library(data.table)
x <- rbindlist(
  idcol = "path",
  lapply(
    stats::setNames(file, file),
    fread,
    sep = ",",
    quote = "",
    strip.white = FALSE,
    na.strings = NULL
  )
)
print(x)
a <- head(x)
b <- tail(x)
c <- x[sample(NROW(x), 100), ]
d <- x[payment_type == "UNK", ]
e <- x[, .(mean(tip_amount)), by = payment_type]