math.R

pollutantmean <- function(directory, pollutant, id = 1:332) {
  files <- list.files(directory, full.names=T)
  frame <- read.csv(files[1])
  
  #for(file in files[2:3]) { # XXX DELETE ME
  for(file in files[2:length(files)]) {
    frame <- rbind(read.csv(file), frame)
  }
  
  #frame <- frame[1:1000,] # XXX delete me!!
  
  F = is.element(frame$ID, id)
  frame <- frame[F,]
  
  mean <- mean(frame[[pollutant]], na.rm = T)
  round(mean, 3) # XXX REMOVE SAID NOT TO ROUND
}  

#  rows <- frame$ID[is.element(frame$ID, id)]
#  
#  mean(if(pollutant == "sulfate") {
#          frame$sulfate
#        } else {
#          frame$nitrate
#        }
#       , na.rm = T)
#}

print(pollutantmean("d:/gdrive/src/datasciencecoursera/specdata", "sulfate", 1:10))
print(pollutantmean("d:/gdrive/src/datasciencecoursera/specdata", "nitrate", 70:72))
print(pollutantmean("d:/gdrive/src/datasciencecoursera/specdata", "nitrate", 23))