Calculating Conditional Entropy in R

conditionalEntropy <- function( graph ) {
   # graph is a 2 or 3 column dataframe
   if (ncol(graph) == 2 ) {
      names(graph) <- c("from","to")
      graph$weight <- 1
   } else if (ncol(graph) == 3)
      names(graph) <- c("from","to","weight")
   max <- length(rle(paste(graph$from, graph$to))$values)
   total <- sum(graph$weight)
   entropy <- data.frame(H = 0, Hmax = 0);
   entropy$H <- sum(graph$weight/total * log(graph$weight/total) / log(2)) * -1
   entropy$Hmax <- log(max * (max-1))/log(2)
   return(entropy)
}