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How to change single value in a data.frame (value being a new level)?

I want to change a single value in a data.frame, which is NA using this code df[307, 1] <- 231. However, I get the warning message

warning message:
In `[<-.factor`(`*tmp*`, iseq, value = 231) :
invalid factor level, NA generated

As I understood right, the level 231 is not within the levels of the variable ([,1] first column). What can I do to solve this problem? Add new level 231? Or another way to change this single value. Thanks for your ideas.

Solution

  • As @akrun noted in the comments:

    x <- factor(c("a", "b"))
x[3] <- "c"
Warning message:
In `[<-.factor`(`*tmp*`, 3, value = "c") :
  invalid factor level, NA generated
# one solution:
x <- factor(c("a", "b"))
x <- factor(c(as.character(x), "c"))

# a second solution:
x <- factor(c("a", "b"))
levels(x) <- c("a", "b", "c")
x[3] <- "c"