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

you want to use the test is.na function:

df$value[is.na(df$value)] <- median(df$value, na.rm=TRUE)

which says for all the values where df$value is NA, replace it with the right hand side. You need the na.rm=TRUE piece or else the median function will return NA

to do this month by month, there are many choices, but i think plyr has the simplest syntax:

library(plyr)
ddply(df, 
      .(months), 
      transform, 
      value=ifelse(is.na(value), median(value, na.rm=TRUE), value))

you can also use data.table. this is an especially good choice if your data is large:

library(data.table)
DT <- data.table(df)
setkey(DT, months)

DT[,value := ifelse(is.na(value), median(value, na.rm=TRUE), value), by=months]

There are many other ways, but there are two!