COP 3223H meeting -*- Outline -*-

* Boolean expressions

** Boolean values
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      BOOLEAN VALUES IN PYTHON

The two values: 
  are the only values of type bool

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        ... False, True
        Note the initial capital letters,
           show what happens when you give true or false to the interpreter!


** Boolean Expressions
*** Boolean operators
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       OPERATORS ON BOOLEANS

>>> True and True

>>> True and False

>>> False and True

>>> False and False

>>> True or True

>>> True or False

>>> False or True

>>> False or False

>>> not True

>>> not False

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    Q: So what does "and" do?
    it tells if both subexpressions are True.
    This is && in C
    Q: What does "or" do?
    it tells whether one subexpression is True
    This is || in C
    Q: What does "not" do?
    it changes True to False and vice versa, negating the expression.
    This is ! in C

*** Boolean-valued expressions
    You rarely need to write True or False explicitly in a program,
    because most boolean values arise from other expressions...
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     BOOLEAN-VALUED EXPRESSIONS

Comparisons have bool values:

>>> 2 < 3

>>> 2 < 3

>>> 3 > 2

>>> 2 > 3

>>> 2 == 2

>>> 2 == 3

>>> 2 = 2  # error!

>>> v = 2
>>> v = v  # what does this do?
>>> v == v

>>> 2 != 3

>>> not (2 == 3)

>>> 2 <= 3

>>> 2 < 3 or 2 == 3

>>> 3 <= 2

>>> 3 <= 3

>>> 2 <= 2

>>> 2 <= 3 and 3 <= 4

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        Note the difference between == and = in Python
        Q: What does <= mean?
        Q: How is <= different from < in expressions?
        Q: What do you think the >= operator does?

        Actually 2 <= 3 and 3 <= 4 can be written 2 <= 3 <= 4 in Python,
           but not in most other languages (like C).

** Tips for writing Boolean expressions

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   DON'T USE TRUE AND FALSE EXPLICITLY

Rule 1 Prefer:



instead of

     e == True

Example:

  if weight <= 160 and height <= 6:
     return "OK"

vs.

  if (weight <= 160 and height <= 6) == True:
     return "OK"
     
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      ... e

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   DON'T USE TRUE AND FALSE EXPLICITLY

Rule 2: Prefer:

     

instead of:

     e == False
     
Example:

     if not (weight <= 160 and height <= 6):
         return "Heavy"
vs.

     if (weight <= 160 and height <= 6) == False:
         return "Heavy"
         
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     ... not e