The R language is object-oriented, meaning that it conforms to the conventions implied by that term. If you are not familiar with these conventions, some explanation may help to avoid confusion. Officially, objects are discrete programming entities that send and receive messages. This may not sound terribly edifying, but the concept was one of the major changes in the way that we think about programming. Traditionally, code (also referred to as functions) and data (structures) were kept separate, even to the point of reserving different areas of memory to store them. To use a function, you had to know how the function operated. Objects, in contrast, typically combine code and data, and the programmer only has to know about the message interface, that is, what sort of message you can send to the object, and what sort of message the object will produce. The message interface, like everything else about an object, is defined by its class. The definition of the class of an object might include something like "an object which when sent a message containing the human-readable text of a number will return the numeric value". So what makes this better than, say, the strtod() function in C?

For one thing, the C function requires that you pass it a pointer to the string. If you pass something else, you may get a software explosion. A properly constructed object should be able to gracefully tell whatever has sent it the wrong information that it has done so. This isn't all, though. In an object-oriented programming environment, objects can easily work out how to deal with a variety of incoming messages that better reflect how humans think.

The classic case is the addition operator '+'. Humans naively think that numbers should be added in the manner that they learned in school, and that strings like "John" and "Smith" should be added by concatenation to "John Smith" or into a list like "John, Smith". This implies that the '+' operator should be able to tell numbers from strings, use the appropriate operation for each, and probably refuse to add numbers to strings or vice versa. That is a much smarter '+' than the '+' in C, for example. If we were to specify its class in a verbose way, it might look like:

	An object which, when passed a message containing two numbers will
	return a message containing their numerical sum, when passed a message 
	containing two strings will return a message containing their 
	concatenation, and when passed a message containing two different data 
	types will display an error message and return nothing.

Unfortunately for the beginner, classes are specified in a much more cryptic manner. Typically, a language specifies very simple building blocks that are assigned fairly simple but very abstract classes, and other classes are spawned from these by inheritance with the addition of more specific abilities. This means that to understand what the class of most useful objects is, you must trace it back through the parent classes.

It isn't quite as bad as it looks, though. In order to use an object, all you have to do is know what sort of message you can send to it, and what sort of message it will return. So for the beginner, it's best to consider the object a "black box" - something that does something useful, but how it does it is unknown. Understanding of any particular object-oriented system generally comes with familiarity and study, and we're here to learn how to use R. The message from this particular object is, before using a function object, know what sort of objects may be sent to it, and what sort of objects are returned. Misunderstandings about the structure of data objects are among the most common problems for the beginner.

Data types

One of the more common pitfalls that the new user encounters is the behavior of factor objects. Although they often look like numbers, and in fact may advertise their mode as "numeric", they will fail the is.numeric() test and cause functions such as mean() to barf. If you want a factor to act like a number, try:

as.numeric(myfactor)

The as...() functions are very handy things to know about when trying to get some functions to work.

For more information, see An Introduction to R: Objects, their modes and attributes.

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