LPC Basics, by Descartes of Borg, Chapter 3

LPC Basics by Descartes of Borg, end ed. May 25 1993

Chapter 3: LPC Data Types

3.1 What you should know by now

LPC object are made up of zero or more variables manipulated by one or more functions. The order in which these functions appear in code is irrelevant. The driver uses the LPC code you write by loading copies of it into memory whenever it is first referenced and additional copies through cloning. When each object is loaded into memory, all the variables initially point to no value. The reset() function in compat muds, and create() in native muds are used to give initial values to variables in objects. The function for creation is called immediately after the object is loaded into memory. However, if you are reading this textbook with no prior programming experience, you may not know what a function is or how it gets called. And even if you have programming experience, you may be wondering how the process of functions calling each other gets started in newly created objects. Before any of these questions get answered, however, you need to know more about what it is the functions are manipulating. You therefore should thouroughly come to know the concept behind LPC data types. Certainly the most boring subject in this manual, yet it is the most crucial, as 90% of all errors (excepting misplaced {} and ()) involve the improper usage of LPC data types. So bear through this important chapter, because it is my feeling that understanding this chapter alone can help you find coding much, much easier.

3.2 Communicating with the computer

You possibly already know that computers cannot understand the letters and numbers used by humans. Instead, the "language" spoken by computers consists of an "alphabet" of 0's and 1's. Certainly you know computers do not understand natural human languages. But in fact, they do not understand the computer languages we write for them either. Computer languages like BASIC, C, C++, Pascal, etc. are all intermediate languages. They allow you to structure your thoughts more coherently for translation into the 0's and 1's of the computer's languages.

There are two methods in which translation is done: compilation and interpretation. These simply are differences betweem when the programming language is translated into computer language. With compiled languages, the programmer writes the code then uses a program called a compiler to translate the program into the computer's language. This translation occurs before the program is run. With interpreted languages however, the process of translation occurs as the program is being run. Since the translation of the program is occurring during the time of the program's running in interpreted languages, interpreted languages make much slower programs than compiled languages.

The bottom line is, no matter what language you are writing in, at some point this has to be changed into 0's and 1's which can be understood by the computer. But the variables which you store in memory are not simply 0's and 1's. So you have to have a way in your programming languages of telling the computer whether or not the 0's and 1's should be treated as decimal numbers or characters or strings or anything else. You do this through the use of data types.

For example, say you have a variable which you call 'x' and you give it the decimal whole number value 65. In LPC you would do this through the statement:

x = 65;

You can later do things like:

write(x+"\n");        /* \n is symbolically represents a carriage return */
y = x + 5;

The first line allows you to send 65 and a carriage return to someone's screen. The second line lets you set the value of y to 70. The problem for the computer is that it does not know what '65' means when you tell it x = 65;. What you think of 65, it might think of as:

00000000000000000000000001000001

But, also, to the computer, the letter 'A' is represented as:

00000000000000000000000001000001

So, whenever you instruct the computer write(x+"\n");, it must have some way of knowing that you want to see '65' and not 'A'.

The computer can tell the difference between '65' and 'A' through the use of data types. A data types simply says what type of data is being stored by the memory location pointed to by a given variable. Thus, each LPC variable has a variable type which guides conversions. In the example given above, you would have had the following line somewhere in the code *before* the lines shown above:

int x;

This one line tells the driver that whatever value x points to, it will be used as the data type "int", which is short for integer, or whole number. So you have a basic introduction into the reason why data types exist. They exist so the driver can make sense of the 0's and 1's that the computer is storing in memory.

3.3 The data types of LPC

All LPMud drivers have the following data types:

void, status, int, string, object, int *, string *, object *, mixed *

Many drivers, but not all have the following important data types which are important to discuss:

float, mapping, float *, mapping *

And there are a few drivers with the following rarely used data types which are not important to discuss:

function, enum, struct, char

3.4 Simple data types

This introductory textbook will deal with the data types void, status, int, float, string, object, mand mixed. You can find out about the more complex data types like mappings and arrays in the intermediate textbook. This chapter deals with the two simplest data types (from the point of view of the LPC coder), int and string.

n An int is any whole number. Thus 1, 42, -17, 0, -10000023 are all type int. A string is one or more alphanumeric characters. Thus "a", "we are borg", "42", "This is a string" are all strings. Note that strings are always enclosed in "" to allow the driver to distinguish between the int 42 and the string "42" as well as to distinguish between variable names (like x) and strings by the same names (like "x").

When you use a variable in code, you must first let the driver know what type of data to which that variable points. This process is called *declaration*. You do this at the beginning of the function or at the beginning of the object code (outside of functions before all functions which use it). This is done by placing the name of the data type before the name of the variable like in the following example:

void add_two_and_two() {
    int x;
    int y;

    x = 2;
    y = x + x;
}

Now, this is a complete function. The name of the function is add_two_and_two(). The function begins with the declaration of an int variable named x followed by the declaration of an in variable named y. So now, at this point, the driver now has two variables which point to NULL values, and it expects what ever values end up there to be of type int.

A note about the data types void and status: Void is a trivial data type which points to nothing. It is not used with respect to variables, but instead with respect to functions. You will come to understand this better later. For now, you need only understand that it points to no value.

The data type status is a boolean data type. That is, it can only have 1 or 0 as a value. This is often referred to as being true or false.

3.5 Chapter summary

For variables, the driver needs to know how the 0's and 1's the computer stores in memory get converted into the forms in which you intend them to be used. The simplest LPC data types are void, status, int, and string. You do not user variables of type void, but the data type does come into play with respect to functions. In addition to being used for translation from one form to the next, data types are used in determining what rules the driver uses for such operations as +, -, etc. For example, in the expression 5+5, the driver knows to add the values of 5 and 5 together to make 10. With strings however, the rules for int addition make no sense. So instead, with "a"+"b", it appends "b" to the string "a" so that the final string is "ab". Errors can thus result if you mistakenly try to add "5"+5. Since int addition makes no sense with strings, the driver will convert the second 5 to "5" and use string addition. The final result would be "55". If you were looking for 10, you would therefore have ended up with erroneous code. Keep in mind, however, that in most instances, the driver will not do something so useful as coming up with "55". It comes up with "55" cause it has a rule for adding a string to an int, namely to treat the int as a string. In most cases, if you use a data type for which an operation or function is not defined (like if you tried to divide "this is" by "nonsense", "this is"/"nonsense"), the driver will barf and report an error to you.

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