4.3) Handling Dataframes: As Arrays
Indexing Arrays
Arrays are indexed in the same way as vectors, but with an index value for each dimension of the array. So a matrix (2-D array) might be indexed as Matrix[1,1:3] to get the first row of the first three columns (rows first, then columns). Leaving one of the indices blank means that all the values are included (e.g. Matrix[,1] to get the entire first column). If your rows and/or columns are named, you can also put the names (in inverted commas) in the square brackets. Now, a dataframe can be coerced to behave as a matrix for this purpose, so you can get rows and columns in all the same ways.
Select the first 10 rows of GRASS. Then (revision from this page and the previous one) show five ways to extract the first column of GRASS. What are the differences in output?
Create a matrix as follows: Matrix <- matrix(c(1:16),nrow=4,ncol=4). How could you extract a diagonal? It should be obvious that the answer is not Matrix[1:4,1:4] (try it!). The diagonal is a vector of numbers, but it's not a 2-D subset of the matrix, so in this case we can't simply use a set of square brackets. Fortunately there is a function diag() for matrices, but if you needed to compile a vector from a matrix using some other set of coordinates, it's more complicated - one way would be to use a for() loop.