```
x <- c(6:-4)
sqrt(x) #- gives warning
sqrt(ifelse(x >= 0, x, NA)) # no warning
## Note: the following also gives the warning !
ifelse(x >= 0, sqrt(x), NA)
## ifelse() strips attributes
## This is important when working with Dates and factors
x <- seq(as.Date("2000-02-29"), as.Date("2004-10-04"), by = "1 month")
## has many "yyyy-mm-29", but a few "yyyy-03-01" in the non-leap years
y <- ifelse(as.POSIXlt(x)$mday == 29, x, NA)
head(y) # not what you expected ... ==> need restore the class attribute:
class(y) <- class(x)
y
## ==> Again a case where it is better *not* to use ifelse(), but
## both more efficient and clear:
y2 <- x
y2[as.POSIXlt(x)$mday != 29] <- NA
stopifnot(identical(y2, y))
## example of different return modes:
yes <- 1:3
no <- pi^(0:3)
typeof(ifelse(NA, yes, no)) # logical
typeof(ifelse(TRUE, yes, no)) # integer
typeof(ifelse(FALSE, yes, no)) # double
```

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