Defining a default proc for Hash


#1

Hello,

Is there a way to redefine or newly define a default proc for Hash way
after
it has been constructed and populated with elements?

There seems to be a #default= but no #default_proc=

Thank you,

Jayanth


#2

Srijayanth S. wrote:

Hello,

Is there a way to redefine or newly define a default proc for Hash way
after
it has been constructed and populated with elements?

There seems to be a #default= but no #default_proc=

Did you read this?

---------------------------------------------------------- Hash#default=
hsh.default = obj => hsh

  ....

  It is not possible to set the a default to a
 +Proc+ that will be executed on each key lookup.

   ...

    # This doesn't do what you might hope...
    h.default = proc do |hash, key|
      hash[key] = key + key
    end
    h[2]       #=> #<Proc:0x401b3948@-:6>
    h["cat"]   #=> #<Proc:0x401b3948@-:6>

#3

Yes, I did. All that tells me is that it isn’t possible to set a default
proc using default or any other calls. Which is why I asked, if there is
any
other way to do it. If not, it simply can’t be done. Of course, it
would
also be nice if someone could explain the rationale for not wanting to
alter
the default proc once it has been defined.

Thank you,

Jayanth


#4

Thanks,

That’s twice in two days that merge has come up wrt Hash.

Jayanth


#5

2009/4/30 Srijayanth S. removed_email_address@domain.invalid:

Yes, I did. All that tells me is that it isn’t possible to set a default
proc using default or any other calls. Which is why I asked, if there is any
other way to do it. If not, it simply can’t be done. Of course, it would
also be nice if someone could explain the rationale for not wanting to alter
the default proc once it has been defined.

It’s there - but not in 1.8.*:

13:14:34 JavaProducts_NGCP_dev_R1.0_PoA$ allruby -e ‘puts
Hash.instance_methods.grep(/proc/)’
CYGWIN_NT-5.1 padrklemme1 1.5.25(0.156/4/2) 2008-06-12 19:34 i686 Cygwin
ruby 1.8.7 (2008-08-11 patchlevel 72) [i386-cygwin]
default_proc
ruby 1.9.1p0 (2009-01-30 revision 21907) [i386-cygwin]
default_proc
default_proc=
13:14:51 JavaProducts_NGCP_dev_R1.0_PoA$

Kind regards

robert


#6

2009/4/30 Srijayanth S. removed_email_address@domain.invalid:

Yes, I did. All that tells me is that it isn’t possible to set a default
proc using default or any other calls. Which is why I asked, if there is any
other way to do it. If not, it simply can’t be done. Of course, it would
also be nice if someone could explain the rationale for not wanting to alter
the default proc once it has been defined.

You can do it like this:

class Hash
def default_proc=(blk)
self.replace(Hash.new(&blk).merge(self))
end
end

h = {1=>2,3=>4}
p h.default_proc
h.default_proc = proc {|h,k| h[k]=3 }
p h.default_proc

Regards,

Park H.