Hi,

I'm reading a program written in perl and I read this statement


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On Wed, Jun 22, 2011 at 6:44 PM, josanabr <john.sanabria [at] gmail.com> wrote:

> Hi,
>
> I'm reading a program written in perl and I read this statement
>
>
> --
> To unsubscribe, e-mail: beginners-unsubscribe [at] perl.org
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>
>
>
Without any more information I would have to say good, and what do you want
form the list? (it really does help if you formulate a question and ask what
it is that you want to know ;-)

I suspect hat you are wondering what this means right?

Lets dissect this a little:

Lets take the inner most thing ($var2) this is obviously a scalar (or a
reference to another variable (I'll explain why I am betting it is a scalar
in a bit))
Then we see the following: $var1{...} this is the way one accesses a
variable in a hash based on the key (the thing that goes between those
brackets). Usually the keys used in a has are scalars of course there is
nothing stopping anyone from using complex data structures as a key but it
is performance wise not the smartest thing to do.
The last bit then [at] {...} basically says treat what is in side the brackets
as an array (which is what one would do if one is expecting an array
reference to be returned from $var1's value associated with key $var2.

So what would the data structure look like?
{
"Hash key 1" => \[
'Array value 1',
'Array value 2',
...
],
"Hash key 2" => \[
'Array value 1',
'Array value 2',
...
],
...
}

Or in text form: $var1 is an hash containing keys associated with values
which are references to arrays.

I hope that explains things a little bit. :-)

Regards,

Rob

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On 6/22/11 Wed Jun 22, 2011 9:44 AM, "josanabr" <john.sanabria [at] gmail.com>
scribbled:

> Hi,
>
> I'm reading a program written in perl and I read this statement

It is best to put all of your post in the body of your message, and use the
title as a description of your question.

Do you have a specific question about the statement appearing in your title?
This one:

[at] {$var1{$var2}}

I can tell you that the syntax of that statement implies that %var1 is a
hash, $var2 is a scalar, the element of %var1 indexed by $var2 is
$var1{$var2} and is a reference to an array, and [at] {$var1{$var2}} is the
dereferenced array.

See 'perldoc perlref' for information about references in Perl.



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>>>>> "JG" == Jim Gibson <jimsgibson [at] gmail.com> writes:

JG> See 'perldoc perlref' for information about references in Perl.

even better for a newbie is to read 'perldoc perlreftut' and later
perllol and perldsc. leave perlref for when you have some experience
with refs under your belt and want more info.

uri

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Hi John,

On Wed, 22 Jun 2011 09:44:56 -0700 (PDT)
josanabr <john.sanabria [at] gmail.com> wrote:

> Hi,
>
> I'm reading a program written in perl and I read this statement
>

Well, assuming you are interested to learn about what [at] {$var1{$var2}} mean,
then:

1. $var1{$var2} is the value of the %var1 hash which is keyed by $var2.

2. [at] {$array_ref} dereferences the array $array_ref into an array.

Regards,

Shlomi Fish

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On 2011-06-22 18:44, josanabr wrote:

> I'm reading a program written in perl and I read this statement

[at] { $var1{ $var2 } }

Such variable names with numbers in them are often a sign of bad code.
What is the context?

--
Ruud

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On Jun 22, 12:49=A0pm, rco... [at] gmail.com (Rob Coops) wrote:
> On Wed, Jun 22, 2011 at 6:44 PM, josanabr <john.sanab... [at] gmail.com> wrote=
:
> ...
> Lets dissect this a little:
>
> Lets take the inner most thing ($var2) this is obviously a scalar (or a
> reference to another variable (I'll explain why I am betting it is a scal=
ar
> in a bit))
> Then we see the following: $var1{...} this is the way one accesses a
> variable in a hash based on the key (the thing that goes between those
> brackets). Usually the keys used in a has are scalars of course there is
> nothing stopping anyone from using complex data structures as a key but i=
t
> is performance wise not the smartest thing to do.

> The last bit then [at] {...} basically says treat what is in side the bracket=
s
> as an array (which is what one would do if one is expecting an array
> reference to be returned from $var1's value associated with key $var2.
>
> So what would the data structure look like?
> {
> =A0"Hash key 1" =3D> \[
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'Array value 1',
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'Array value 2',
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0...
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ],
> =A0"Hash key 2" =3D> \[
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'Array value 1',
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0'Array value 2',
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0...
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 ],
> =A0...
>
> }

Hm, I think it's worth noting though if you really want to
"treat what is in side the brackets as an array", you'd
likely be using a simpler data structure such as:

"Hash key 2" =3D [ "Array value 1", "Array value 2", ... ]

rather than:

"Hash key 2" =3D \[ "Array value 1", "Array value 2", ... ]

since the latter actually creates a ref to an anonymous
array. And, if it is a "ref to a ref", you'd need to deref
the original like this:

[at] { ${$var1{$var2}} }

rather than just: [at] { $var1{$var2} }

See: perldoc perlref

>
> Or in text form: $var1 is an hash containing keys associated with values
> which are references to arrays.
>

--
Charles DeRykus


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I prefer to decode this kind of code from outside to inside.

1. The sigil [at] indicates the result is an array
2. Thus, $var1{$var2} should be an array reference
3. The structure of $var1{$var2} suggests var1 means a hash, or %var1
4. Thus, $var2 is a key of the hash %var1

So, %var1 is a hash of array reference, or the values of the hash %var1
are array references. [at] {$var1{$var2}} is trying to dereference the
references and get the arrays.

On 2011-6-23 0:44, josanabr wrote:
> Hi,
>
> I'm reading a program written in perl and I read this statement


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