parens:

(y == 0) & (z == 0)

Answer 2

Here's a shorter and easier-to-debug, although maybe less clear, way to approach it (assumes all inputs are 0,1, or 2, and that you do in fact mean or and and rather than bitwise):

def _tnry(x,y,z):

    if len({x,y,z}) < 3: #at least two are the same:
        return (sorted([x,y,z])[1] + 1) % 3

If at least two are the same, then the middle one when they're sorted will be from the majority. Returns None by default, like all Python functions.

Answer 3

use and for logical and.

def _tnry(x,y,z):
    a = None
    if((y == 0 and z == 0) | (x == 0 and y == 0) | (x == 0 and z == 0)):
        a = 1
    if((y == 1 and z == 1) | (x == 1 and y == 1) | (x == 1 and z == 1)):
        a = 2
    if((y == 2 and z == 2) | (x == 2 and y == 2) | (x == 2 and z == 2)):
        a = 0
    print(x,y,z,'ternary =',a )

if there's no need to constrain the input to 0,1,2 I would write it like this:

def _tnry(x,y,z):
    if y == z or x == z :
        a = z
    elif x == y : 
        a = x
    else:
        a = None
    print(x,y,z,'ternary =',a )