Boolean Expressions

[Steve D'Aprano]

On Wed, 27 Sep 2017 08:23 am, Cai Gengyang wrote:

> 
> I'm trying to understand the logic behind AND. I looked up Python logic tables
> 
> False and False gives False
> False and True gives False
> True and False gives False
> True and True gives True.
> 
> So does that mean that the way 'and' works in Python is that both terms must
> be True (1) for the entire expression to be True ? Why is it defined that way,
> weird ? I was always under the impression that 'and' means that when you have
> both terms the same, ie either True and True or False and False , then it
> gives True

No, your impression is wrong. Python's AND is the same as boolean AND
everywhere: every programming language that supports boolean AND, in Boolean
Algebra and in logic.

In C++ https://msdn.microsoft.com/en-us/library/c6s3h5a7.aspx

In Javascript

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_Operators

Boolean algebra

https://en.wikipedia.org/wiki/Boolean_algebra#Basic_operations

Consider that today I spent the day sitting at home watching movies. If I said:

"Today, I climbed to the top of Mount Everest."

That would be False.

If I said:

"Today, I swam across the English Channel."

That would be False.

If I said:

"Today, I climbed to the top of Mount Everest, AND I swam across the English
Channel."

that is still False.


What you are thinking of is best describes as "equals":

False equals False gives True
False equals True gives False
True equals False gives False
True equals True gives True



-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.