r - Replace NAs with mean of the same column of a data.table

Question

I want to replace NAs present in a column of a DATA TABLE with the mean of the same column. I am doing the following. But it is not working.

ww <- data.table(iris)

ww <- ww[1:5 , ]

ww[1,1] <- NA

   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1:           NA         3.5          1.4         0.2  setosa
2:          4.9         3.0          1.4         0.2  setosa
3:          4.7         3.2          1.3         0.2  setosa
4:          4.6         3.1          1.5         0.2  setosa
5:          5.0         3.6          1.4         0.2  setosa


ww[is.na(Sepal.Length) , Sepal.Length:= mean(Sepal.Length, na.rm = T)]

   Sepal.Length Sepal.Width Petal.Length Petal.Width Species
1:          NaN         3.5          1.4         0.2  setosa
2:          4.9         3.0          1.4         0.2  setosa
3:          4.7         3.2          1.3         0.2  setosa
4:          4.6         3.1          1.5         0.2  setosa
5:          5.0         3.6          1.4         0.2  setosa

Why am I getting NaN in place of NA when it should have been the mean of the rest of the values (4.9, 4.7, 4.6, 5.0)?

What is the alternate of acheiving this in case something is wrong with this syntax?

I want to the syntax for the data table.

Answer 1

na.aggregate in the zoo package replaces NAs with the mean of the non-NAs in the same column:

library(zoo)

ww[, Sepal.Length := na.aggregate(Sepal.Length)]