How to replace the missing values with median

f=function(x){
   x<- span="">as.numeric(as.character(x)) #first convert each column into numeric if it is from factor
   x[is.na(x)] =median(x, na.rm=TRUE) #convert the item with NA to median value from the column
   x #display the column
}
ss=data.frame(apply(df,2,f))