From: Hal Fulton <hal9000@...>
Date: 2003-09-03T13:54:45+09:00
Subject: Re: Possible to make compound if statements...

Kurt Euler wrote:
> Thanks Hal. Question: What do you mean by use ||? Could you 
 > give an example? (I'm new to Ruby and programming.)

|| is essentially the same as or
&& is essentially the same as and

So:
    if this == that || this =~ /another/
is the same as
    if this == that or this =~ /another/

History lesson: 'and' and 'or' and 'not' are used in
languages like BASIC, Algol, and Pascal. && and || and !
are used in languages like C, C++, and Java. Ruby happens
to know both forms.

And by the way, don't confuse the &&/|| operators with
the &/| operators. The latter are "bitwise" -- they act
on individual bits in a number. For example. 5 | 6 is 7,
since you're doing a bitwise OR of binary 101 and 110,
giving binary 111.

Whirlwind tour of truth and falsehood in Ruby:
Two rules -
   1. false and nil test as false
   2. everything else tests as true
Example:
   if this       # prints "no" if this is nil or false;
     puts "yes"  # prints "yes" if it's 0 or "" or []
   else          # or 237 or "foobar" or [1,2,3] or
     puts "no"   # any other value imaginable.
   end

Also remember that these expressions are "short-circuited" --
as soon as we know the result, we stop executing. Especially
remember this if you're calling a method with side effects
(in other words -- don't).
Example:
   x = true || foo()  # foo doesn't get called -- we already
                      # know the result is true
   x = true && foo()  # foo gets called -- it's necessary in
                      # order to get the correct answer
   x = false || foo() # foo is called
   x = false && foo() # foo is not called

This makes an interesting idiom possible. Suppose we
want to assign 5 to x, but ONLY if it doesn't already
have a value.

An unassigned variable effectively has a value of nil.
So we can say
   x = x || 5
to do this. Or using the shorthand that Ruby borrowed from
C, we can say:
   x ||= 5

It's worth mentioning that this is a little strange on
second thought, but makes sense on third thought.

Variables in Ruby aren't declared. Ruby knows something is
a variable when it sees an assignment to it.

Now in the statement  x = x || 5  you would take it for
granted that the right hand side is evaluated first so that
we can assign to the variable on the left hand side. But
how can x || 5 make sense when x is undefined??? How does it
even know x is a variable?

The answer is that the interpreter "sees" the assignment to
x at parse time and realizes that x is a variable. But since
it has never been assigned, it will have the value nil. So
the right hand side evaluates to  nil || 5  (which is just 5).
So then x gets its first "real" value.

Confused yet??

Ask all the questions you want, we'll take turns on them.

Cheers,
Hal