I would like to refer to another value in the same tibble and mutate my initial value into that. For example if I have a tibble,

```
tibble(x = 200:203, y = c("a","b","c", "d"), z = c(NA, 202,201,NA))

```

I would like to get
`tibble(x = 200:203, y = c("a","b","c", "d"), z = c(NA, "c","b",NA))`

as a result.

I have tried `mutate(tbl,z, ifelse(!is.na(z), tbl[[which(fixed_messages$id == z),2]], NA))` but it doesn't work.

Also how do I insert a tibble into a stackoverflow question?