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Boolean Expressions

Cai Gengyang gengyangcai at gmail.com
Tue Sep 26 23:55:26 EDT 2017 
On Wednesday, September 27, 2017 at 6:45:00 AM UTC+8, Cameron Simpson wrote:
> On 26Sep2017 14:43, Cai Gengyang <gengyangcai at gmail.com> wrote:
> >Help check if my logic is correct in all 5 expressions
> 
> Why not just run some code interactively? Unless this is entirely a thought 
> exercise to verify that you have a solid mental understanding of Python 
> semantics, all your reasoning is easy to test.
> 
> In order to be honest to yourself, you could write does all your answers 
> (exactly as you have just done), _then_ go and run the expressions by hand in 
> python to see which are correct. You can also run the subparts of the 
> expressions, so that you can see which you have misevaluated versus which you 
> have made correct/incorrect logic reasoning about.
> 
> >A) Set bool_one equal to the result of
> >False and False
> >
> >Entire Expression : False and False gives True because both are False
> 
> No. False and anything gives False. An "and" only gives True if both sides are 
> true.
> 
> >B) Set bool_two equal to the result of
> >-(-(-(-2))) == -2 and 4 >= 16 ** 0.5
> >
> >-(-(-(-2))) is equal to 2, and 2 is not equal to -2, hence the first term       -(-(-(-2))) == -2 is False. 4 >= 16 ** 0.5 is True because 16 ** 0.5 is equal to 4, and 4 is greater then or equal to 4, hence the 2nd term 4 >= 16 ** 0.5 is True.
> >
> >Entire expression : False because False and True gives False
> 
> In python, "and" and "or" short circuit. So if you know enough to evaluate the 
> condition from the left hand side, the right hand side is not evaluated at all.
> 
> So since 2 == -2 gives False, the expresion is False and the right hand is not 
> evaluated.
> 
> Note, BTW, that 16 ** 0.5 returns a floating point value. While that particular 
> example works nicely, probably because it is all powers of 2 and doesn't hit 
> rounding issues, testing equality with floating point is a dangerous area.
> 
> >C) Set bool_three equal to the result of
> >19 % 4 != 300 / 10 / 10 and False
> >
> >19 % 4 = 3 which is equal to 300 / 10 / 10 = 3, hence the first term is False. Entire expression is then equal to True, because False and False = True
> 
> Entire expression is False because the left hand side is False.
> 
> >D) Set bool_four equal to the result of
> >-(1 ** 2) < 2 ** 0 and 10 % 10 <= 20 - 10 * 2
> >
> >-(1 ** 2) is equal to -1 , which is less than 2 ** 0 = 1, hence the first term is True. 2nd term 10 % 10 is equal to 0 , which is less than or equal to 20 - 10 * 2 , hence 2nd term is True.
> >
> >Entire expression : True and True = True
> 
> Correct. (In my head; haven't run the code.)
> 
> >E) Set bool_five equal to the result of
> >True and True
> >
> >Entire Expression : True and True = True
> 
> Correct.
> 
> Cheers,
> Cameron Simpson <cs at cskk.id.au>
> 
> ERROR 155 - You can't do that.  - Data General S200 Fortran error code list



> 19 % 4 = 3 which is equal to 300 / 10 / 10 = 3, hence the first term is False. Entire expression is then equal to True, because False and False = True
> 
> Entire expression is False because the left hand side is False.


Am I missing something here ? 19 % 4 = 19 modulo 4 equals to 3 right ? which equals the right hand side , hence first term is True
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