[R] Using files as connections

[maj@stats.waikato.ac.nz]

I nearly forgot to thank Andy Liaw and Tony Plate for their help with this
problem. BTW Andy's method does run faster than the natural fix-up of my
original code.

Murray Jorgensen


> You are using the connection the wrong way.  You need to do something
> like:
>
> fcon <- file("c:/data/perry/data.csv", open="r")
> for (iline in 1:slines) {
>     isel <- isel + 1
>     cline <- readLines(fcon, n=1)
>     ...
> }
> close(fcon)
>
> BTW, here's how I'd do it (not tested!):
>
> strvec <- rep("",slines)
> selected <- sort(sample(flines, slines))
> skip <- c(0, diff(selected) - 1)
> fcon <- file("c:/data/[erry/data.csv", open="r")
> for (i in 1:length(skip)) {
>     ## skip to the selected line
>     readLines(fcon, n=skip[i])
>     strvec[i] <- readLines(fcon, n=1)
> }
> close(fcon)
>
> HTH,
> Andy
>
>
>> -----Original Message-----
>> From: maj at stats.waikato.ac.nz [mailto:maj at stats.waikato.ac.nz]
>> Sent: Wednesday, August 27, 2003 7:19 PM
>> To: r-help at stat.math.ethz.ch
>> Subject: [R] Using files as connections
>>
>>
>> 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
>>
>> ______________________________________________
>> R-help at stat.math.ethz.ch mailing list
>> https://www.stat.math.ethz.ch/mailman/listinfo> /r-help
>>
>
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