# Exercise: 9. This and That (or This, But Not That!)

#1

This exercise became really confusing. Even though the instructions do say to refer to the “Stuck? Get a Hint”, I was still lost . And still am. The Hint says “True or not False and False. not gets evaluated first, so we have True or True and False. and goes next, so we get True or False. As we’ve seen, True or False is True, so the value finally returned is True!” All I see is “True, False, True, Not…” So on. Can someone clarify in more simpler terms?

#2

Precedence

``````not
and
or
``````

in that order. `not` applies to the value immediately to its right.

#3

I’m still confused, but I’ll try to read the previous exercise to understand. Why does coding get so complicated??? Quick question though; Can you give me an example other than the one one the exercise? Because I think maybe listening the someone else response can give me another viewpoint on this situation.

#4

Like most things, it gets easier as you go, once the concepts are clear.

Consider the basic truth tables.

``````A AND B
T  F
T  T  F
F  F  F

A OR B
T  F
T  T  T
F  T  F
``````

So,

``````True and True   => True
True and False  => False
False and True  => False
False and False => False

True or True    => True
True or False   => True
False or True   => True
False or False  => False
``````

As we can see, only when both operands are True is `A and B` True, otherwise False; and, only when both operands are False is `A or B` False, otherwise True. This is known as short-circuiting. Any operand that is False in an AND expression will short-circuit to False as the result. Any operand that is True in an OR expression will short-circuit to True as a result.

``````A or B or C or D
``````

will be True if any one of the operands is True.

``````A and B and C and D
``````

will be False if any one of the operands is False.

This gets a little more complicated when we consider casting which is when a value or expression is evaluated as a boolean. Truthy values will cast to True, and Falsy values will cast to False.

``````0, '', "", None
``````

are all Falsy, so cast to False in an expression:

``````0 or '' or "" or None     => False

0 and '' and "" and None  => False
``````

Everything else is Truthy.

``````1 and 'a' and "A" and not None  => True
``````

See where we snuck in the `not`? That negates a boolean, so if a value is True, it becomes False, and if it is False, it becomes True.

Let’s use the bool() class to test the above.

``````>>> bool(0)
False
>>> bool('')
False
>>> bool("")
False
>>> bool(None)
False
>>>
``````

Above we used the `bool` function to convert (cast) the values to their boolean equivalent.

``````>>> bool(1)
True
>>> bool('a')
True
>>> bool("A")
True
>>> not None
True
>>>
``````

Notice how `not` behaves just like the `bool` function? That’s right, we can use `not` to coerce (force cast) a boolean from an expression or value.

``````>>> not 1
False
>>> not 0
True
>>> not 'real'
False
>>> not not 'real'
True
>>>
``````

This takes a little while to get straight so work on lots of exercises to cement the ideas in your head. It may not seem important but it is at the very core of logic and is vital to the cause of programming. Programs make all their decisions using IF, AND, OR and NOT conditional expressions.

I’ll use another post to go over the lesson material with you.

#5

Set bool_one equal to the result of

``````False or not True and True
``````

Set bool_two equal to the result of

``````False and not True or True
``````

Set bool_three equal to the result of

``````True and not (False or False)
``````

Set bool_four equal to the result of

``````not not True or False and not True
``````

Set bool_five equal to the result of

``````False or not (True and True)
``````

`bool_one`

``````False or not True and True
``````

Start with NOT, `not True` => `False`

Rewriting gives,

``````False or False and True
``````

Next work with the AND, `False and True` => `False`

Rewriting gives,

``````False or False
``````

which we know to be False.

`bool_two`

``````False and not True or True
``````

``````False and False or True,
``````

Next the AND,

``````False or True
``````

which we know to be True.

`bool_three`

``````True and not (False or False)
``````

Brackets always come first so,

``````True and not False
``````

then NOT,

``````True and True
``````

which we know to be True.

`bool_four`

``````not not True or False and not True
``````

First the NOT,

``````True or False and False
``````

then the AND,

``````True or False
``````

which we know to be True.

`bool_five`

``````False or not (True and True)
``````

Brackets come first,

``````False or not True
``````

Then the NOT,

``````False or False
``````

which we know to be False.

#6

Wow, so much more simple. Thanks a lot Mr. Roy. This helped very much.

#7

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