06. Logical operators, missing values

1. Logical operators

1<2

2<=1

1>2

1>=2

2>=1

1==2

1 !=2

(1<2) & (1>4)

(1<2) | (1>4)

2. The missing value marker, NA

When an element or value is "not available", a place within a vector may be reserved for it by assigning it a special value NA.

In general, any operation on an NA becomes an NA. The function is.na(x), where x is a vector, gives a logical vector of the same length as x with values set to be true if the corresponding element in x is NA.

z<-NA;

is.na(z);

z==NA

Note the differences shown in the above.