From: Josh Cheek <josh.cheek@...>
Date: 2011-12-09T15:40:09+09:00
Subject: Re: How can I do h["foo"] += "bar" if h["foo"] does not exist?

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On Fri, Dec 9, 2011 at 12:11 AM, Ryan Davis <ryand-ruby@zenspider.com>wrote=
:

>
> On Dec 8, 2011, at 22:03 , Peter Zotov wrote:
>
> > Andrew S. =D0=C9=D3=C1=CC 09.12.2011 09:58:
> >> Hi there,
> >>
> >> I have a hash with various keys and values that are strings.
> >>
> >> If the key exists, the following works: h["foo"] +=3D "bar"
> >>
> >> But if the key does not exist, I get an error.
> >>
> >> Is it possible _in one statement_ to cover both the cases where the ke=
y
> >> does exist (as above) AND the case where the key does not exist - and =
in
> >> this situation I'd like the new key and value pair to be created in th=
e
> >> hash.
> >>
> >> Any help gratefully received!
> >>
> >> - A
> >
> > $ irb
> > ruby-1.9.3-p0 :001 > hash =3D Hash.new("")
> > =3D> {}
> > ruby-1.9.3-p0 :002 > hash[:a] =3D "test"
> > =3D> "test"
> > ruby-1.9.3-p0 :003 > hash[:a] +=3D " - one"
> > =3D> "test - one"
> > ruby-1.9.3-p0 :004 > hash[:b] +=3D "nothing - two"
> > =3D> "nothing - two"
> > ruby-1.9.3-p0 :005 > hash
> > =3D> {:a=3D>"test - one", :b=3D>"nothing - two"}
>
> This is bad. Very bad.
>
> irb(main):007:0> hash[:d] << "blah"
> =3D> "blah"
> irb(main):008:0> hash[:e] +=3D " blah"
> =3D> "blah blah"
> irb(main):009:0> hash
> =3D> {:b=3D>"nothing - two", :e=3D>"blah blah", :c=3D>"blah", :a=3D>"test=
 - one"}
>
> Make sure you use the block form initializer whenever you have a mutable
> default object.
>
>
>
>
Additionally:

hash =3D Hash.new("")

# modifying the key doesn't set the key
hash[:foo] << 'blah'
hash.keys  # =3D> []

# it modifies the default
hash.default # =3D> "blah"
hash[:foo] # =3D> "blah"
hash[:bar] # =3D> "blah"

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