Replace in a factor column

Question

I want to replace values in a factors column with a valid value. But I can not find a way. This example is only for demonstration. The original data

Answer 1

The problem is that NA is not a level of that factor:

> levels(df$a)
[1] "2"  "4"  "5"  "9"  "10"

You can't change it straight away, but the following will do the trick:

df$a <- as.numeric(as.character(df$a))
df[is.na(df$a),1] <- 88
df$a <- as.factor(df$a)
> df$a
 [1] 9  88 3  9  5  9  88 8  3  9 
Levels: 3 5 8 9 88
> levels(df$a)
[1] "3"  "5"  "8"  "9"  "88"