0是假
NULL、NA無法辨認真假
除了以上3個其他的都是真
> if (NULL) print("OK") else print("Error")
Error in if (NULL) print("OK") else print("Error") :
argument is of length zero
> if (NA) print("OK") else print("Error")
Error in if (NA) print("OK") else print("Error") :
missing value where TRUE/FALSE needed
> if (0) print("OK") else print("Error")
[1] "Error"
> if (1) print("OK") else print("Error")
[1] "OK"
> if (-1) print("OK") else print("Error")
[1] "OK"