I've never programmed before and am trying to learn. I'm following that "coursera" course that I've seen other people post about — a course offered by Johns Hopkins on R programming. Anyway, this was supposed to be my first function. Yet, it doesn't work! But when I type out all the steps individually, it runs just fine... Can anyone tell me why? > pollutantmean <- function(directory, pollutant, id = 1:332){ + x<- list.files("/Users/mike******/Desktop/directory", full.names=TRUE) + y<- lapply(x, read.csv) + z<- do.call(rbind.data.frame, y[id]) + + mean(z$pollutant, na.rm=TRUE) + } > pollutantmean(specdata,nitrate,1:10) [1] NA Warning message: In mean.default(z$pollutant, na.rm = TRUE) : argument is not numeric or logical: returning NA #### > x<- list.files("/Users/mike******/Desktop/specdata",full.names=TRUE) > y<- lapply(x,read.csv) > z<- do.call(rbind.data.frame,y[1:10]) > mean(z$nitrate,na.rm=TRUE) [1] 0.7976266