[R] hash or other quick lookup function?

[Duncan Murdoch]

Duncan Murdoch wrote:
> 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)
>   

Whoops, that assignment should have been in the cache:

        assign(bits, value, envir=cache)
>   return(value)
> }
>
>
> Duncan Murdoch
>
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