From: Phillip Gawlowski <pg@...>
Date: 2009-12-31T19:53:13+09:00
Subject: Re: Why can't I use "or" here?

On 31.12.2009 11:35, Albert Schlef wrote:
> Then why can I do this?
>
>      puts (123 or true)
>
> Are you going to tell me that "123 or true", unlike "123 if true",
> *isn't* a statement?

I'm afraid I am. It's an evaluation. Boolean evaluation (with "and" / 
"or" having lower precedence than && / ||).

> If "123 or true" isn't a statement, then it's a simple expression. And
> then I would be able to do:

Take a third option: It's an evaluation. ;)


Crash course in Boolean logic:

We have two items A and B.

Case "AND":
return true if, and only if, both "a" *and* "b" are true.

Example:
irb(main):004:0> a, b = true, true
=> [true, true]
irb(main):005:0> a and b
=> true
irb(main):006:0> b = false
=> false
irb(main):007:0> a and b
=> false
irb(main):008:0>

As you see, the evaluation spits back a result: True, if both "a" and 
"b" are true, false if only "a" or "b" is true.

Case "OR":
return true if *either* "a" *or* "b" are true

Example:
irb(main):008:0> a, b = false, false
=> [false, false]
irb(main):009:0> a or b
=> false
irb(main):010:0> a = true
=> true
irb(main):011:0> a or b
=> true
irb(main):012:0> b = false
=> false
irb(main):013:0> a or b
=> true

As you can see, if both "a" and "b" are false, the evaluation returns 
false ("returns false" meaning "the result is false").
If either "a" *or* "b" are true, the evaluation returns true.
Same if both "a" *and* "b" are true.

> No, no, no. By `puts (123 if true)` I mean: output the result of `123 if
> true`.

That's what I said, I was just verbose about it. ;)

--
Phillip Gawlowski