A quick R function to speed up reading in selected columns from a large csv file.

extractCols.R

#####################
### COLS FUNCTION ###
#####################

## Given the file, fields, and their classes, R will import only the relevant columns
extractCols = function(file="", fields = c(), fields.class = c(), nrows=-1){
  
  # Read the first line of the file
  first.line = read.csv(file, nrows=1)
  
  # Check that the fields are present and get their indices
  if(length(fields %in% names(first.line)) == length(fields)){
    fields.ind = which(names(first.line) %in% fields)
  } 
  else{
    return("The specified fields are not present")
  }
  
  # Create vector to exclude unnecessary columns
  cols.want = c()
  for(i in 1:length(names(first.line))){
    if(i %in% fields.ind){ 
      cols.want[i] = fields.class[which(names(first.line)[i] == fields)]
    } else {
      cols.want[i] = "NULL"
    }
  }
  return(read.csv(file, colClasses=cols.want, nrows=nrows))
}