From: Ehsanul Hoque <ehsanul_g3@...>
Date: 2009-09-11T09:07:37+09:00
Subject: Re: populating a hash from an array using inject

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I don't have an answer=2C but trying out the code in JRuby=2C I can see tha=
t the default value for bar becomes simply the products array after the inj=
ect() method is used. One more thing that requires an explanation. You can =
see this by appending this line to your code:

pp bar["random string here"]


> Date: Fri=2C 11 Sep 2009 08:30:07 +0900
> From: glennj@ncf.ca
> Subject: populating a hash from an array using inject
> To: ruby-talk@ruby-lang.org
>=20
> I was looking at this problem on Stack Overflow (this one:
> http://stackoverflow.com/questions/1405657/reorganizing-ruby-array-into-h=
ash
>=20
> The question is how to "convert" an array of objects into a hash.
>=20
> Consider this code:
>=20
>     require 'pp'
>     p RUBY_VERSION
>=20
>     Product =3D Struct.new(:name=2C :category)
>     products =3D [
>         ['Apple'=2C'Golden Delicious']=2C
>         ['Apple'=2C'Granny Smith']=2C
>         ['Orange'=2C'Navel']
>     ].collect {|cat=2C name| Product.new(name=2C cat)}
>=20
>     foo =3D products.inject({}) {|h=2Cp| h[p.category] ||=3D []=3B h[p.ca=
tegory] << p=3B h}
>     pp foo
>=20
>     bar =3D products.inject(Hash.new([])) {|h=2Cp| h[p.category] << p=3B =
h}
>     pp bar
>=20
>     baz =3D products.inject(Hash.new([])) {|h=2Cp| h[p.category] +=3D p=
=3B h}
>     pp baz
>=20
> It outputs:
>=20
>     "1.8.7"
>     {"Orange"=3D>[#<struct Product name=3D"Navel"=2C category=3D"Orange">=
]=2C
>      "Apple"=3D>
>       [#<struct Product name=3D"Golden Delicious"=2C category=3D"Apple">=
=2C
>        #<struct Product name=3D"Granny Smith"=2C category=3D"Apple">]}
>     {}
>     {"Orange"=3D>[#<struct Product name=3D"Navel"=2C category=3D"Orange">=
]=2C
>      "Apple"=3D>
>       [#<struct Product name=3D"Golden Delicious"=2C category=3D"Apple">=
=2C
>        #<struct Product name=3D"Granny Smith"=2C category=3D"Apple">]}
>=20
> My question: why is bar empty?
>=20
> --=20
> Glenn Jackman
>     Write a wise saying and your name will live forever. -- Anonymous
>=20

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