Take the csv files above the threshold of complete file(rows without any NA) and check the covariance of them all together

Assignment1_Part3.R

corr <- function(directory, threshold = 0) {
        result <- NULL
        #for each csv file, use function complete() to obtain the complete number and
        #compare with threshold
        for (i in 1:332) {
                # get the number into 000 format, exp: 1 -> 001
                num <- sprintf("%03d",i)
                # read the file and saved to data
                path <- paste(directory,num,".csv",sep="")
                data <- read.csv(path)
                t <- complete(directory,i)
                # get the nobs number and compare to threshold.
                if (t[,2] > threshold){
                        correlations <- cor(data$sulfate,data$nitrate,use="complete.obs")
                        result <- c(result,correlations)
                }
        }
        result
}
# this function requires the previous function: complete().