On Thu, 26 Feb 2004 19:31:15 +0000 (GMT), Prof Brian Ripley
<[EMAIL PROTECTED]> wrote :
>On Thu, 26 Feb 2004, Duncan Murdoch wrote:
>> One way to solve this problem would be to create a connection that did
>> nothing except keep track of a file position, then do the save to that
>> connection. However, it's not easy to define new connection types.
>> Might be a nice package to write to allow such a thing.
>
>I don't think it is worth it. Just use an anonymous file connection and
>seek to the end to measure the length. (Similar to Luke's use of a pipe,
>but more portable.)
I didn't mean to write a package to count bytes, I meant to write a
package that allowed for user-defined connections. The various
functions (read, write, seek, etc.) needed by the connection would be
supplied by the user in R code.
Then you'd do something like this for the special case of counting
bytes:
count <- 0
countbytes <- function(num, bytes) {
count <<- count + num
}
f <- userconnection(write=countbytes)
save(list=ls(all=TRUE),file=f)
cat('Byte count is ',count)
Other possible connections you might want:
Circular connections that cycle through a buffer of bytes again and
again.
Encrypting connections which do something to the bytes on their way to
another connection.
Compressing connections which compress or uncompress another
connection.
Bit-level connections that let you read or write one bit at a time.
Concatenating connections that join two inputs to make them look like
a single stream.
Limiting connections that only show the caller a subset of the stream,
within a certain byte range.
Connections that calculate a checksum or CRC on the bytes as they pass
through.
Many years ago I wrote a Turbo Pascal unit containing all of these
things for "streams", the TP equivalent of connections, and it ended
up being surprisingly useful.
Duncan Murdoch
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