[R] how to skip last lines while reading the data in R

[Prof Brian Ripley]

On Mon, 28 Jan 2008, mrafi wrote:

> but then the number of levels would reamain the same...!!

Please explain: the levels of factors are taken from the data which is 
actually read.

>
>
> Prof Brian Ripley wrote:
>>
>> On Mon, 28 Jan 2008, Barry Rowlingson wrote:
>>
>>> Henrique Dallazuanna wrote:
>>>> Perhaps:
>>>>
>>>> data <-
>>>> read.table(textConnection(rev(rev(readLines('data.txt'))[-(1:2)])))
>>>>
>>>
>>>  Euurgh! Am I the only one whose sense of aesthetics is enraged by
>>> this? To get rid of the last two items you reverse the vector, remove
>>> the first two items, then reverse the vector again?
>>>
>>>  One liners are fine for R Golf games, but in the real world, I'd get
>>> the length of the vector and cut directly off the end. Consider these:
>>>
>>> # reverse/trim/reverse:
>>> rev1 <- function(x,n=100,m=5){
>>>   for(i in 1:n){
>>>     y=rev(rev(x)[-(1:m)])
>>>   }
>>>   return(y)
>>> }
>>>
>>> # get length, trim
>>> rev2 <- function(x,n=100,m=5){
>>>   for(i in 1:n){
>>>     y=x[1:(length(x)-m)]
>>>   }
>>>   return(y)
>>> }
>>>
>>> > system.time(rev1(1:1000,10000,5))
>>>  [1] 1.864 0.008 2.044 0.000 0.000
>>> > system.time(rev2(1:1000,10000,5))
>>>  [1] 0.384 0.008 0.421 0.000 0.000
>>>
>>>
>>>  Result: faster, more directly readable code.
>>
>> And if you know the file size, just use
>>
>> read.table('data.txt', nrows=<#file_rows>-2)
>>
>> (and wc -l will tell you the number of rows more efficiently that using a
>> text connection: if you must use a temporary home use file(), no
>> arguments, as that is much more efficient).
>>
>> --
>> Brian D. Ripley,                  ripley at stats.ox.ac.uk
>> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
>> University of Oxford,             Tel:  +44 1865 272861 (self)
>> 1 South Parks Road,                     +44 1865 272866 (PA)
>> Oxford OX1 3TG, UK                Fax:  +44 1865 272595
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>>
>
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>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595