Anime Junkie ha escrito:

> I'm using the "Programming Ruby" book and find that a lot of the examples
> don't work.  They use too much pseudo code in my opinion.

That's funny.  There is no pseudo-code AT ALL in Programming Ruby.
What you see IS actual running Ruby code.
It is a testament to how easy ruby is that it LOOKS like pseudo code.

> def duration_in_mins=(new_duration)
>   @duration = (new_duration*60).to_i
> end

This is a function that takes a duration in minutes and converts
(internally) into seconds.
Its use is:

    # To specify a 3.2min song
     song.duration_in_mins = 3.2
     puts song.duration  # returns the song in seconds

>
> puts song.duration_in_mins  # I assumed that this would print 2 but
> produces the error "undefined method `duration_in_mins'"
>

And that's correct.  There's no duration_in_mins function.
You just defined a function duration_in_mins=  (notice the EQUAL sign).
 That's a setter function, not a getter.


> //I know that duration_in_mins is a virtual attribute but if I can use
> it for assigning value shouldn't I be able to print that?

No.  Setters and Getters are two different functions.  You need to
specify both yourself, just like in any other language.
Note, also, that in that example the actual setter never stores the
result in minutes into the class.  Thus, you can never get the result
back as minutes, without an additional calculation.

Now... if you add...(you can do this from irb):

class Song
    def duration_in_mins
          @duration / 60.0
    end
end

You'll get what you want.  Hope that helps.

---

Adding setters and getters can be rather tedious.
Thus, for simple attributes that don't do any calculation, Ruby
provides attr_accessor, attr_reader and attr_writer functions, that
just add a getter/setter, just a getter or just a setter function
automatically.  Thus, your duration attribute can also be written as:

class NewSong
    attr_accessor :duration
    attr_writer      :another
end

song = NewSong.new
song.duration = 20
puts song.duration
song.another = 40
puts song.another ## fails, no reader, just a writer