# Double assignation in a hash **URL:** \ **Category:** Ruby\ **Created:** [June 5, 2008, 1:24am UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571 "2008-06-05T01:24:48Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 5, 2008, 1:24am UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/1 "2008-06-05T01:24:48Z")
hi ! I’m new to ruby, i’d like to know if it is possible to make the following kind of assignation in a hash, like i used to do in php : a={} a[:b][:c]=“value” the goal for me is to create a hash by iterating on keys, like hash={} [a,b,c].each do |k1| [d,e,f].each do |k2| hash[k1][k2] = afunction(k1,k2) end end this last code lead to a nil.[] error Could you help me ? . ---
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 5, 2008, 1:43am UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/2 "2008-06-05T01:43:23Z")
On Jun 4, 2008, at 7:23 PM, nico Itkin wrote: > hash={} > [a,b,c].each do |k1| > [d,e,f].each do |k2| > hash[k1][k2] = afunction(k1,k2) > end > end > > this last code lead to a nil.[] error > > Could you help me ? . Your problem is that a[:b] is not a hash. You can get this effect by defining a default for the hash ‘a’ like so: irb\> a = Hash.new {|h,k| h[k] = {} } =\> {} irb\> a[:b][:c] = ‘value’ =\> “value” irb\> a =\> {:b=\>{:c=\>“value”}} The block supplied to Hash.new is called with the hash and the key when the key doesn’t exist. So the effect is as if you had: a = {} a[:b] = {} a[:b][:c] = ‘value’ -Rob Rob B. [http://agileconsultingllc.com](http://agileconsultingllc.com) removed\_email\_address@domain.invalid ---
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 5, 2008, 9:59am UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/3 "2008-06-05T09:59:02Z")
On Thu, Jun 5, 2008 at 1:23 AM, nico Itkin [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > hi ! > > I’m new to ruby, i’d like to know if it is possible to make the > following kind of assignation in a hash, like i used to do in php : > > a={} > a[:b][:c]=“value” For arbitrary hash nesting, try this: irb(main):023:0\> a = Hash.new {|h,k| h[k] = Hash.new(&h.default\_proc)} irb(main):024:0\> a[1][2][3][4] = “hello” irb(main):025:0\> a =\> {1=\>{2=\>{3=\>{4=\>“hello”}}}} irb(main):026:0\> a[1][2][3][4] =\> “hello” Hope this helps, Jesus. ---
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 5, 2008, 4:21am UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/4 "2008-06-05T04:21:34Z")
On Thu, Jun 05, 2008 at 08:23:53AM +0900, nico Itkin wrote: > hash={} > [a,b,c].each do |k1| > [d,e,f].each do |k2| > hash[k1][k2] = afunction(k1,k2) > end > end > > this last code lead to a nil.[] error > > Could you help me ? . You may find this of interest: [http://redcorundum.blogspot.com/2007/05/just-nifty-meta-meta-tidbit.html](http://redcorundum.blogspot.com/2007/05/just-nifty-meta-meta-tidbit.html) –Greg ---
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 6, 2008, 1:10pm UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/5 "2008-06-06T13:10:53Z")
Jesús Gabriel y Galán wrote: > For arbitrary hash nesting, try this: > > irb(main):023:0\> a = Hash.new {|h,k| h[k] = Hash.new(&h.default\_proc)} > irb(main):024:0\> a[1][2][3][4] = “hello” > irb(main):025:0\> a > =\> {1=\>{2=\>{3=\>{4=\>“hello”}}}} > irb(main):026:0\> a[1][2][3][4] > =\> “hello” Or this (longer but clearer IMO): a = {} a[1] = {} a[1][2] = {} a[1][2][3] = {} a[1][2][3][4] = “hello” print a[1][2][3][4] # “hello” ---
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 9, 2008, 2:19pm UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/6 "2008-06-09T14:19:08Z")
On Sun, Jun 8, 2008 at 10:25 PM, nico Itkin [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Thanks evebody for yours answers, helps a lot 🙂 ! > > Jesús : looks fantastic, but could you give me fex explanation how it > works? > > > a = Hash.new {|h,k| h[k] = Hash.new(&h.default\_proc)} > > is this a way to override the method new for the object a? > > You’re doing so by passing the following block but then i’m lost, do you > have any links to share which detail that ? Hash.new receives a block, which is executed whenever you try to access a non-existing key. In the block you can assign a value to the key in the hash. The value that we assign above is a hash. If we did this: a = Hash.new {|h,k| h[k] = Hash.new} Then we would achieve a two-level hash, but the second level hash, wouldn’t be able to assign a next level hash to a non-existing key, because a block wasn’t provided. Hash has a default\_proc variable that contains the block (in proc form) that was passed to the constructor, so if we assign this proc as the block for the constructor, we get a hash that can assign a hash to a non-existing key, in a recursive way, cause each hash will pass its default proc to the hash created in the constructor block. So many words that I don’t know if I made myself clear… Jesus. ---
**Author:** ![itkin](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/i/22d042/32.png) [@itkin](https://www.ruby-forum.com/u/itkin)\ **Post date:** [June 8, 2008, 10:26pm UTC](https://www.ruby-forum.com/t/double-assignation-in-a-hash/139571/7 "2008-06-08T22:26:27Z")
Thanks evebody for yours answers, helps a lot 🙂 ! Jesús : looks fantastic, but could you give me fex explanation how it works? > a = Hash.new {|h,k| h[k] = Hash.new(&h.default\_proc)} is this a way to override the method new for the object a? You’re doing so by passing the following block but then i’m lost, do you have any links to share which detail that ? Nico (so much to learn…)