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Conditionals And Control Flows

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  Conditionals And Control Flows Cai Gengyang <gengyangcai@gmail.com> - 2016-05-04 07:41 -0700
    Re: Conditionals And Control Flows Bob Gailer <bgailer@gmail.com> - 2016-05-04 10:54 -0400
    Re: Conditionals And Control Flows Michael Selik <michael.selik@gmail.com> - 2016-05-04 14:56 +0000
    Re: Conditionals And Control Flows Chris Angelico <rosuav@gmail.com> - 2016-05-05 00:58 +1000
    Re: Conditionals And Control Flows Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-05-04 18:10 +0300
      Re: Conditionals And Control Flows Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-05-04 16:21 +0100
      Re: Conditionals And Control Flows Cai Gengyang <gengyangcai@gmail.com> - 2016-05-04 08:23 -0700
        Re: Conditionals And Control Flows Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-05-04 19:41 +0300
    Re: Conditionals And Control Flows Stephen Hansen <me+python@ixokai.io> - 2016-05-04 11:54 -0700

#108133 — Conditionals And Control Flows

I am trying to understand the boolean operator "and" in Python. It is supposed to return "True" when the expression on both sides of "and" are true 

For instance,

1 < 3 and 10 < 20 is True --- (because both statements are true)
1 < 5 and 5 > 12 is False --- (because both statements are false)

bool_one = False and False --- This should give False because none of the statements are False
bool_two = True and False --- This should give False because only 1 statement is True 
bool_three = False and True --- This should give False because only 1 statement is True
bool_five = True and True --- This should give True because only 1 statement is True 

Am I correct ?

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#108135

On May 4, 2016 10:45 AM, "Cai Gengyang" <gengyangcai@gmail.com> wrote:
>
> I am trying to understand the boolean operator "and" in Python. It is
supposed to return "True" when the expression on both sides of "and" are
true
>
> For instance,
>
> 1 < 3 and 10 < 20 is True --- (because both statements are true)
> 1 < 5 and 5 > 12 is False --- (because both statements are false)
>
> bool_one = False and False --- This should give False because none of the
statements are False
> bool_two = True and False --- This should give False because only 1
statement is True
> bool_three = False and True --- This should give False because only 1
statement is True
> bool_five = True and True --- This should give True because only 1
statement is True
>
> Am I correct ?
Yes. Caveat : python Boolean operators return the value of the last
argument necessary to determine the outcome.
Example :
2 or 3 results in 2
2 and 3 results in 3.
0 and 3 results in 0.

HTH.

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#108136

On Wed, May 4, 2016 at 10:46 AM Cai Gengyang <gengyangcai@gmail.com> wrote:

> I am trying to understand the boolean operator "and" in Python. It is
> supposed to return "True" when the expression on both sides of "and" are
> true
>

Not exactly, because they will short-circuit. Take a look at the docs. (
https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not
)

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#108137

On Thu, May 5, 2016 at 12:41 AM, Cai Gengyang <gengyangcai@gmail.com> wrote:
> I am trying to understand the boolean operator "and" in Python. It is supposed to return "True" when the expression on both sides of "and" are true
>
> For instance,
>
> 1 < 3 and 10 < 20 is True --- (because both statements are true)
> 1 < 5 and 5 > 12 is False --- (because both statements are false)
>
> bool_one = False and False --- This should give False because none of the statements are False
> bool_two = True and False --- This should give False because only 1 statement is True
> bool_three = False and True --- This should give False because only 1 statement is True
> bool_five = True and True --- This should give True because only 1 statement is True
>
> Am I correct ?

Not entirely so, but very close. So long as you stick to the exact
values True and False (or simple conditional expressions, like your
examples), yes, that's what you'd get.

The best way to try these out is the interactive interpreter. On
Windows, look in your Start menu for "IDLE"; on other platforms, open
up a terminal and type "python3". Then just start messing around:

>>> 1 < 3 and 10 < 20
True
>>> 1 < 5 and 5 > 12
False

This is far and away the easiest way to learn how Python works. You
can even play with some other things, and learn how Python's 'and'
operator handles other types of data:

>>> 1 and 4
4
>>> 0 and 3
0

Confused? Keep messing around. Build up a theory as to what's going
on, test your theory, then go check your theory against the
documentation. (Or come and ask here, if you can't find it in the
docs.) Python doesn't mind how much you poke around with it, and you
will learn ever so much more than we can explain!

ChrisA

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#108138

Cai Gengyang writes:

> I am trying to understand the boolean operator "and" in Python. It is
> supposed to return "True" when the expression on both sides of "and"
> are true
>
> For instance,
>
> 1 < 3 and 10 < 20 is True --- (because both statements are true)

Yes.

> 1 < 5 and 5 > 12 is False --- (because both statements are false)

No :)

> bool_one = False and False --- This should give False because none of the statements are False
> bool_two = True and False --- This should give False because only 1 statement is True 
> bool_three = False and True --- This should give False because only 1 statement is True

Yes.

> bool_five = True and True --- This should give True because only 1 statement is True 

No :)

> Am I correct ?

Somewhat.

In a technical programming-language sense, these are "expressions", not
"statements". Technically, if the first expression evaluates to a value
that counts as true in Python, the compound expression "E and F"
evaluates to the value of the second expression.

Apart from False, "empty" values like 0, "", [] count as false in
Python, and all the others count as true.

But it's true that "E and F" only evaluates to a true value when both E
and F evaluate to a true value.

Your subject line is good: Python's "and" is indeed a conditional,
control-flow operator.

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#108139

Jussi Piitulainen <jussi.piitulainen@helsinki.fi> writes:

> Cai Gengyang writes:
>
>> I am trying to understand the boolean operator "and" in Python. It is
>> supposed to return "True" when the expression on both sides of "and"
>> are true
>>
>> For instance,
>>
>> 1 < 3 and 10 < 20 is True --- (because both statements are true)
>
> Yes.
>
>> 1 < 5 and 5 > 12 is False --- (because both statements are false)
>
> No :)
>
>> bool_one = False and False --- This should give False because none
>> of the statements are False
>> bool_two = True and False --- This should give False because only 1
>> statement is True
>> bool_three = False and True --- This should give False because only
>> 1 statement is True
>
> Yes.
>
>> bool_five = True and True --- This should give True because only 1
>> statement is True
>
> No :)
>
>> Am I correct ?
>
> Somewhat.

Just an observation on the language...  Change "only 1 statement" to
"only statement 1" and you get much closer to a correct explanation.

<snip>
-- 
Ben.

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#108140

Sorry I mistyped , this should be correct :

bool_one = False and False --- This should give False because none of the statements are True 
bool_two = True and False --- This should give False because only 1 statement is True 
bool_three = False and True --- This should give False because only 1 statement is True 
bool_five = True and True --- This should give True because both statements are True 






On Wednesday, May 4, 2016 at 11:10:28 PM UTC+8, Jussi Piitulainen wrote:
> Cai Gengyang writes:
> 
> > I am trying to understand the boolean operator "and" in Python. It is
> > supposed to return "True" when the expression on both sides of "and"
> > are true
> >
> > For instance,
> >
> > 1 < 3 and 10 < 20 is True --- (because both statements are true)
> 
> Yes.
> 
> > 1 < 5 and 5 > 12 is False --- (because both statements are false)
> 
> No :)
> 
> > bool_one = False and False --- This should give False because none of the statements are False
> > bool_two = True and False --- This should give False because only 1 statement is True 
> > bool_three = False and True --- This should give False because only 1 statement is True
> 
> Yes.
> 
> > bool_five = True and True --- This should give True because only 1 statement is True 
> 
> No :)
> 
> > Am I correct ?
> 
> Somewhat.
> 
> In a technical programming-language sense, these are "expressions", not
> "statements". Technically, if the first expression evaluates to a value
> that counts as true in Python, the compound expression "E and F"
> evaluates to the value of the second expression.
> 
> Apart from False, "empty" values like 0, "", [] count as false in
> Python, and all the others count as true.
> 
> But it's true that "E and F" only evaluates to a true value when both E
> and F evaluate to a true value.
> 
> Your subject line is good: Python's "and" is indeed a conditional,
> control-flow operator.

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#108143

Cai Gengyang writes:

> Sorry I mistyped , this should be correct :
>
> bool_one = False and False --- This should give False because none of the statements are True 
> bool_two = True and False --- This should give False because only 1 statement is True 
> bool_three = False and True --- This should give False because only 1 statement is True 
> bool_five = True and True --- This should give True because both statements are True 

Yes.

(That is, afterwards bool_one, bool_two, ... evaluate to the stated
truth values, for the stated reasons.)

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#108148

On Wed, May 4, 2016, at 07:41 AM, Cai Gengyang wrote:
> I am trying to understand the boolean operator "and" in Python. It is
> supposed to return "True" when the expression on both sides of "and" are
> true 

The thing is, its kinda dubious to think of 'and' as a 'boolean
operator', because once you go down that road, some people start wanting
it to be a *pure* boolean operator. Something that always returns True
or False. Instead, 'and' and 'or' return something that is true, or
something that is false. Notice the lower case. (I know the docs call
them Boolean Operations, but I still think saying 'boolean' is
unhelpful)

Python defines false things as False, None, 0 (of any numeric type), an
empty container (lists, tuples, mappings, something else that defines
__len__ and it returns 0), and instances of classes that define
__nonzero__ that return 0 or False.

Everything else is a true thing.

If you see "x and y", the rule is: if x is a false thing, it'll return
something false. As it happens, it has x handy, and since its decided x
is false, it'll return that. Therefore, "x and y" is false. If x is
true, though, it'll return y. In this case, "x and y" will be a true
thing if y is a true thing, and a false thing if y is a false thing.

As you can see, all of this logic happens without ever using True or
False. 

-- 
Stephen Hansen
  m e @ i x o k a i . i o

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