x=[]; x[:bla][:some_key] does not work?

Discussion in 'Ruby' started by forum@josh.ch, Nov 3, 2007.

  1. Guest

    Hi all

    PHP lets me easily create multidimensional hashes using the following
    syntax:

    x = array();
    x["bla"]["some_key"] = true;

    Is Ruby not capable of doing this?

    x = []
    x[:bla][:some_key] = true

    gives me a nil error!

    What wrong here?

    Thanks Josh
    --
    Posted via http://www.ruby-forum.com/.
    , Nov 3, 2007
    #1
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  2. Guest

    Hi --

    On Sun, 4 Nov 2007, Joshua Muheim wrote:

    > Hi all
    >
    > PHP lets me easily create multidimensional hashes using the following
    > syntax:
    >
    > x = array();
    > x["bla"]["some_key"] = true;
    >
    > Is Ruby not capable of doing this?
    >
    > x = []
    > x[:bla][:some_key] = true
    >
    > gives me a nil error!


    Are you sure it's not giving you a "Symbol as array index" error?

    > What wrong here?


    A couple of things. You're indexing an array with a symbol, but you
    have to use an integer. You're also expecting a non-existent array
    element to be something other than nil, but it isn't:

    a = []
    a[1] # nil

    You're then trying (or would be, if you used an integer instead of
    :bla) to call the method [] on nil, and nil has no such method.
    (Remember that the index syntax, a[n], is actually a method call:
    a.[](n).)


    David

    --
    Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Advancing With Rails, Edison, NJ, November 6-9
    * Advancing With Rails, Berlin, Germany, November 19-22
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
    See http://www.rubypal.com for details!
    , Nov 3, 2007
    #2
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  3. Guest

    Re: x=[:some_key] does not work?

    Ups I'm sorry, I messed things up because PHP uses [] for both arrays
    and hashes.

    x = array();
    x["bla"]["some_key"] = true;

    Is Ruby not capable of doing this?

    x = {}
    x[:bla][:some_key] = true
    --
    Posted via http://www.ruby-forum.com/.
    , Nov 3, 2007
    #3
  4. Guest

    On 11/3/07, Joshua Muheim <> wrote:
    > Hi all
    >
    > PHP lets me easily create multidimensional hashes using the following
    > syntax:
    >
    > x = array();
    > x["bla"]["some_key"] = true;
    >
    > Is Ruby not capable of doing this?
    >
    > x = []
    > x[:bla][:some_key] = true
    >
    > gives me a nil error!


    As David said, if you want to create hashes, you have to create Hashes ;-)

    Then, you can tell Hash.new what is the default value, so you can
    create hash, that will contain by default empty hashes:

    x = Hash.new { Hash.new }

    This will add an "automatic" two level hash. I don't know quickly how
    to make this indefinitly deep, you can at least repeat the pattern.

    Please note that it is not enough to write Hash.new { {} } as the
    inner will create one particular Hash instance, that all keys will
    reference. You need Hash.new to create a new hash for each key.

    Jano
    , Nov 3, 2007
    #4
  5. Guest

    Re: x=[:some_key] does not work?

    Joshua Muheim wrote:
    > Ups I'm sorry, I messed things up because PHP uses [] for both arrays
    > and hashes.
    >
    > x = array();
    > x["bla"]["some_key"] = true;
    >
    > Is Ruby not capable of doing this?
    >
    > x = {}
    > x[:bla][:some_key] = true
    >

    For a 2-dimensional Hash you can use:

    h = Hash.new({})

    See the documentation of the initialize method of the Hash class. Here
    it basically tells the object to use a default value (an empty hash) for
    uninitialized keys.

    Lionel
    , Nov 3, 2007
    #5
  6. Guest

    Jano Svitok wrote:
    >
    > Please note that it is not enough to write Hash.new { {} } as the
    > inner will create one particular Hash instance, that all keys will
    > reference. You need Hash.new to create a new hash for each key.
    >


    Ooopps, my bad.

    Lionel
    , Nov 3, 2007
    #6
  7. Guest

    Joshua Muheim wrote:
    > PHP lets me easily create multidimensional hashes[...]
    > Is Ruby not capable of doing this?
    >
    > x = []


    That's an array. A hash would be x={}

    > x[:bla][:some_key] = true
    >
    > gives me a nil error!


    The code above should give you a different error. If you use {} it should give
    you a nil error because x[:bla] would return nil and nil doesn't have a method
    [].
    To fix that you have to make x return a hash for non-existant keys. That would
    work as such:
    x = Hash.new {|h,k| h[k] = Hash.new}
    x[:bla][:some_key] = true

    Or if you want an arbitrary amount of nesting:

    blk = lambda {|h,k| h[k] = Hash.new(&blk)}
    x = Hash.new(&blk)
    x[:la][:li][:lu][:chunky][:bacon][:foo] = "bar"


    HTH,
    Sebastian
    --
    Jabber:
    ICQ: 205544826
    , Nov 3, 2007
    #7
  8. Guest

    Re: x=[:some_key] does not work?

    Lionel Bouton wrote:
    > For a 2-dimensional Hash you can use:
    >
    > h = Hash.new({})


    That won't result in the behaviour most people would expect from a
    2-dimensional hash:
    >> h = Hash.new({})

    => {}
    >> h[:a][:foo] = "bar"

    => "bar"
    >> h[:a]

    => {:foo=>"bar"}
    >> h

    => {}
    >> h[:b][:chunky] = "bacon"

    => "bacon"
    >> h[:b]

    => {:chunky=>"bacon", :foo=>"bar"}
    >> h[:c]

    => {:chunky=>"bacon", :foo=>"bar"}
    >> h

    => {}

    You want to a) use the block-form of Hash.new to not use the same instance of
    a hash everytime and b) assign the new Hash instead of just returning it.


    --
    Jabber:
    ICQ: 205544826
    , Nov 3, 2007
    #8
  9. Guest

    Hi --

    On Sun, 4 Nov 2007, Jano Svitok wrote:

    > On 11/3/07, Joshua Muheim <> wrote:
    >> Hi all
    >>
    >> PHP lets me easily create multidimensional hashes using the following
    >> syntax:
    >>
    >> x = array();
    >> x["bla"]["some_key"] = true;
    >>
    >> Is Ruby not capable of doing this?
    >>
    >> x = []
    >> x[:bla][:some_key] = true
    >>
    >> gives me a nil error!

    >
    > As David said, if you want to create hashes, you have to create Hashes ;-)
    >
    > Then, you can tell Hash.new what is the default value, so you can
    > create hash, that will contain by default empty hashes:
    >
    > x = Hash.new { Hash.new }
    >
    > This will add an "automatic" two level hash. I don't know quickly how
    > to make this indefinitly deep, you can at least repeat the pattern.
    >
    > Please note that it is not enough to write Hash.new { {} } as the
    > inner will create one particular Hash instance, that all keys will
    > reference. You need Hash.new to create a new hash for each key.


    That's not quite right. The only-one-object thing is when you do this:

    h = Hash.new({})

    If you use a block, it gets executed each time -- so in your example,
    a new hash would get created.

    However, it's important to remember that what you're setting is the
    default value (or behavior) for *non-existent* keys. So if you do:

    h = Hash.new { {} }
    a = h[1]

    a is now a hash, but h still has no keys.

    The block is automatically passed the hash itself, and the key:

    h = Hash.new {|hash,key| # do stuff with hash and key }

    and you can use that fact to actually add the key to the hash.


    David

    --
    Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Advancing With Rails, Edison, NJ, November 6-9
    * Advancing With Rails, Berlin, Germany, November 19-22
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
    See http://www.rubypal.com for details!
    , Nov 3, 2007
    #9
  10. Guest

    Joshua Muheim wrote:
    > Hi all
    >
    > PHP lets me easily create multidimensional hashes using the following
    > syntax:
    >
    > x = array();
    > x["bla"]["some_key"] = true;
    >
    > Is Ruby not capable of doing this?
    >
    > x = []
    > x[:bla][:some_key] = true
    >
    > gives me a nil error!
    >


    x = []
    x[:blah][:some_key] = true

    --output:--
    `[]': Symbol as array index (TypeError)
    from r5test.rb:2

    In ruby, array indexes must be non-negative integers-not symbols, not
    strings, not floats...err this actually 'works':

    x = []
    x[3.5] = true
    puts x[3.5]

    --output:--
    true

    But, I think that ruby must convert the float to an int. However, this
    works:

    puts x.values_at(3.5)

    --output:--
    true

    But, then so does this:

    puts x.values_at(3)

    --output:--
    true

    So, I can't really figure out a way to prove that the index value can't
    be a float. Anyway...


    To create nested hashes you can do this:

    x = {}
    x['a'] = 10
    p x

    x[:bla] = {1=>10, 2=>20}
    p x

    --output:--
    {"a"=>10}
    {"a"=>10, :bla=>{1=>10, 2=>20}}


    Writing this is problematic, though:

    x = {}
    x[:bla][:some_key] = true

    --output:--
    undefined method `[]=' for nil:NilClass (NoMethodError)

    That is equivalent to:

    x = {}
    lookup1 = x[:bla]
    lookup1[:some_key] = true

    and when you lookup a non-existent key for a hash, it returns nil. As
    a result, lookup1 is assigned nil.

    But in ruby you can make hashes to return whatever you want when you
    lookup a non-existent key. You do that by specifying what you want
    returned in the Hash constructor:

    x = Hash.new {|hash, key| hash[key] = {} }
    x[:bla][:some_key] = true
    p x

    --output:--
    {:bla=>{:some_key=>true}}





    --
    Posted via http://www.ruby-forum.com/.
    , Nov 3, 2007
    #10
  11. Guest

    Re: x=[:some_key] does not work?

    Jano Svitok wrote:
    >
    > x = Hash.new { Hash.new }
    >
    > This will add an "automatic" two level hash. I don't know quickly how
    > to make this indefinitly deep, you can at least repeat the pattern.
    >
    > Please note that it is not enough to write Hash.new { {} } as the
    > inner will create one particular Hash instance, that all keys will
    > reference. You need Hash.new to create a new hash for each key.
    >
    > Jano


    As far as I can tell neither of the following does anything:

    x = Hash.new { Hash.new }
    y = Hash.new{ {} }

    x[:bla][:some_key] = true
    y[:bla][:some_key] = true

    p x, y

    --output:--
    {}
    {}

    From the docs for Hash.new:

    If a block is
    specified, it will be called with the hash object and the key, and
    should return the default value. It is the block's responsibility
    to store the value in the hash if required.

    Your lookup of a non-existent key returns a hash, but the returned hash
    never gets stored in the original hash, so the original hash remains
    empty.


    --
    Posted via http://www.ruby-forum.com/.
    , Nov 3, 2007
    #11
  12. Guest

    , Nov 3, 2007
    #12
  13. Ken Bloom Guest

    On Sat, 03 Nov 2007 15:44:48 -0500, sepp2k wrote:

    > Joshua Muheim wrote:
    >> PHP lets me easily create multidimensional hashes[...] Is Ruby not
    >> capable of doing this?
    >>
    >> x = []

    >
    > That's an array. A hash would be x={}
    >
    >> x[:bla][:some_key] = true
    >>
    >> gives me a nil error!

    >
    > The code above should give you a different error. If you use {} it
    > should give you a nil error because x[:bla] would return nil and nil
    > doesn't have a method [].
    > To fix that you have to make x return a hash for non-existant keys. That
    > would work as such:
    > x = Hash.new {|h,k| h[k] = Hash.new}
    > x[:bla][:some_key] = true
    >
    > Or if you want an arbitrary amount of nesting:
    >
    > blk = lambda {|h,k| h[k] = Hash.new(&blk)} x = Hash.new(&blk)
    > x[:la][:li][:lu][:chunky][:bacon][:foo] = "bar"
    >
    >
    > HTH,
    > Sebastian


    sepp2k's answer is the only correct answer in this thread.

    --Ken

    --
    Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
    Department of Computer Science. Illinois Institute of Technology.
    http://www.iit.edu/~kbloom1/
    Ken Bloom, Nov 4, 2007
    #13
  14. Ken Bloom Guest

    On Sat, 03 Nov 2007 15:44:48 -0500, sepp2k wrote:

    > Joshua Muheim wrote:
    >> PHP lets me easily create multidimensional hashes[...] Is Ruby not
    >> capable of doing this?
    >>
    >> x = []

    >
    > That's an array. A hash would be x={}
    >
    >> x[:bla][:some_key] = true
    >>
    >> gives me a nil error!

    >
    > The code above should give you a different error. If you use {} it
    > should give you a nil error because x[:bla] would return nil and nil
    > doesn't have a method [].
    > To fix that you have to make x return a hash for non-existant keys. That
    > would work as such:
    > x = Hash.new {|h,k| h[k] = Hash.new}
    > x[:bla][:some_key] = true
    >
    > Or if you want an arbitrary amount of nesting:
    >
    > blk = lambda {|h,k| h[k] = Hash.new(&blk)} x = Hash.new(&blk)
    > x[:la][:li][:lu][:chunky][:bacon][:foo] = "bar"
    >
    >
    > HTH,
    > Sebastian


    Or use Hash#autonew from the facets gem, which does precisely the same
    thing.

    --Ken

    --
    Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
    Department of Computer Science. Illinois Institute of Technology.
    http://www.iit.edu/~kbloom1/
    Ken Bloom, Nov 4, 2007
    #14
  15. Guest

    Hi --

    On Sun, 4 Nov 2007, Ken Bloom wrote:

    > sepp2k's answer is the only correct answer in this thread.


    Could you point out the mistakes in my answers for me? I'd be
    interested. Thanks --


    David

    --
    Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Advancing With Rails, Edison, NJ, November 6-9
    * Advancing With Rails, Berlin, Germany, November 19-22
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
    See http://www.rubypal.com for details!
    , Nov 4, 2007
    #15
  16. Ken Bloom Guest

    On Sat, 03 Nov 2007 20:46:00 -0500, dblack wrote:

    > Hi --
    >
    > On Sun, 4 Nov 2007, Ken Bloom wrote:
    >
    >> sepp2k's answer is the only correct answer in this thread.

    >
    > Could you point out the mistakes in my answers for me? I'd be
    > interested. Thanks --
    >
    >
    > David


    The Hash.new constructor has several different behaviors depending on
    what you pass it:

    Hash.new(param)
    creates a new hash where the object in param is returned whenever a
    specific key is not found. No new key is created in the hash:
    a=Hash.new(0) => {}
    a[:foo] => 0
    a => {}

    Additionally, this same object is constructed once, and used every time
    you call the hash so:
    Foo=Struct.new:)bar) => Foo
    h=Hash.new(Foo.new) => {}
    h[:baz].bar="Hello" => "Hello"
    h => {}
    h[:baz] => #<struct Foo bar="Hello">
    h[:furby] => #<struct Foo bar="Hello">

    I haven't created any keys, here, but I've changed the default value of
    the hash.

    Hash.new{code}
    creates a new hash where default values are generated by executing the
    block {code}. When used as
    Hash.new{param}
    the code inside the block is called every time, and a new object is
    constructed every time. (because {} and [] construct a new object every
    time they are encountered in execution, here they are encountered every
    time the block is called, whereas before they were encountered only once
    when the hash was constructed). But the semantics of key creation are the
    same as before -- no new keys are created until you explicitly assign to
    the hash.

    a=Hash.new{0} => {}
    a[:foo] => 0
    a => {}

    Foo=Struct.new:)bar) => Foo
    h=Hash.new{Foo.new} => {}
    h[:baz].bar="Hello" => "Hello"
    h => {}
    h[:baz] => #<struct Foo bar=nil>
    h[:furby] => #<struct Foo bar=nil>


    Hash.new{|h,k| h[k]=param}
    this finally assigns to the hash when you access the hash.

    Foo=Struct.new:)bar) => Foo
    h=Hash.new{|h,k| h[k]=Foo.new} => {}
    h[:baz].bar="Hello" => "Hello"
    h => {:baz=>#<struct Foo bar="Hello">}
    h[:baz] => #<struct Foo bar="Hello">
    h[:furby] => #<struct Foo bar=nil>
    h => {:baz=>#<struct Foo bar="Hello">, :furby=>#<struct Foo bar=nil>}

    Personally, I think that Hash might benefit from having a few more named
    constructors so that people can guess the expected behavior by name.
    Particularly one that automatically does the whole
    Hash.new{|h,k| h[k]=param}
    thing, through judicious use of #dup.

    class Hash
    def self.new_add obj
    Hash.new{|h,k| h[k]=obj.dup}
    end
    end

    --
    Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
    Department of Computer Science. Illinois Institute of Technology.
    http://www.iit.edu/~kbloom1/
    Ken Bloom, Nov 4, 2007
    #16
  17. Guest

    Re: x=[:some_key] does not work?

    David A. Black wrote:
    > Hi --
    >
    > On Sun, 4 Nov 2007, Ken Bloom wrote:
    >
    >> sepp2k's answer is the only correct answer in this thread.

    >
    > Could you point out the mistakes in my answers for me? I'd be
    > interested. Thanks --
    >



    In your first post, you misspelled the op's name:

    >David A. Black wrote:
    > Hi --


    and then you followed that up with the very same mistake in your second
    post:

    >David A. Black wrote:
    > Hi --


    Can you blame the indignation felt by subsequent readers?
    --
    Posted via http://www.ruby-forum.com/.
    , Nov 4, 2007
    #17
  18. Guest

    Re: x=[:some_key] does not work?

    Ken Bloom wrote:
    > On Sat, 03 Nov 2007 15:44:48 -0500, sepp2k wrote:
    >
    >>> gives me a nil error!

    >>
    >> blk = lambda {|h,k| h[k] = Hash.new(&blk)} x = Hash.new(&blk)
    >> x[:la][:li][:lu][:chunky][:bacon][:foo] = "bar"
    >>
    >>
    >> HTH,
    >> Sebastian

    >
    > sepp2k's answer is the only correct answer in this thread.
    >



    Darn it! I knew someone would spot the error here:

    x = Hash.new {|hash, key| hash[key] = {} }
    x[:bla][:some_key] = true
    p x

    --output:--
    {:bla=>{:some_key=>true}}

    But I couldn't figure out how to do it, so I fudged the output. It was
    really:

    --output:--
    Nuh uh, shorty. Don't bring that stepped on hash round here no mo'.

    --
    Posted via http://www.ruby-forum.com/.
    , Nov 4, 2007
    #18
  19. Guest

    Re: x=[:some_key] does not work?

    > x=[]; x[:bla][:some_key] does not work?
    > Posted by Joshua Muheim (josh) on 03.11.2007 21:21
    > Hi all
    >
    > PHP lets me easily create multidimensional hashes using the following
    > syntax:
    >
    > x = array();
    > x["bla"]["some_key"] = true;
    >
    > Is Ruby not capable of doing this?
    >
    > x = []
    > x[:bla][:some_key] = true
    >
    > gives me a nil error!
    >
    > What wrong here?



    How about this approach:

    a = Hash.new{|h,k| h[k]=Hash.new(&h.default_proc) }

    a[2][1]=2
    a[2][2][3]=4
    a[3][1][1][1]=1

    p a #=> {2=>{1=>2, 2=>{3=>4}}, 3=>{1=>{1=>{1=>1}}}}

    ( http://snippets.dzone.com/posts/show/4146 )

    Cheers,

    j. k.
    --
    Posted via http://www.ruby-forum.com/.
    , Nov 4, 2007
    #19
  20. Guest

    Hi --

    On Sun, 4 Nov 2007, Ken Bloom wrote:

    > On Sat, 03 Nov 2007 20:46:00 -0500, dblack wrote:
    >
    >> Hi --
    >>
    >> On Sun, 4 Nov 2007, Ken Bloom wrote:
    >>
    >>> sepp2k's answer is the only correct answer in this thread.

    >>
    >> Could you point out the mistakes in my answers for me? I'd be
    >> interested. Thanks --
    >>
    >>
    >> David

    >
    > The Hash.new constructor has several different behaviors depending on
    > what you pass it:


    I didn't say it didn't. I'm still wondering what you found in my posts
    that was incorrect, if you wouldn't mind sharing.


    David

    --
    Upcoming training by David A. Black/Ruby Power and Light, LLC:
    * Advancing With Rails, Edison, NJ, November 6-9
    * Advancing With Rails, Berlin, Germany, November 19-22
    * Intro to Rails, London, UK, December 3-6 (by Skills Matter)
    See http://www.rubypal.com for details!
    , Nov 4, 2007
    #20
    1. Advertising

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