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/.
Hi --
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] = truegives 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
On Sun, 4 Nov 2007, Joshua Muheim wrote:
--
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* 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!
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
On 11/3/07, Joshua Muheim <forum@josh.ch> 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] = truegives me a nil error!
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: sepp2k@jabber.org
ICQ: 205544826
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] = truegives 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/\.
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/.
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
Hi --
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] = truegives 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
On Sun, 4 Nov 2007, Jano Svitok wrote:
On 11/3/07, Joshua Muheim <forum@josh.ch> wrote:
--
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!
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/\.
sepp2k's answer is the only correct answer in this thread.
--Ken
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] = trueOr 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
--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/
Or use Hash#autonew from the facets gem, which does precisely the same
thing.
--Ken
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] = trueOr 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
--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/
Very nice, Sebastian and Jimmy.
Leading to the obvious...
class NestedHash < Hash
def initialize
blk = lambda {|h,k| h[k] = NestedHash.new(&blk)}
super(&blk)
end
end
class Hash
def Hash.new_nested_hash
Hash.new{|h,k| h[k]=Hash.new(&h.default_proc) }
end
end
... and with no comment on propriety.
I've been using Ruby for years and I'm still startled by what you can do when passing blocks to initializers. A few weeks ago I stumbled on this:
Hash.new{ | h, k | h[k] = Array.new }
which is just a hash of arrays, completely trivial really, but I'd never thought of it before, and now I'm using it all over the place.
And I was really impressed with Ruby when I discovered using blocks with gsub.
Cheers,
Bob
On 3-Nov-07, at 4:44 PM, Sebastian Hungerecker wrote:
blk = lambda {|h,k| h[k] = Hash.new(&blk)}
x = Hash.new(&blk)
x[:la][:li][:lu][:chunky][:bacon][:foo] = "bar"
On 4-Nov-07, at 4:34 AM, Jimmy Kofler wrote:
a = Hash.new{|h,k| h[k]=Hash.new(&h.default_proc) }
----
Bob Hutchison -- tumblelog at http://www.recursive.ca/so/
Recursive Design Inc. -- weblog at http://www.recursive.ca/hutch
http://www.recursive.ca/ -- works on http://www.raconteur.info/cms-for-static-content/home/
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
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!
Ken Bloom 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,
Sebastiansepp2k'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'.
Hi --
On Mon, 5 Nov 2007, Bob Hutchison wrote:
I've been using Ruby for years and I'm still startled by what you can do when passing blocks to initializers. A few weeks ago I stumbled on this:
Hash.new{ | h, k | h[k] = Array.new }
which is just a hash of arrays, completely trivial really, but I'd never thought of it before, and now I'm using it all over the place.
My favorite is:
class NonexistentKeyError < StandardError
end
hash = Hash.new {|h,k| raise NonexistentKeyError, "No such key: #{k}" }
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!
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: sepp2k@jabber.org
ICQ: 205544826
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
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
--
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/
David A. Black wrote:
Hi --
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?