The R language is
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
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,
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.
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
to barf. If you want a factor to act like a number, try:
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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