How to fill NA with median?

Question

Example data:

set.seed(1)
df <- data.frame(years=sort(rep(2005:2010, 12)), 
                 months=1:12, 
                 value=c(rnorm(60),NA,NA,NA,NA,         


        

Answer 1

Here's the most robust solution I can think of. It ensures the years are ordered correctly and will correctly compute the median for all previous months in cases where you have multiple years with missing values.

# first, reshape your data so it is years by months:
library(reshape2)
tmp <- dcast(years ~ months, data=df)  # convert data to years x months
tmp <- tmp[order(tmp$years),]          # order years
# now calculate the running median on each month
library(caTools)
# function to replace NA with rolling median
tmpfun <- function(x) {
  ifelse(is.na(x), runquantile(x, k=length(x), probs=0.5, align="right"), x)
}
# apply tmpfun to each column and convert back to data.frame
tmpmed <- as.data.frame(lapply(tmp, tmpfun))
# reshape back to long and convert 'months' back to integer
res <- melt(tmpmed, "years", variable.name="months")
res$months <- as.integer(gsub("^X","",res$months))