pollutantmean.R

     ## 'directory' is a character vector of length 1 indicating
        ## the location of the CSV files

        ## 'pollutant' is a character vector of length 1 indicating
        ## the name of the pollutant for which we will calculate the
        ## mean; either "sulfate" or "nitrate".

        ## 'id' is an integer vector indicating the monitor ID numbers
        ## to be used

        ## Return the mean of the pollutant across all monitors list
        ## in the 'id' vector (ignoring NA values)
       
pollutantmean <- function(directory, pollutant, id = 1:332) {
       id <- as.integer(id)
       pollutant <- as.character(pollutant)
       directory <- as.character(directory)
       setwd(paste("./", directory, sep= ""))
       x <-c()
       for (i in id) {
          	 if (i < 10){
          	 	z <-paste("00", i, sep="")
          	 }else if (10 <= i && i < 100){
          	   z <- paste(0, i, sep ="")
          	 }else {
               z <- as.character(i)
             }
       	   data <- read.csv(paste(z, ".csv", sep =""))
       	   if (pollutant == "sulfate"){
       	   	 y <- c(data[,2])
       	   } else if (pollutant == "nitrate") {
       	   	 y <- c(data[,3])
       	   }
       	   x <- c(x, y)
       }
      mean_data <- mean (x, na.rm = TRUE)
      round(mean_data, 3)
}