Two questions about hash declaration

[whn]

Experts,

I have two questions about "hash declaration".

1) How to declare a constant HASH? Once it's declared, the total numbers keys can not be changed and the values for each key also can not be changed.

One way I can think of is to use Readonly module. However, it is not a core module so that my code is not very portable.

2) See the example below first:

my %h1 = (
  'x1', 'y1',
  'x2', 'y2',
  'x3', 'y3',
  ...
  'xn', 'yn',
);

my %h2 = (
  'x1', 'y1',
  'x2', 'y2',
  'x3', 'y3',
  ...
  'xn', 'yn',
  'a1', 'b1',
  'a2', 'b2',
  'a3', 'b3',
  ...
  'an', 'bn',
);

Both %h1 and %h2 are static. In reality, '%h1' can have many many keys. To make the program short, here is what I did:

my %h2;
foreach (keys(%h1)) {
  $h2{$_} = $h1{$_};
}
$h2{'a1'} = 'b1';
$h2{'a2'} = 'b2';
...
$h2{'an'} = 'bn';

I wish there could be something like operator overloding in java/c++:

my %tmp = (
  'a2', 'b2',
  'a3', 'b3',
  ...
  'an', 'bn',
);

my %h2 = %h1 + %tmp;

Is there an elegant way to do this?

 

[KevinADC]

my %h2 = (%h1,%tmp);

but any duplicate hash keys will be lost doing that.

------------------------------------------
- Kevin, perl coder unexceptional!

 

[prex1]

1) Can't see a way to build a constant hash, nor can I see the use of it.
In fact you can create a reference to a hash where the reference is a constant, but you can still change the underlying hash via its constant reference, and I assume this is not what you want.
On the other side a constant hash is nothing less and nothing more than a collection of constants with their corresponding (single, not list) values: that's why I don't see the use for it, unless you can explain better your goal.
A collection of constants is created with

use constant {
  X1 => 'y1',
  X2 => 'y2',
  ..
  XN => 'yn',
};

Of course in this way you can't use the [tt]each, values, keys, exists[/tt] functions, all depends on what you need to do.
If you only need to be sure that no change will be done on the constant hash, you should write a module where the hash is defined (with [tt]my[/tt] of course), that contains also all the functions required to return a value from a key, check whether a key exists, list the key values, and so on.
2)Kevin has the answer on how to fill a hash with another one. However once again I can't see the use of it, especially if both are used as static (constant) key-value pair lists.

Franco

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[whn]

Thank you, Kevin and Franco.

Currently, I implemented my hash exactly in the way you suggested -- declare it as a private hash in a module and another subroutine returns it.

Now, I have another question, when a subroutine returns a hash (only one hash), should the subroutine returne it by value or by reference? I always return by reference. Butin this case, I guess I'd better return it by value. Am I right?

With regarding to why I want to use a constant hash, my company's software support certain linux platforms. The combination of flavor/release/processor/kernel is huge. no one can remember all of them. And worse, this matrix is written in a PDF file and managed by human. I am writing a perl tool to manage it. The first step is to declare a multi-dimension hash to map this matrix. I was thinking to use a constant hash. Since I could not figure it out how, I put it in a module just as the way as you suggested.

Again, thank you both.

 

[prex1]

Well, it all depends on what you want to do.
Generally speaking, one would prefer to use a reference, as passing a hash by value may slow down your code.
However, knowing that reference, your code could change that hash, even if it declared and populated in a separate module.
So, if your main goal is to make that hash unmodifiable, you should return everything by value.

Franco

http://www.xcalcs.com

: Online engineering calculations

http://www.megamag.it

: Magnetic brakes for fun rides

http://www.levitans.com

: Air bearing pads

 

[KevinADC]

I could have sworn there is a module on CPAN that "locks" a hash but maybe I am wrong.

------------------------------------------
- Kevin, perl coder unexceptional!

 

[whn]

To Kevin,

Yes, CPAN has a non-core module called 'Readonly.pm'. Because my tool is meant to be used in any newly configured servers, non-core modules are not portable. Now, I'm thinking to put those non-core ones in a local dir and tar them together with my tool so that they can be portable.