9.3 Splitting

Sometimes it is useful to split a vector or data frame into pieces according to the value of a variable. For example, from m111survey we might like to have separate data frames for each of the three seating preferences. We can accomplish this with the split() function:

bySeat <- split(m111survey, f = m111survey$seat)

If you run the command str(bySeat), you find that bySeat is a list consisting of three data frames:

• 1_front: the frame of all subjects who prefer the Front;
• 2_middle: the frame of all subjects who prefer the Middle;
• 3_back: the frame of all subjects who prefer the Back.

Now you can carry on three separate analyses, working with one frame at a time.

There is a pitfall which of you should be aware. If you try to access any one of the frames by its name, you will get an error:

bySeat$1_front

## Error: unexpected numeric constant in "bySeat$1"

The reason is that variable names cannot begin with a number! You have two options, here. You could access a single frame by using the name in quotes:

bySeat[["1_front"]]

Your second option is to use the index of the element you want:

bySeat[[1]]