asterisk users - Sep 2008 - Can someone give a plain english explanation of the HASH function?

Can someone explain what the HASH function does and how to use it?


   -= Info about function 'HASH' =-

[Syntax]
HASH(hashname[,hashkey])

[Synopsis]
Implementation of a dialplan associative array

[Description]
In two argument mode, gets and sets values to corresponding keys  
within a named
associative array.  The single-argument mode will only work when  
assigned to from
a function defined by func_odbc.so.


--
Eric Chamberlain
Founder
RF.com
http://RF.com/

There is no "plain English" explanation that is not made with
reference
to the concept of a hash, which is an algorithmic and programmatic 
technique.  The HASH() function is designed for people that understand 
what hashing is and need to make use of it.

So, the answer is, "it provides a hashing mechanism to the user."

The simplest way to discover what hashing is would be to educate 
yourself about hash tables and associative arrays (which provide 
high-level, usually type-safe syntactical bindings around the use of 
hash tables) in other contexts, after which it should be clear. 
Soliciting any sort of Asterisk-specific explanation is going to give 
you an incomplete, oversimplified, and/or reductionist viewpoint.

I would start here:

   http://en.wikipedia.org/wiki/Hash_function

Cheers,

-- Alex


Eric Chamberlain wrote:
> Can someone explain what the HASH function does and how to use it?
> 
> 
>    -= Info about function 'HASH' =-
> 
> [Syntax]
> HASH(hashname[,hashkey])
> 
> [Synopsis]
> Implementation of a dialplan associative array
> 
> [Description]
> In two argument mode, gets and sets values to corresponding keys  
> within a named
> associative array.  The single-argument mode will only work when  
> assigned to from
> a function defined by func_odbc.so.
-- 
Alex Balashov
Evariste Systems
Web    : http://www.evaristesys.com/
Tel    : (+1) (678) 954-0670
Direct : (+1) (678) 954-0671
Mobile : (+1) (706) 338-8599

On Saturday 13 September 2008 19:40:45 Eric Chamberlain
wrote:
> Can someone explain what the HASH function does and how to use it?
func_odbc.conf:
[FOO]
dsn=mysql
readsql=SELECT * FROM foo WHERE somefield='${SQL_ESC(${ARG1})}'

extensions.conf:

Set(HASH(foo)=${ODBC_FOO(${bar})})

Now you can reference ${HASH(foo,somefield)} or ${HASH(foo,someotherfield)}.
You can even add things to the HASH foo with:
Set(HASH(foo,notinthetable)=baz).

Basically, it's a way to retrieve multiple values from a database without
using ARRAY() to enumerate each field, but also without polluting the
variable namespace, which could cause unexpected behavior, such as when
somebody adds a field to a database table whose name happens to coincide
with an existing variable.

In short, it's a way to be able to use a 'SELECT *' query which
doesn't break
when somebody adds columns to a table.

-- 
Tilghman

> There is no "plain English" explanation that is not made with
reference
> to the concept of a hash, which is an algorithmic and programmatic
> technique.  The HASH() function is designed for people that understand
> what hashing is and need to make use of it.
> ... 
> I would start here:
> 
>    http://en.wikipedia.org/wiki/Hash_function
That is a good detailed article, but for a higher level overview of what
the HASH() function is meant to achieve, I would recommend the following
article:
 
http://en.wikipedia.org/wiki/Associative_array 

(A hash table is a common way to implement an associate array, but the
article on hash tables goes into a lot of implementation detail that is
not really necessary for understanding how to use the asterisk HASH()
function) 

Others have already listed some good examples of why you might want to
use such a feature in your dialplan.

-Matt.