title: Opening files in R
body: <p>Suppose a directory has 100 files. How should I open files say in the range 50-60 through a R code?
  I tried using a for loop but how do i open files with the name 50,51,52...60 through a for loop?</p>
  
  <p>Write a function named 'pollutantmean' that calculates the mean of a pollutant (sulfate or nitrate) across a specified list of monitors. The function 'pollutantmean' takes three arguments: 'directory', 'pollutant', and 'id'. Given a vector monitor ID numbers, 'pollutantmean' reads that monitors' particulate matter data from the directory specified in the 'directory' argument and returns the mean of the pollutant across all of the monitors, ignoring any missing values coded as NA.</p>