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
Hi –
On Sun, 4 Nov 2007, Joshua M. wrote:
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..)
David
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
Joshua M. wrote:
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
Jano S. 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
On 11/3/07, Joshua M. [email protected] wrote:
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.
Jano
Lionel B. 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.
Hi –
On Sun, 4 Nov 2007, Jano S. 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.
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
Joshua M. 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
Joshua M. 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}}
lol. Waaaaaaaaaay too late.
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.
–Ken
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
Or use Hash#autonew from the facets gem, which does precisely the same
thing.
–Ken
Hi –
On Sun, 4 Nov 2007, Ken B. 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
On Sat, 03 Nov 2007 20:46:00 -0500, dblack wrote:
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] => #
h[:furby] => #
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] => #
h[:furby] => #
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=>#}
h[:baz] => #
h[:furby] => #
h => {:baz=>#, :furby=>#}
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
Jano S. 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.
David A. Black wrote:
Hi –
On Sun, 4 Nov 2007, Ken B. 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?
Ken B. 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,
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’.
x=[]; x[:bla][:some_key] does not work?
Posted by Joshua M. (josh) on 03.11.2007 21:21
Hi allPHP 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!
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.
Hi –
On Sun, 4 Nov 2007, Ken B. wrote:
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