This is one of the functions I wrote for R programming Coursera assignment

corr.R

# R assignment 1 - Air Poluttion

corr <- function(directory, threshold = 0){
        # 'threshold' is a numeric vector of length 1 indicating the 
        # number of complete cases required to compute the correlation
        
        setwd(paste("/Users/lucky1eva/Downloads/", directory, sep = '' ))
        cr <- NULL
        dlist <- complete(directory, 1:332)
        oklist <- dlist[dlist[,2] >= threshold , 1]
        
        if (length(oklist) != 0) {
                for (i in 1:length(oklist)) {
                        
                        if (oklist[i] < 10) {
                                idd[i] <- paste("00", oklist[i], sep = '')
                                
                        } else if (oklist[i] >= 10 && oklist[i] < 100) { 
                                idd[i] <- paste("0", oklist[i], sep = '')
                        } else {
                                idd[i] <- oklist[i]
                        }
                        
                        
                } 
        } else {
                return(cr <- vector(mode = "numeric", length = 0))
        }
        
        # read tables and compute the vector of cr in each selected table
        for (k in 1:length(idd)) {
                myTable <- read.csv(paste(idd[k], ".csv", sep = ''), header = TRUE)
               if (!any(complete.cases(myTable))) {
                       next
               } else 
                {cr[k] <- cor(myTable[, 2], myTable[, 3], use = "complete.obs")}
        }
        
   return (cr)
        
}