R devel - Jul 2008 - head() for files

Hi!

I was thinking about using the head and tail functions also for files i.e.
show me the first/last few lines of a file. For example:

zz <- file("ex.data", "w")  # open an output file
connection
cat("TITLE extra line", "2 3 5 7", "", "11 13
17", file = zz, sep = "\n")
cat("One more line\n", file = zz)
close(zz)
readLines("ex.data")

head.character <- function(x, n=6, ...) readLines(con=x, n=n, ...)

head.character("ex.data")

As can be seen, this is essentially a wrapper for readLines, but a handy one.
Of course, S3 "linking" is needed, perhaps also for class
"connection".

I am not prefectly satisfied with it as I would like to see printout line by
line.
Any ideas?

--
Lep pozdrav / With regards,
    Gregor Gorjanc
----------------------------------------------------------------------
University of Ljubljana     PhD student
Biotechnical Faculty        www: http://gregor.gorjanc.googlepages.com
Zootechnical Department     blog: http://ggorjan.blogspot.com
Groblje 3                   mail: gregor.gorjanc <at> bfro.uni-lj.si
SI-1230 Domzale             fax: +386 (0)1 72 17 888
Slovenia, Europe            tel: +386 (0)1 72 17 861

On 25/07/2008 7:30 AM, Gorjanc Gregor wrote:
> Hi!
> 
> I was thinking about using the head and tail functions also for files i.e.
> show me the first/last few lines of a file. For example:
> 
> zz <- file("ex.data", "w")  # open an output file
connection
> cat("TITLE extra line", "2 3 5 7", "",
"11 13 17", file = zz, sep = "\n")
> cat("One more line\n", file = zz)
> close(zz)
> readLines("ex.data")
> 
> head.character <- function(x, n=6, ...) readLines(con=x, n=n, ...)
> 
> head.character("ex.data")
> 
> As can be seen, this is essentially a wrapper for readLines, but a handy
one.
> Of course, S3 "linking" is needed, perhaps also for class
"connection".
> 
> I am not prefectly satisfied with it as I would like to see printout line
by line.
> Any ideas?
I think you should have called it head.connection, and then

head(file("ex.data"))

would sort of work.  The problems with this are open connections:  what 
should it do on one of those?  Seeking to the start would make sense, 
but not all connections can do that.

Duncan Murdoch

>> I was thinking about using the head and tail functions also for files
i.e.
>> show me the first/last few lines of a file. For example:
>>
>> zz <- file("ex.data", "w")  # open an output
file connection
>> cat("TITLE extra line", "2 3 5 7", "",
"11 13 17", file = zz, sep = "\n")
>> cat("One more line\n", file = zz)
>> close(zz)
>> readLines("ex.data")
>>
>> head.character <- function(x, n=6, ...) readLines(con=x, n=n, ...)
>>
>> head.character("ex.data")
>>
>> As can be seen, this is essentially a wrapper for readLines, but a
handy one.
>> Of course, S3 "linking" is needed, perhaps also for class
"connection".
>>
>> I am not prefectly satisfied with it as I would like to see printout
line by line.
>> Any ideas?
>
> I think you should have called it head.connection, and then
>
> head(file("ex.data"))
Yes, but typing head(file("ex.data")) is not that different than
readLines(file="ex.data").
However, I do agree that head would work with "character" and
"connection" classes.
> would sort of work.  The problems with this are open connections:  what
> should it do on one of those?  Seeking to the start would make sense,
> but not all connections can do that.
Hmm, I am not sure about that since I do not use them often. After a quick look
at
?file, I think that file(), url(), gzfile(), bzfile() should work out of the box
- see showcases
bellow. unz() can not work without passing a filename, but that is a matter of
unz() and
not head.

I also do not know anything about fifos, while pipe could also be useful - btw.
I can not find
example for pipe() in ?pipe although the docs say

"For pipe the description is the command line to be piped to or from (see
the Examples). This
command line is run in the shell specified by the COMSPEC environment
variable."

Regards, gregor

## Some showcases for head()

head.character <- head.connection <- function(x, n=6, ...)
readLines(con=x, n=n, ...)
tmp <- function(x)
{
    cat("TITLE extra line", "2 3 5 7", "",
"11 13 17", file=x, sep = "\n")
    cat("One more line\n", file=x)
    close(x)
}

head(file("ex.data"))

## file()
zz <- file("ex.data", "w"); tmp(zz)
head.character("ex.data")
head(file("ex.data"))
unlink("ex.data")

## url()
head(url("http://www.r-project.org/index.html"))

## gzfile()
zz <- gzfile("ex.gz", "w"); tmp(zz)
head(gzfile("ex.gz"))
unlink("ex.gz")

## bzfile()
zz <- bzfile("ex.bz2", "w"); tmp(zz)
head(bzfile("ex.bz2"))
unlink("ex.bz2")

## unz()