r - creating a function which extracts a user specified column from a set of files -

i have set of csv files. of them have same structure. want create function extracts particular column files. finds mean of values in column , store in vector. column name should passed user.

i have coded following program. somehow can not identify "pollutant" contains name of column.

   pollutantmean<-function(pollutant) {   file_names<-dir("c:/users/keval/desktop/project r/r_courseera_programming_exercise/specdata",pattern= glob2rx("*.csv"))    for(file_name in file_names)   {     file_reader<-read.csv(file_name)     pollutant_data<-file_reader$pollutant   }   pollutant_data   pollutant }`enter code here` 

use string, e.g., call function with

pollutantmean(pollutant = "mercury") 

and use [ (which accepts strings) instead of $, doesn't:

# replace line pollutant_data <- file_reader$pollutant # this: pollutant_data <- file_reader[, pollutant] 

this won't error out, still need take mean , store it. i'm pretty sure want list.files, not dir.

pollutantmean<-function(pollutant) {     file_names <- list.files("c:/users/keval/desktop/projectr/r_courseera_programming_exercise/specdata",       pattern= glob2rx("*.csv"))    # initialize mean vector @ correct length   my_means = numeric(length(file_names)   # make loop indexed number   for(i in seq_along(file_names)) {     file_reader <- read.csv(file_names[i])     pollutant_data <- file_reader[, pollutant]     # using number index     my_means[i] = mean(pollutant_data)   }   return(my_means) }