Calculate the mean of one column from several CSV files

Question

I have over 300 CSV files in a folder (named 001.csv, 002.csv and so on). Each contains a data frame with a header. I am writing a function that will take three arguments: t

Answer 1

You do not need more functions. The solution can be simpler from my understanding in 6 lines:

pollutantmean <- function(directory, pollutant, id = 1:10) {
filenames <- sprintf("%03d.csv", id)
filenames <- paste(directory, filenames, sep="/")
ldf <- lapply(filenames, read.csv)
df=ldply(ldf)
# df is your list of data.frames
mean(df[, pollutant], na.rm = TRUE)
}

Answer 2

I think your major problem is listing the files in your working directory and reading them into R. Try list.files function in R Example code which may work for you is

  files <- list.files(pattern = ".csv") ## creates a vector with all file names in your folder
polmean <- rep(0,length(files))
for(i in 1:length(files)){
   data <- read.csv(files[i],header=T)
   polmean[i] <- mean(data$pollutant)
 }
result <- cbind(files,polmean)
write.csv(result,"result_polmeans.csv")

This program gives you the data with name of file in the first column and corresponding means in the second column.