Re: [R] unexpected behavior from gzfile and unz

From: Matthew Suderman <matthewsuderman_at_yahoo.com>
Date: Wed, 19 Dec 2007 11:10:18 -0800 (PST)


I figured out the problem: functions
gzfile/unz/file/... create a connection to a file but do not *open* the connection unless the 'open' argument is specified.

... a little R gotcha for people who use other programming languages and expect similar concepts/behavior.

> I get unexpected behavior from "readLines()" and
> "scan()" depending on how the file is opened with
> "gzfile" or "unz". More specifically:
>
> > file <- gzfile("file.gz")
> > readLines(file,1)
> [1] "a\tb\tc"
> > readLines(file,1)
> [1] "a\tb\tc"
> > close(file)
>
> It seems that the stream is rewound between calls to
> readLines. The same is true if I replace readLines
> with scan.
>
> However, if I set argument 'open="r"', then
> rewinding
> does not occur:
>
> > file <- gzfile("file.gz",open="r")
> > readLines(file,1)
> [1] "a\tb\tc"
> > readLines(file,1)
> [1] "1\t2\t3"
> > close(file)
>
> Once again, I get the same behavior for scan. The
> rewinding behavior just described also appears if I
> open a zip file with "unz".
>
> > file <- unz("file.zip", "file.txt")
> > readLines(file,1)
> [1] "a\tb\tc"
> > readLines(file,1)
> [1] "a\tb\tc"
> > close(file)
>
> If I add the 'open="r"' argument to the call then I
> get an error from readLines:
>
> > file <- unz("file.zip", "file.txt", open="r")
> > readLines(file,1)
> Error in readLines(file, 1) : seek not enabled for
> this connection
> > close(file)
>
>
> > file <- unz("file.zip", "file.txt", open="rb")
> > readLines(file,1)
> Error in readLines(file, 1) : seek not enabled for
> this connection
> > close(file)
>
> However, if I instead use "scan" to read the file,
> then there are no errors and I get the rewind/no
> rewind behavior described above.
>
> > file <- unz("file.zip", "file.txt")
> > scan(file,nlines=1,sep="\t",what=character(0))
> Read 3 items
> [1] "a" "b" "c"
> > scan(file,nlines=1,sep="\t",what=character(0))
> Read 3 items
> [1] "a" "b" "c"
> > close(file)
>
>
> > file <- unz("file.zip", "file.txt", open="r")
> > scan(file,nlines=1,sep="\t",what=character(0))
> Read 3 items
> [1] "a" "b" "c"
> > scan(file,nlines=1,sep="\t",what=character(0))
> Read 3 items
> [1] "1" "2" "3"
> > close(file)
>
> Is this a bug?
>
> Matt
>
>
>



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