Re: Unusual behavior of a Ruby hash

In fact, Jesus should be the program saviour (i'm kidding).

2011/10/25 Jes=FAs Gabriel y Gal=E1n <[e-mail]>

> Well, first of all, it seems that essential_params has a key :params_hash=
,
> so I guess it's something like:
>

> essential_params =3D {:params_hash =3D> {:val_1 =3D> 0, :val_2 =3D> [x,y,=
z],
>
:val_3 =3D>0}}
>

I agree. In fact,  :params_hash in essential_params is an array of hashes.

So, modify your line 15 to this:
>
>      new_data[:params_hash]  =3D essential_params[:params_hash].dup
>
> and let us know if it worked for you.
>
> Hope this helps,
>

I should say, it took me this long to get all this was getting messed up. A=
s
it was rightly put by Jesus, I needed to duplicate
essential_params[:params_hash]. Since essential_params[:params_hash] is als=
o
an array of hashes, at every iteration I have to duplicate each hash elemen=
t
of the "essential_params[:params_hash]" array. Though I do not love the
repetitive "semi-manual" duplication, but it is much more efficient than th=
e
repetitive database querying.

In due course, I have learned two good methods and their differences: *clon=
e
* and *dup* (
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/6326).

Thanks folks!!


---
*Edmond*



2011/10/25 Jes=FAs Gabriel y Gal=E1n <[e-mail]>

> On Tue, Oct 25, 2011 at 3:50 PM, Edmond Kachale <[e-mail]>
> wrote:
> > Rubysters,
> >
> > I had the following code working nicely:
> >
> > (a) http://pastie.org/private/8qn4f36mfeppbobm7b2wsa
> >
> > At the beginning of every iteration, I expect "essential_params" hash (=
in
> > line 8) to look like this: essential_params  =3D {:val_1 =3D> 0, :val_2=
 =3D>
> > [x,y,z], :val_3 =3D>0}. However it does repetitive work to initialize a
> hash
> > from a database (in line 8).
> >
> > So as to reduce repetitive database querying (at every iteration), I ha=
ve
> > just moved hash initialization to (line 7) outside a loop:
> >
> > (b) http://pastie.org/private/iudfe0q2s41vszxby6vva
> >
> > Now the code acts wierd: At every iteration,  my "essential_params" has=
h
> > contains data accumulated from the previous iterations. For example, at
> the
> > nth iteration, essential_params may look like this: {:val_1 =3D> 25, :v=
al_2
> =3D>
> > [x,y,z], :val_3 =3D>24} which are results from (n-1)th iteration (inste=
ad
> of
> > the default values: {:val_1 =3D> 0, :val_2 =3D> [x,y,z], :val_3 =3D>0})=
.
> >
>
> Well, first of all, it seems that essential_params has a key
> :params_hash, so I guess it's something like:
>
> essential_params =3D {:params_hash =3D> {:val_1 =3D> 0, :val_2 =3D> [x,y,=
z],
> :val_3 =3D>0}}
>
> If this is the case, then you are overwriting the values of the
> params_hash in lines 22 and 23. When you do this:
>
> new_data[:params_hash]  =3D essential_params[:params_hash]
>
> you are referencing one hash (essential_params[:params_hash]) from the
> other hash. So when, after that, you do this:
>
> new_data[:params_hash][:val_1] =3D 25
>
> you are changing the original hash object.
>
> > Does anyone have a better explanation about this? Are Ruby hashed
> assigned
> > by value or by reference? (Specifically, why is the pre-populated hash =
at
> > line 7 in (b) getting values from lines 22 and 23 ? )
>
> Everything in Ruby is an object. When you assign to a variable, you
> are making the variable reference the object. When you call a method
> through the variable, you are sending that message to the object. An
> object can be referenced by several variables. Check this:
>
> ruby-1.8.7-p334 :001 > a =3D {}
>  =3D> {}
> ruby-1.8.7-p334 :002 > a.object_id
>  =3D> -611696338
> ruby-1.8.7-p334 :003 > b =3D a
>  =3D> {}
> ruby-1.8.7-p334 :004 > b.object_id
>  =3D> -611696338
>
> Both a and b are variables that point to the same object. There's only
> one hash. So when you change the hash, be it through a or through b
> you are changing the same object:
>
> ruby-1.8.7-p334 :005 > b[:val] =3D 3
>  =3D> 3
> ruby-1.8.7-p334 :006 > a
>  =3D> {:val=3D>3}
>
> This is basically what you are seeing in your code (a is
> essential_params[:params_hash] and b is new_data[:params_hash]).
> You might want to create a copy of the hash so that the original hash
> is not modified. You can do that with Object#dup.
>
> ruby-1.8.7-p334 :007 > a =3D {:val =3D> 3}
>  =3D> {:val=3D>3}
> ruby-1.8.7-p334 :008 > b =3D a.dup
>  =3D> {:val=3D>3}
> ruby-1.8.7-p334 :009 > b[:val] =3D 5
>  =3D> 5
> ruby-1.8.7-p334 :010 > a
>  =3D> {:val=3D>3}
> ruby-1.8.7-p334 :011 > b
>  =3D> {:val=3D>5}
>
> So, modify your line 15 to this:
>
>      new_data[:params_hash]  =3D essential_params[:params_hash].dup
>
> and let us know if it worked for you.
>
> Hope this helps,
>
> Jesus.
>
>