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.
If Set A contains{1, 2} and Set B contains {1, 2, 3, 4}, then A is a subset of B because each element of A, the numbers 1 and 2, are also elements of B. The empty set has no elements, so we can say that all the elements of the empty set are elements of any other set. Therefore, the empty set is a subset of any set.