1 Correlation<- function(x,y) {
 2   len<-length(x)
 3   if( len != length(y))
 4     stop("length not equal!")
 5    
 6   x2 <- unlist(lapply(x,function(a) return(a^2)))
 7   y2 <- unlist(lapply(y,function(a) return(a^2)))
 8   xy <- x*y
 9    
10   a <- sum(xy)*len - sum(x)*sum(y)
11   b <- sqrt(sum(x2)*len - sum(x)^2)*sqrt(sum(y2)*len - sum(y)^2)
12   if( b == 0)
13     stop("data is incorrect!")
14   return(a/b)
15 }