pollutantmean.R

pollutantmean <- function(directory, pollutant, id = 1:332) {
        ## '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)
        ## NOTE: Do not round the result!
        
        df <- NA
        
        fileIdNames <- dir(directory)
        
        for(fileIdName in fileIdNames){
                fileIdStr <- strsplit(fileIdName,".csv",fixed = TRUE)
                fileId <- as.numeric(fileIdStr)
                if(fileId %in% id){
                        tempDataFrame <- read.csv(file.path(directory,fileIdName))
                        df <- rbind(df,tempDataFrame)
                }
        }
        
        filteredDf <- df[which(!is.na(df[pollutant])),pollutant]
        mean(filteredDf)
}