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.
Number of subsets with 4 members = (5 x 4 x 3 x 2)/(4 x 3 x 2) = 5. Number of subsets with 5 members = 1 Total subsets = 1 + 5 + 10 + 10 + 5 + 1= 32. A set with n elements has 2n subsets. In this case n = 5 and 25 = 32.