RE: [R] skip lines on a connection

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From: Vadim Ogranovich (vograno@evafunds.com)
Date: Mon 03 May 2004 - 07:26:45 EST


Message-id: <C698D707214E6F4AB39AB7096C3DE5A54B32DB@phost015.intermedia.net>


> From: Prof Brian Ripley [mailto:ripley@stats.ox.ac.uk]
> Sent: Saturday, May 01, 2004 11:44 PM
> You will be telling us next you think the default nmax=-1
> means to read a negative number of lines!

No, I won't. Your extrapolation is inaccurate.

> ... So reading no
> lines would mean not calling scan at all, and what would be
> the point of that?

It would mean skipping the number of lines specified in the skip
argument thus advancing the read point on the connection to where I want
it to be. I guess you wouldn't argue that seek(con, where) has no
meaning.

>
> nmax <= 0 and nlines <= 0 are ignored.
>
> Note carefully what nmax actually means, and it is not what `nlines'
> means!

I had noted that. If one reads no "data value" one reads no line, so the
two should have the same effect in the case at hand.

> Do read the documentation for scan, too, please.

I had. For your convenience this is what it says about nmax.

    nmax: the maximum number of data values to be read, or if 'what' is
          a list, the maximum number of records to be read. If omitted
          (and 'nlines' is not set to a positive value), 'scan' will
          read to the end of 'file'.

It is hard to see from the text that nmax=0 is ignored since "omitted"
means leaving it set to -1.

BTW, the paragraph regarding 'nlines' doesn't mention that nlines=0 is a
special case either.

  nlines: the maximum number of lines of data to be read.

> Note that to read *lines* you do need to read every byte on
> the file to
> find the EOL marker(s) so readLines() or scan() with NULL in
> "what" are as
> good as anything. You can use them in blocks of lines, in a loop.

This is a very nice trick indeed! Just what I've been looking for.

Thank you very much,
Vadim

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