The Art of R Programming

Other Matrix-Like Operations

Various matrix operations also apply to data frames. Most notably and usefully, we can do filtering to extract various subdata frames of interest.

Extracting Subdata Frames

As mentioned, a data frame can be viewed in row-and-column terms. In particular, we can extract subdata frames by rows or columns. Here’s an example:

> examsquiz[2:5,]
  Exam.1 Exam.2 Quiz
2    3.3      2  3.7
3    4.0      4  4.0
4    2.3      0  3.3
5    2.3      1  3.3
> examsquiz[2:5,2]
[1] 2 4 0 1
> class(examsquiz[2:5,2])
[1] "numeric"
> examsquiz[2:5,2,drop=FALSE]
  Exam.2
2      2
3      4
4      0
5      1
> class(examsquiz[2:5,2,drop=FALSE])
[1] "data.frame"

Note that in that second call, since examsquiz[2:5,2] is a vector, R created a vector instead of another data frame. By specifying drop=FALSE ...