R Language Missing values TRUE/FALSE and/or NA
Example
NA is a logical type and a logical operator with an NA will return NA if the outcome is ambiguous. Below, NA OR TRUE evaluates to TRUE because at least one side evaluates to TRUE, however NA OR FALSE returns NA because we do not know whether NA would have been TRUE or FALSE
NA | TRUE
# [1] TRUE
# TRUE | TRUE is TRUE and FALSE | TRUE is also TRUE.
NA | FALSE
# [1] NA
# TRUE | FALSE is TRUE but FALSE | FALSE is FALSE.
NA & TRUE
# [1] NA
# TRUE & TRUE is TRUE but FALSE & TRUE is FALSE.
NA & FALSE
# [1] FALSE
# TRUE & FALSE is FALSE and FALSE & FALSE is also FALSE.
These properties are helpful if you want to subset a data set based on some columns that contain NA.
df <- data.frame(v1=0:9,
v2=c(rep(1:2, each=4), NA, NA),
v3=c(NA, letters[2:10]))
df[df$v2 == 1 & !is.na(df$v2), ]
# v1 v2 v3
#1 0 1 <NA>
#2 1 1 b
#3 2 1 c
#4 3 1 d
df[df$v2 == 1, ]
v1 v2 v3
#1 0 1 <NA>
#2 1 1 b
#3 2 1 c
#4 3 1 d
#NA NA NA <NA>
#NA.1 NA NA <NA>
