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Re: Large-scale enumerations and memory problems

"Mark E. Berres" wrote:

you need OpenAppen[] and must Write[] your results
sequential to the file. Export[] will always
use the complete expression ..


> Hi,
>     I am performing some enumerations (v4.1 under Win-XP with 768 MB
> physical memory and
> 4GB swap space) with the following simple code:
> Enum[l_List, (k_Integer)?Positive] := Block[{minusone = Enum[l, k - 1], i,
>      n = Length[l]}, Join @@ Table[(Prepend[#1, l[[i]]] & ) /@ minusone, {i,
> n}]].
> What this does is to take a supplied list of objects, l, and generate all
> possible combinations of
> these objects in a string of length k. Thus, even for small l and moderate
> values of k, the number
> of combinations becomes very large.
> I am exporting the results to a file by
> Export["file.txt", Enum[{G , A , T, C}, 6], "List"] (I want one entry per
> line of text)
> but when k becomes "too large", for example if l=4
> and k=14, 4^14 = ~268 million, the kernal eventually crashes complaining
> about memory. I estimated
> that approx. 1.61 GB is required to store all strings in memory before
> writing to disk - plenty of room
> available on my machine, but still it crashes. It appears that the
> calculation first does its thing and then
> when initiating a write to disk, crashes. Is there a way to write the
> results one-at-a-time, rather than
> the whole-thing-at-once? Or does my problem lie elsewhere?
>                 Sincerely,
>                 Mark

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