fileNames <- c("sample1.csv", "sample2.csv")

Finding Files with a Specific Extension

fileNames <- Sys.glob("*.csv")

Iterating Across All Files

for (fileName in fileNames) {

  # read data:
  sample <- read.csv(fileName,
    header = TRUE,
    sep = ",")

  # add more stuff here

}

for (fileName in fileNames) {

  # read old data:
  sample <- read.csv(fileName,
    header = TRUE,
    sep = ",")

  # add one to every widget value in every file:
  sample$Widgets <- sample$Widgets + 1
  
  # overwrite old data with new data:
  write.table(sample, 
    fileName,
    append = FALSE,
    quote = FALSE,
    sep = ",",
    row.names = FALSE,
    col.names = TRUE)

}

Or we could do the same thing, but create a new copy of each file:

extension <- "csv"

fileNames <- Sys.glob(paste("*.", extension, sep = ""))

fileNumbers <- seq(fileNames)

for (fileNumber in fileNumbers) {

  newFileName <-  paste("new-", 
    sub(paste("\\.", extension, sep = ""), "", fileNames[fileNumber]), 
    ".", extension, sep = "")

  # read old data:
  sample <- read.csv(fileNames[fileNumber],
    header = TRUE,
    sep = ",")

  # add one to every widget value in every file:
  sample$Widgets <- sample$Widgets + 1
  
  # write old data to new files:
  write.table(sample, 
    newFileName,
    append = FALSE,
    quote = FALSE,
    sep = ",",
    row.names = FALSE,
    col.names = TRUE)

}

In the above example, we used the paste and sub functions from the base package to automatically create new file names based on the old file names.

Or we could instead use each dataset to create an entirely new dataset, where each row is based on data from one file:

fileNames <- Sys.glob("*.csv")

for (fileName in fileNames) {

  # read original data:
  sample <- read.csv(fileName,
    header = TRUE,
    sep = ",")

  # create new data based on contents of original file:
  allWidgets <- data.frame(
    File = fileName,
    Widgets = sum(sample$Widgets))
  
  # write new data to separate file:
  write.table(allWidgets, 
    "Output/sample-allSamples.csv",
    append = TRUE,
    sep = ",",
    row.names = FALSE,
    col.names = FALSE)

}

In the above example, data.frame is used to create a new data row based on each data file. Then the append option of write.table is set to TRUE so that row can be added to the other rows created from other data files.

Those are just a few examples of how you can use R to perform the same function(s) on a large number of files without having to manually run each one. I'm sure you can think of more uses.

All the files are available on GitHub. You can see how eachFile.R, eachfile-newNames.R, and eachFile-append.R each do something different to the sample datasets.