If you are mining with a text source, performing some language modelling, you need to sample from a text corpus. I was wondering how to have a function to randomly selects chunks of a text file efficiently and is doing the job fast. I wanted to keep the function as simple as possible and using basic S3 methods in R.

Solution

I heavily encourage you to use scan function that is a S3 basic R function:

sampleText <- function (filename, total, sampleSize) {  
  lineNumber = sample(total, sampleSize)
  sample <- list()
  for (line in lineNumber) {
    result <- scan(filename, what="character", skip= line, 
         nlines=1, sep="\n", strip.white = TRUE, fileEncoding = 'UTF-8')
    sample <- list(sample, result)
  }
  return(unlist(sample))
}

And here's how to use it:

sample <- createPartition('path_to_file.txt', 2000, 10)  

Inspecting result:

> class(sample)
[1] "character"
> sample
 [1] "It's a cloudy day"                                                                                                                                                
 [2] "But I did want her to have at least PART of my imaginary Paris experience, so I used her pretty Paris stamp to make her birthday card."                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
 ...
[10] "Our smartest friend (Zachary)- was nice enough to study the injection and give us some information to share with everyone..." 

NOTE: The reason that this function is fast is that you send in a file connection. If you are working with different encoding, to change values accordingly. Also, make sure that in case you want sentences back, set sep='.'.

Hope you find this function useful! ?