LPC Basics

Written by Descartes of Borg

23 april 1993

CHAPTER 3: LPC Data Types

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;

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

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;

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.

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;
}

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.