R help - May 2008 - hash or other quick lookup function?

Hello all,

I have a matrix of bit values.

I compute certain values based on the bits in each row. There may be
*duplicate* entries in the matrix, ie several rows may be identical.
These rows change over time, and identical entries may not be next to
each other.

Computing these values is time consuming, so I would like a way to store a
value once it's computed. That way I would be able to look up the value
based on a given bitstring (row in the matrix) and avoid having to re-compute
the same value.

So, my thought is a hash function based on a bit string - does R have such
a thing? Or is there an alternative approach that would do the same in R?
The lookup should be quick, otherwise the gains from avoiding
recomputing identical values would be lost to some extent.

I wasn't able to find anything via searches, I am hoping someone can
point me in the right direction.

Thanks
Esmail

Esmail Bonakdarian wrote:
> Hello all,
>
> I have a matrix of bit values.
>
> I compute certain values based on the bits in each row. There may be
> *duplicate* entries in the matrix, ie several rows may be identical.
> These rows change over time, and identical entries may not be next to
> each other.
>
> Computing these values is time consuming, so I would like a way to store a
> value once it's computed. That way I would be able to look up the value
> based on a given bitstring (row in the matrix) and avoid having to
re-compute
> the same value.
>
> So, my thought is a hash function based on a bit string - does R have such
> a thing? Or is there an alternative approach that would do the same in R?
> The lookup should be quick, otherwise the gains from avoiding
> recomputing identical values would be lost to some extent.
>
> I wasn't able to find anything via searches, I am hoping someone can
> point me in the right direction.
>   
Environments can be hashed. To use this, you'd convert the bit string 
into a character string, and use that as the name of a variable in an 
environment.

For example,

cache <- new.env(hash=TRUE, parent=emptyenv())
 ...
bits <- '0101'
if (exists(bits, cache)) {
  return(get(bits, cache))
} else {
  value <- long.computation()
  assign(bits, value)
  return(value)
}


Duncan Murdoch