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.
1. 1  Note that in the first definition S is a nonempty set, and the span of the empty set has to be defined as a special case. The second definition works when S is empty, since every subspace of V considered as a set includes the empty set, and {0} is the smallest subspace of any vector space.