Note that a much more detailed overview of subsetting can be found in the Subsetting chapter of Advanced R.

There are 6 different ways to subset and 3 subsetting operators.

Vectors

  1. Positive integers

  2. Negative integers

  3. Logical vectors

  4. Nothing

  5. Zero returns a zero length vector

  6. Character vector

Lists

Lists work in the same way as vectors. We’ll look at how the different subsetting operators [, [[, and $ change what you pull out of the list.

Matrices

Matrices have two dimensions, each one is subset like a one-dimensional vector. Rows are first, then columns.

Matrices can be indexed with a single vector and in this case the matrix will behave like a vector.

Data frames

Data frames can be indexed with a single vector like list (remember data frame columns are like a list). They can (and are more commonly) indexed with two vectors like a matrix.

Subsetting operators

  • You can also [[ and $ to subset.
  • The difference is the type of object they return.
  • [[ is most useful for lists because [ always returns a list and not the actual value.
  • However, [[ only returns a single value so it can only be used with a single positive integer or a string.

From @hadleywickham

The different operators are useful in different situations and you often find yourself using multiple operators in a single subsetting operation

Read the chapter on Subsetting from Advanced R for more details. In particular the section on simplifying versus preserving will save you many headaches in the future.