[R] Fast reading of hex data?

[R. Michael Weylandt]

I'd imagine there are better tricks, but I know you can use
as.numeric() if you signal to R that you've got a hex value. See,
e.g.,

http://tolstoy.newcastle.edu.au/R/help/06/08/33758.html

Best,
Michael

On Tue, May 8, 2012 at 5:44 AM, Fang <zhou.zfang at gmail.com> wrote:
> Hi all,
>
> Basically, I have data in the format of (up to 1 gig in size) text files
> containing stuff like:
>
> F34060F81000F28055F8A000F2E05EF8F000F34 (...)
>
> The data is basically strings denoting hex values (9 = 9, A = 10, B = 11,
> ...) organised in fixed, small blocks. What I want to do is to read in a
> specified segment of the string, break it up into blocks, and convert it
> into a vector of integers for further processing. And I want to do this
> fast, and hopefully without using masses of memory. So, I'm wondering if
> anyone has any better ideas than what I'm doing - well, anything that would
> make a sizable difference anyway.
>
> Right now, my methodology is the following:
>
> Use mmap (from library mmap) to map the file to a memory mapped variable,
> reading in each byte as uint8 integer.
> obj <- mmap("file.txt", mode = uint8())
> tmp <- obj[bytepos]
> Converting the integer representations of each byte into the appropriate
> integer by
> tmp <- tmp - 48 - 7*(tmp>64)
> Collating blocksize values together by
> tmp<- matrix(tmp, ncol = blocksize, byrow = T) %*% 16^(blocksize: 1 - 1)
>
> Now, my question is, is there a better way? My attempts with rawToChar and
> strtoi seems to take drastically longer for reasonably lengthy bytepos,
> presumeably because of string manipulations/storage, but possibly I am doing
> it wrong somehow. If there is no better way in R, would there be much value
> in implementing this in C, for example, or would the computational
> improvement be small?
>
> Thanks,
>
> Zhou
>
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