[R] Using files as connections

Tony Plate tplate at blackmesacapital.com
Thu Aug 28 01:54:12 CEST 2003

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)

(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)
>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
>sel.flows <- read.table(textConnection(strvec), header=FALSE, sep=",")
>There was also an error "no applicable method"  for close.
>Comments gratefully received.
>Murray Jorgensen
>R-help at stat.math.ethz.ch mailing list

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