extractCols2.R

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

require(readr)

## Given the file, fields, and their classes, R will import only the relevant columns
## Instead of full class names, you should use the following naming conventions...
## c = character, d = double, i = integer, l = logical 
## see read_csv documentation for more information

extract_Cols = function(file="", fields, fields.class, n_max=-1){
  
  # Read the first line of the file
  first.line = read_csv(file, n_max=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{
    print("The specified fields are not present")
  }
  
  # Create vector to exclude unnecessary columns
  skip_code = ""
  for(i in 1:length(names(first.line))){
    if(i %in% fields.ind){ 
      skip_code = paste0(skip_code, fields.class[which(names(first.line)[i] == fields)])
    } else {
      skip_code = paste0(skip_code, "_")
    }
  }
  return(read_csv(file, col_types = skip_code, n_max=n_max))
}