[R-sig-Geo] reading select values from a binary file

[Barry Rowlingson]

On Wed, Dec 9, 2009 at 6:20 PM, rick reeves <reeves at nceas.ucsb.edu> wrote:
> Hello All:
> Faced with a similar challenge, and NOT wanting to resort to writing a C
> language function
> employing fseek(), I just used two readBin calls: One to read (and
> implicitly discard) data up
> to the spot I actually wanted to read, and a second call to read the desired
> information.
> here is a code sample:
>
>                     fileCon = file(sTsFileName,"rb")
> #
> # Offset has been calculated above: number of NumericByte-sized elements
> preceeding
> # the data of interest.
> #
>                     if (iOffset > 0)
>                     {
>
>                        TestVec = readBin(fileCon,numeric(),size =
> iNumericBytes,n=iOffset)
>                     }
>                       dTimeSeries = readBin(fileCon,numeric(),size =
> iNumericBytes,n=iNumElements)                     close(fileCon)
>
> Crude, but effective. I WIS that we could add a seekBin() function that
> positions the file pointer
> to the desired spot.
>

 The R seek function does this. I hadn't tested it when I posted,
hoping the original poster would work it from my message. So I just
now tested it:

 I created a file with ascii a to z in, and then:

 > con = file("file.tst","rb")
 # jump to 13th letter:
 > seek(con,13)
 [1] 0
 # read it:
 > readBin(con,what="raw",n=1)
 [1] 6e     # thats ascii 'n'
 # jump back:
 > seek(con,1)
 [1] 14 # returns where we were, we've now moved...
 > readBin(con,what="raw",n=1)
 [1] 62  # ascii 'b'. Things start at zero....

 Is that what you are trying to do?

Barry