correl <- function(prah=0) {
    ret_vec <- vector()

    file_ids <- 1:332
    for (file_id in file_ids) {
        filename <- sprintf("%03d.csv")
        file <- read.csv(filename)
        val_entries <- nrow(file[complete.cases(file), ])

        if (val_entries >= prah) {
            # file <- read.csv(filename)
            sulf_vec <- file$sulfate
            nitr_vec <- file$nitrate

            correlation <- cor(sulf_vec, nitr_vec, na.rm=TRUE)
            append(ret_vec, correlation)
        }
    }

    return ret_vec
}