corr.R

## Write a function that takes a directory of data files and a threshold for
## complete cases and calculates the correlation between sulfate and nitrate for
## monitor locations where the number of completely observed cases (on all
## variables) is greater than the threshold.
## The function should return a vector of correlations for the monitors that
## meet the threshold requirement. If no monitors meet the threshold
## requirement, then the function should return a numeric vector of length 0.
corr <- function(directory, threshold = 0) {

    completes <- complete(directory, 1:332)
    completes <- subset(completes, nobs > threshold )

    ## Initialize variables
    correlations <- vector()

    ## Loop over the passed id's
    for(i in completes$id ) {

        ## Pad the i to create a filename
        filename <- sprintf("%03d.csv", i)
        filepath <- paste(directory, filename, sep="/")

        ## Load the data
        data <- read.csv(filepath)

        ## Calculate and store the count of complete cases
        completeCases <- data[complete.cases(data),]
        count <- nrow(completeCases)

        ## Calculate and store the count of complete cases
        ## if threshhold is reached
        if( count >= threshold ) {
            correlations <- c(correlations, cor(completeCases$nitrate, completeCases$sulfate) )
        }
    }

    ## Return a numeric vector of correlations
    correlations
}