0是假
NULL、NA无法辨认真假
除了以上3个其他的都是真
 
> 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"