r - 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,NA,NA,NA,NA,NA,NA,NA,NA))

head(df)
  years months      value
1  2005      1 -0.6264538
2  2005      2  0.1836433
3  2005      3 -0.8356286
4  2005      4  1.5952808
5  2005      5  0.3295078
6  2005      6 -0.8204684

Tell me please, how i can replace NA in df$value to median of others months? "value" must contain the median of value of all previous values for the same month. That is, if current month is May, "value" must contain the median value for all previous values of the month of May.

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!