Improving missing values operation explanation in boolean algebra section

[davidrsch]

Previous explanation

To understand what's going on, think about NA | TRUE (NA or TRUE). A missing value in a logical vector means that the value could either be TRUE or FALSE. TRUE | TRUE and FALSE | TRUE are both TRUE because at least one of them is TRUE. NA | TRUE must also be TRUE because NA can either be TRUE or FALSE. However, NA | FALSE is NA because we don't know if NA is TRUE or FALSE. Similar reasoning applies with NA & FALSE

The last sentence of previous explanation implies that NA & FALSE returns NA as NA | FALSE, explained in previous sentence. Therefore a more detailed explanation explaining how the similar reasoning is applied when using & but taking into consideration that both conditions must be fulfilled.

To understand what's going on, think about NA | TRUE (NA or TRUE). A missing value in a logical vector means that the value could either be TRUE or FALSE. TRUE | TRUE and FALSE | TRUE are both TRUE because at least one of them is TRUE. NA | TRUE must also be TRUE because NA can either be TRUE or FALSE. However, NA | FALSE is NA because we don't know if NA is TRUE or FALSE. Similar reasoning applies for & considering that both conditions must be fulfilled. Therefore NA & TRUE is NA because NA can either be TRUE or FALSE and NA & FALSE is FALSE because at least one of the conditions is FALSE.