corr.R

###Coursera, R programming, week2, assignement 
##Part_3
##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.
setwd("/Users/YvonneGong/Documents/Spring_2017/Coursera/R_programming/Assignment/Week2")##after every run, tje wd should be reset
corr<-function(directory, threshold=0){
    setwd(file.path(getwd(),directory))
    correlationVector=NULL
    for (i in 1:332){
        if (i<10){
            data<-read.csv(paste("0","0",as.character(i),".csv", sep=""),
                           header = T,
                           na.strings=c("NA","NaN",""))
        }
        else if (i>=10&i<100){
            data<-read.csv(paste("0",as.character(i),".csv", sep=""),
                           header = T,
                           na.strings=c("NA","NaN",""))
        }
        else {
            data<-read.csv(paste(as.character(i),".csv",sep=""),
                           header=T,
                           na.strings = c("NA","NaN"," "))
        }
        data=na.omit(data)
        if (nrow(data)>threshold){
            correlationVector=c(correlationVector,cor(data[,2],data[,3]))##cal correlation 
        }
    }
    return(correlationVector)
}    

# examples
cr <- corr("specdata", 150)
head(cr)
cr <- corr("specdata", 400)
head(cr)