r data structures

Whereas for a matrix or data frame, one must supply *two* index values
(even if one of them may be omitted)
  mydf[ 1 , 3 ]
  mydf[   , 5 ]

  mymat[ 2:5 ,     ]
  mymat[ 3   , 4:6 ]
are valid statements.

Not quite:

x <- matrix(rev(1:9), ncol = 3)

x[5]

x[2:6]

x <- data.frame(x)

x[3]

I understand this as meaning, when you don't use special 2D indexing,
R falls back on 1D indexing behavior given by the relevant superclass:
vector for matrix and list for data.frame.

Cheers,
RMW