What do two commas mean?

Question

First I have a slightly embarrassing question. What do the commas in R represent? For example, whenever there's code that's like unique[x3,] or something similar, What is the comma doing before the bracket?

Second,

mosaicplot(UCBAdmissions[,,i],)

what do the two commas inside the square bracket mean?

Answer 1

The best way to understand these things is to try them out on your own and see what they do!

In general:

mydf[1, ] ## Get the first row
mydf[, 3] ## Get the third column

The UCBAdmissions has more than two dimensions, so

UCBAdmissions[, , 1] ## Get the first table in the 3D array

Of course, these can be combined. The UCBAdmissions sample data is a set of 6 two-by-two table:

dim(UCBAdmissions)
# [1] 2 2 6

Let's imagine you wanted the first row from just the first two tables:

UCBAdmissions[1, , c(1, 2)]
#         Dept
# Gender     A   B
#   Male   512 353
#   Female  89  17