pollutantmean <- function(directory="specdata", pollutant, id=1:332) {
  fileslist <- list.files("specdata", full.names=TRUE)
  frame <- data.frame()
  for (i in 1:332) {
     frame <- rbind(frame, read.csv(fileslist[i]))
  }
  fsub <- frame[which(frame[,"ID"] == id), ]
  if (pollutant == "sulfate") {
     sulfate <- mean(fsub[,"sulfate"], na.rm=TRUE)
     return(sulfate)
  }else {
     nitrate <- mean(fsub[,"nitrate"], na.rm=TRUE)
     return(nitrate)
  }
}

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