Hands-On Programming with R

Chapter 4. R Notation

Now that you have a deck of cards, you need a way to do card-like things with it. First, you’ll want to reshuffle the deck from time to time. And next, you’ll want to deal cards from the deck (one card at a time, whatever card is on top—we’re not cheaters).

To do these things, you’ll need to work with the individual values inside your data frame, a task essential to data science. For example, to deal a card from the top of your deck, you’ll need to write a function that selects the first row of values in your data frame, like this:

deal(deck)
##  face   suit value
##  king spades    13

You can select values within an R object with R’s notation system.

Selecting Values

R has a notation system that lets you extract values from R objects. To extract a value or set of values from a data frame, write the data frame’s name followed by a pair of hard brackets:

deck[ , ]

Between the brackets will go two indexes separated by a comma. The indexes tell R which values to return. R will use the first index to subset the rows of the data frame and the second index to subset the columns.

You have a choice when it comes to writing indexes. There are six different ways to write an index for R, and each does something slightly different. They are all very simple and quite handy, so let’s take a look at each of them. You can create indexes with:

• Positive integers

• Negative integers

• Zero

• Blank spaces

• Logical values

• Names

The simplest of these to use is positive integers. ...