Modifying Select Elements in Array of Objects

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?

On Mar 20, 3:07 pm, Glenn R. [email protected] wrote:

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.

See Array#select and Array#find.

e.g.
albums.select{ |a| a.band==“The Beatles” }.each{ |a| a.like=true }

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 = 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 = true }

On 03/20/2010 10:07 PM, Glenn R. wrote:

attr_writer :like
albums << Album.new(“St. Peppers”, “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

Robert K. 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.

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