[R] Using files as connections

[Tony Plate]

You need to save the connection object returned by file() and then use that 
object in other functions.

You need to change the appropriate lines to the following (at least):

>con <- file("c:/data/perry/data.csv",open="r")
>   cline <- readLines(con,n=1)
>close(con)

(I don't know if more changes are needed to get it working.)

Note that using the connection object in other functions can have side 
effects on the connection object (which is how a connection "remembers" its 
point in the file.) (Perhaps more accurately, the side effect is on the 
internal system data referred to by the R connection object.)

 > con <- textConnection(letters)
 > con
      description            class             mode             text
        "letters" "textConnection"              "r"           "text"
           opened         can read        can write
         "opened"            "yes"             "no"
 > readLines(con, 1)
[1] "a"
 > readLines(con, 1)
[1] "b"
 > con.saved <- con
 > readLines(con, 1)
[1] "c"
 > readLines(con.saved, 1)
[1] "d"
 > readLines(con, 1)
[1] "e"
 > identical(con, con.saved)
[1] TRUE
 > showConnections()
   description class            mode text   isopen   can read can write
3 "letters"   "textConnection" "r"  "text" "opened" "yes"    "no"
 >
 >

hope this helps,

Tony Plate

At Thursday 11:19 AM 8/28/2003 +1200, you wrote:
>I have been trying to read a random sample of lines from a file into a
>data frame using readLines(). The help indicates that readLines() will
>start from the current line if the connection is open, but presented with
>a closed connection it will open it, start from the beginning, and close
>it when finished.
>
>In the code that follows I tried to open the file before reading but
>apparently without success, because the result was repeated copies of the
>first line:
>
>flines <- 107165
>slines <- 100
>selected <- sort(sample(flines,slines))
>strvec <- rep("",slines)
>file("c:/data/perry/data.csv",open="r")
>isel <- 0
>for (iline in 1:slines) {
>   isel <- isel + 1
>   cline <- readLines("c:/data/perry/data.csv",n=1)
>   if (iline == selected[isel]) strvec[isel] <- cline else
>     isel <- isel - 1
>}
>close("c:/data/perry/data.csv")
>sel.flows <- read.table(textConnection(strvec), header=FALSE, sep=",")
>
>
>There was also an error "no applicable method"  for close.
>
>Comments gratefully received.
>
>Murray Jorgensen
>
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