Why use a symbol in place of a variable?

Hi,
Say you have a method in a class that’s defined as this:
(working from Peter C.'s Beginning Ruby book, p. 155 of the book)

class Dungeon
.
.
.

def start(location)
@player.location = location
show_current_description
end

why is it that when you place the player in the large cave, it’s
my_dungeon.start(:largecave)

instead of
my_dungeon.start(largecave)
?
why is largecave a symbol?
Thanks so much!

A symbol and a variable are two different things. A symbol is
essentially a special kind of literal, just like a number or a string
is. You cannot assign values to symbols, just like you can’t assign
values to numbers or strings – they are their own values. That is, it
makes no sense to say 42 = "banana", just as it makes no sense to
say :banana = 42.

In this case, the author is using the symbol :largecave to represent a
particular location. The reason why he might prefer a symbol literal
to a string literal is that symbols are immutable. “Immutable” means
that you can’t do operations on symbols to change them (unlike, say,
strings). Immutability is a good property because it decreases that
number of surprises you can have, and because it makes reasoning about
your program easier.

~ jf

John F.
Principal Consultant, BitsBuilder
LI: http://www.linkedin.com/in/johnxf
SO: http://stackoverflow.com/users/75170/

This is quite good tutorial about symbols and strings here
http://www.robertsosinski.com/2009/01/11/the-difference-between-ruby-symbols-and-strings/

Along with what John F. said about symbols’ immutability is the
advantage they have over the string literals that they also often
replace,
and that is the fact that each new instance of a string literal will
refer
to a different Ruby object while each reference to the equivalent symbol
will refer to the same object instance with the added bonus that less
memory
is used. See the example below.

ruby-1.9.2-p136 :001 > “hi”.object_id
=> 71175750
ruby-1.9.2-p136 :002 > “hi”.object_id
=> 71172980
ruby-1.9.2-p136 :003 > :hi.object_id
=> 185928
ruby-1.9.2-p136 :004 > :hi.object_id
=> 185928

Another way to look at it is a symbol is it is an emnumerated scalar.

I don’t think that’s an accurate characterization, imo, since symbols
have no relationship or knowledge of other symbols. They are not a
collection of anything, and thus by definition can’t be an
enumeration. Using your example, nothing stops you from writing

my_dungeon.start(:bananas)

and that would be a perfectly valid parameter to supply. However, if
you wrote something like

my_dungeon.start(DungeonSizes[:bananas])

that would clue you into the problem. In this case, DungeonSizes would
be more like the enumeration, into which a symbol would be the index
for the element of the enumeration you wanted to select.

~ jf

John F.
Principal Consultant, BitsBuilder
LI: http://www.linkedin.com/in/johnxf
SO: http://stackoverflow.com/users/75170/

Another way to look at it is a symbol is an emnumerated scalar.

You could have

cavesize = 345
my_dungeon.start(cavesize)

However if caves are only ever Small, Medium and Large, then you can
represent that finite set of options by passing :smallcave, :mediumcave
or :largecave.

A symbol can therefore more closely represent the logic of the problem.

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