Question

I am currently using this code to input data from numerous files into R:

library(foreign)

setwd("/Users/ericbrotto/Desktop/A_Intel/")

filelist <-list.files()

#assuming tab separated values with a header    
datalist = lapply(filelist, function(x)read.table(x, header=T, sep=";", comment.char="")) 

#assuming the same header/columns for all files
datafr = do.call("rbind", datalist) 

The headers look like this:

TIME ;POWER SOURCE ;qty MONITORS ;NUM PROCESSORS ;freq of CPU Mhz ;SCREEN SIZE ;CPU LOAD ;BATTERY LEVEL ; KEYBOARD MVT ; MOUSE MVT ;BATTERY MWH ;HARD DISK SPACE ;NUMBER PROCESSES ;RAM ;RUNNING APPS  ;FOCUS APP ;BYTES IN ;BYTES OUT ;ACTIVE NETWORKS ; IP ADDRESS ; NAMES OF FILES ; 

and an example of the data looks like this:

 2010-09-11-19:28:34.680 ; BA ; 1 ; 2 ; 2000 ; 1440 : 900  ; 0.224121 ; 92 ; NO ; NO ; NULL ; 92.581558  ;  57    ; 196.1484375   ; +NULL  ; loginwindow-#35  ;  5259  ;  4506  ; en1 :   ;  192.168.1.3  ;  NULL  ;    

Rather then input all of the columns into a data frame I would like to just grab one, say, FOCUS APP.

Answer 1

If you just want to read in a particular column from your files, then colClasses is the way to go. For example, suppose your data looked like this:

a,b
1,2
3,4

Then

## Use colClasses to select columns
## "NULL" means skip the column
## "numeric" means that the column is numeric
## Other options are Date, factor - see ?read.table for more
## Use NA to let R decide
data = read.table("/tmp/tmp.csv", sep=",", 
                  colClasses=c("NULL", "numeric"), 
                  header=TRUE)

gives just the second column.

> data
  b
1 2
2 4