Hi,

I have an array of objects, and the objects have some attributes.  I'd
like to be able to modify selected elements in the array based on any of
those attributes.

For example, suppose I have a have an array of Albums, like this:

class Album
  attr_reader :name, :band, :like
  attr_writer :like

  def initialize(name, band)
    @name = name
    @band = band
    @like = nil
  end
end

albums = []
albums << Album.new("Exile on Main Street", "The Rolling Stones")
albums << Album.new("St. Peppers", "The Beatles")
albums << Album.new("The White Album", "The Beatles")

I want to be able to selectively modify the elements of the albums array
based on, for example, the band.  In other words, I want to be able to
able to change the like instance variable in each object to true with a
method.

Here's what I have so far:

puts albums.collect { |a| a.like = true if a.band == "The Beatles";
a}.inspect

Using collect like this works, but it seems awkward to me to have to
iterate through the whole array just to find the element (or elements)
that I want to change.  I'd like to be able just to select the elements
I want, make the changes, and leave the rest of the elements unchanged.
Is there a better way to do this?
-- 
Posted via http://www.ruby-forum.com/.

On Mar 20, 3:07=A0pm, Glenn Ritz <glenn_r...@yahoo.com> wrote:
> I have an array of objects, and the objects have some attributes. =A0I'd
> like to be able to modify selected elements in the array based on any of
> those attributes.

See Array#select and Array#find.

e.g.
albums.select{ |a| a.band=3D=3D"The Beatles" }.each{ |a| a.like=3Dtrue }

> Using collect like this works, but it seems awkward to me to have to
> iterate through the whole array just to find the element (or elements)
> that I want to change.

Without some previously-established alternative lookup mechanism, such
as a Hash, any implementation is going to have to walk through the
entire array to find multiple matching items. Array#find will stop as
soon as it finds one item, though.

For example (assuming you still need your albums array):

# Do this once
albums_by_band =3D albums.group_by{ |a| a.band }

# Do this for each time you need to look up an array of albums by a
band
albums_by_band["The Beatles"].each{ |a| a.like =3D true }

On 03/20/2010 10:07 PM, Glenn Ritz wrote:
> Hi,
> 
> I have an array of objects, and the objects have some attributes.  I'd
> like to be able to modify selected elements in the array based on any of
> those attributes.
> 
> For example, suppose I have a have an array of Albums, like this:
> 
> class Album
>   attr_reader :name, :band, :like
>   attr_writer :like
> 
>   def initialize(name, band)
>     @name = name
>     @band = band
>     @like = nil
>   end
> end
> 
> albums = []
> albums << Album.new("Exile on Main Street", "The Rolling Stones")
> albums << Album.new("St. Peppers", "The Beatles")
> albums << Album.new("The White Album", "The Beatles")
> 
> I want to be able to selectively modify the elements of the albums array
> based on, for example, the band.  In other words, I want to be able to
> able to change the like instance variable in each object to true with a
> method.
> 
> Here's what I have so far:
> 
> puts albums.collect { |a| a.like = true if a.band == "The Beatles";
> a}.inspect
> 
> Using collect like this works, but it seems awkward to me to have to
> iterate through the whole array just to find the element (or elements)
> that I want to change.  I'd like to be able just to select the elements
> I want, make the changes, and leave the rest of the elements unchanged.
> Is there a better way to do this?

There is a different way but it's not really better because now you 
iterate the elements that you want to change twice:

albums.select {|a| a.band == "The Beatles"}.each {|a| a.like = true}

If your Albums are stored in this single Array only you have no other 
choice than to iterate all.  If you want mo re efficient operation then 
you need to create a more complex data structure which non only manages 
the list of Albums but also one or more indexes (typically Hashes) 
depending on your typical queries.

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

Robert Klemme wrote:

> If your Albums are stored in this single Array only you have no other
> choice than to iterate all.  If you want mo re efficient operation then
> you need to create a more complex data structure which non only manages
> the list of Albums but also one or more indexes (typically Hashes)
> depending on your typical queries.
> 
> Kind regards
> 
>   robert

Thanks, guys.  This helped.
-- 
Posted via http://www.ruby-forum.com/.