[R] Replace values based on neither condition

[Luigi Marongiu]

Hello,
I have a data frame with a string column. All data that are neither
"POS" nor "NEG" should've replaced by an NA. How can I implement that
(even with extra libraries)? My attempts actually wipe out POS and
NEG...
Thank you

```
df = data.frame(a = 1:5, b = c("",      "31.35", "POS",   "20.61", "NEG"),
                stringsAsFactors = F)
df$b[!(df$b == "POS") & (df$b == "NEG")] = NA
df$b[(df$b == "POS") | (df$b == "NEG")] = NA
```

-- 
Best regards,
Luigi