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

[mrafi]

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


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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> 

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