Proper way to either create or append to an array?

ruby-talk
1

Hi all.

Let's say I want to either create a new named array or append to that
array if it already exists. Is there a more elegant way of doing the
following?

@foo[:bar].nil? ? foo[:bar] = ["snafu"] : foo[:bar] << "snafu"

Thanks!

/afb

···

--
Posted via http://www.ruby-forum.com/\.

2

Adam Block wrote:

Hi all.

Let's say I want to either create a new named array or append to that
array if it already exists. Is there a more elegant way of doing the
following?

@foo[:bar].nil? ? foo[:bar] = ["snafu"] : foo[:bar] << "snafu"

  I would use:

@foo[:bar] ||=
@foo[:bar] << "snafu"

  There probably is more compact, but I find this writing very readable
when you're used to it (though, admittedly somehow puzzling for the
beginners).

  Cheers,

  Vince

···

--
Vincent Fourmond, PhD student
http://vincent.fourmond.neuf.fr/

3

Adam Block wrote:

Let's say I want to either create a new named array or append to that
array if it already exists. Is there a more elegant way of doing the
following?

>> @foo[:bar].nil? ? foo[:bar] = ["snafu"] : foo[:bar] << "snafu"

Dunno if you'd call it elegant, but I frequently write:

( @foo[:bar] ||= ) << "snafu"

4

Since you're using a Hash the block constructor is the most elegant solution IMHO:

def initialize
   @foo = Hash.new {|h,k| h[k]=}
   ...
end

def other_method
   ...
   @foo[:bar] << "snafu"
   ...
end

Kind regards

  robert

···

On 20.01.2007 18:55, Adam Block wrote:

Hi all.

Let's say I want to either create a new named array or append to that
array if it already exists. Is there a more elegant way of doing the
following?

@foo[:bar].nil? ? foo[:bar] = ["snafu"] : foo[:bar] << "snafu"

5

Robert Klemme wrote:

Since you're using a Hash the block constructor is the most elegant
solution IMHO:

def initialize
   @foo = Hash.new {|h,k| h[k]=}
   ...
end

To be clear for those unfamiliar with this: any time you ask for the
value of a key that doesn't exist, the block will be called (which
creates a new empty array). This means you can never see this
particular Hash has a key, since:
  if @foo[ :bar ]
will always succeed, and create a (possibly unwanted) array instance in
the process.

6

Robert Klemme wrote:

Since you're using a Hash the block constructor is the most elegant
solution IMHO:

def initialize
   @foo = Hash.new {|h,k| h[k]=}
   ...
end

To be clear for those unfamiliar with this: any time you ask for the
value of a key that doesn't exist, the block will be called (which
creates a new empty array). This means you can never see this
particular Hash has a key, since:
  if @foo[ :bar ]
will always succeed, and create a (possibly unwanted) array instance in
the process.

That's only because you're not checking for the presence of a key correctly:

>> myhash = Hash.new { |h,k| h[k] = Array.new }
=> {}
>> if myhash[:oops]
>> puts "nothing"
>> end
nothing
=> nil
>> myhash
=> {:oops=>}
>> if myhash.has_key? :missing
>> puts ":missing contains #{myhash[:missing] * ', '}"
>> end
=> nil
>> myhash
=> {:oops=>}

If you know that your code isn't using the block initialization, then the first form might be OK. However, the #has_key? will always do what you expect. This is particularly true if the value can be nil or false.

-Rob

Rob Biedenharn http://agileconsultingllc.com
Rob@AgileConsultingLLC.com

···

On Jan 20, 2007, at 1:15 PM, Phrogz wrote:

7

Phrogz wrote:

Robert Klemme wrote:
> Since you're using a Hash the block constructor is the most elegant
> solution IMHO:
>
> def initialize
> @foo = Hash.new {|h,k| h[k]=}
> ...
> end

To be clear for those unfamiliar with this: any time you ask for the
value of a key that doesn't exist, the block will be called (which
creates a new empty array). This means you can never see this
particular Hash has a key, since:
  if @foo[ :bar ]
will always succeed, and create a (possibly unwanted) array instance in
the process.

What if foo[:bar] has been assigned a value of false?
That method of checking for the presence of a key is
too crude even for awk. Note below that the mere
attempt to access the value of a key creates an
entry for that key if it doesn't already exist.

BEGIN {
  SUBSEP = "^"
  a[22,"yes"] = 88
  a[33] = 99

  # The wrong way.
  if ( a["bar"] )
    print "How did 'bar' get here?"
  # The right way.
  if ( "foo" in a )
    print "'foo' too?"
  # Any unauthorized entries?
  for (key in a)
    print key
}

--- output -----
22^yes
33
bar

8

Rob Biedenharn wrote:

···

On Jan 20, 2007, at 1:15 PM, Phrogz wrote:

> Robert Klemme wrote:
>> Since you're using a Hash the block constructor is the most elegant
>> solution IMHO:
>>
>> def initialize
>> @foo = Hash.new {|h,k| h[k]=}
>> ...
>> end
>
> To be clear for those unfamiliar with this: any time you ask for the
> value of a key that doesn't exist, the block will be called (which
> creates a new empty array). This means you can never see this
> particular Hash has a key, since:
> if @foo[ :bar ]
> will always succeed, and create a (possibly unwanted) array
> instance in
> the process.

That's only because you're not checking for the presence of a key
correctly:

>> myhash = Hash.new { |h,k| h[k] = Array.new }
=> {}
>> if myhash[:oops]
>> puts "nothing"
>> end
nothing
=> nil
>> myhash
=> {:oops=>}
>> if myhash.has_key? :missing

I prefer to use include?, since it also works on arrays:

irb(main):006:0> ['foo','bar'].include? 'bar'
=> true