While editing the maps, you may notice that some of the roads are not labeled properly. This means, the road type might be wrong (a highway is marked as a street) or that one road has a few road types (a highway is marked as Freeway, highway and primary at different segments). The importance of proper labeling is in two aspects:
Other sites. R 101: The Subset Function. The subset function is available in base R and can be used to return subsets of a vector, martix, or data frame which meet a particular condition. In my three years of using R, I have repeatedly used the subset() function and believe that it is the most useful tool for selecting elements of a data structure.
A subset is a portion of a set. is a subset of (written ) iff every member of is a member of . If is a proper subset of (i.e., a subset other than the set itself), this is written . If is not a subset of , this is written . (The notation is generally not used, since automatically means that and cannot be the same.) The subsets (i.e., power set) of a given set can be found using Subsets[list].