CHAPTER 6: Variable Handling

                           LPC Basics
                  Written by Descartes of Borg

                  first edition: 23 april 1993

                  second edition: july 5 1993

CHAPTER 6:

Variable Handling

6.1 Review By now you should be able to code some simple objects using your muds standard object library. Inheritance allows you to use functions defined in those objects without having to go and define yourself. In addition, you should know how to declare your own functions. This chapter will teach you about the basic elements of LPC which will allow you to define your own functions using the manipulation of variables.

6.2 Values and objects

Basically, what makes objects on the mud different are two things:

Now, all player objects have the same functions. They are therefore differentiated by the values they hold. For instance, the player named "Forlock" is different from "Descartes" *at least* in that they have different values for the variable true_name, those being "descartes" and "forlock".

Therefore, changes in the game involve changes in the values of the objects in the game. Functions are used to name specific process for manipulating values. For instance, the create() function is the function whose process is specifically to initialize the values of an object. Within a function, it is specifically things called instructions which are responsible for the direct manipulation of variables.

6.3 Local and global variables

Like variables in most programming language, LPC variables may be declared as variables "local" to a specific function, or "globally" available to all functions. Local variables are declared inside the function which will use them. No other function knows about their existence, since the values are only stored in memory while that function is being executed. A global variable is available to any function which comes after its declaration in the object code. Since global variables take up RAM for the entire existence of the object, you should use them only when you need a value stored for the entire existence of the object. Have a look at the following 2 bits of code: 
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int x;

int query_x() { return x; }

void set_x(int y) { x = y; }
-----

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void set_x(int y) {
    int x;

    x = y;
    write("x is set to x"+x+" and will now be forgotten.\n");
}
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In the first example, x is declared outside of any functions, and therefore will be available to any function declared after it. In that example, x is a global variable. In the second example, x is declared inside the function set_x(). It only exists while the function set_x() is being executed. Afterwards, it ceases to exist. In that example, x is a local variable.

6.4 Manipulating the values of variables

Instructions to the driver are used to manipulate the values of variables. An example of an instruction would be: 
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x = 5;
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The above instruction is self-explanatory. It assigns to the variable x the value 5. However, there are some important concepts in involved in that instruction which are involved in instructions in general. The first involves the concept of an expression. An expression is any series of symbols which have a value. In the above instruction, the variable x is assigned the value of the expression 5. Constant values are the simplest forms in which expressions can be put. A constant is a value that never changes like the int 5 or the string "hello". The last concept is the concept of an operator. In the above example, the assignment operator = is used.

There are however many more operators in LPC, and expressions can get quite complex. If we go up one level of complexity, we get: 

-----
y = 5;
x = y +2;
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The first instruction uses the assignment operator to assign the value of the constant expression 5 to the variable y. The second one uses the assignment operator to assign to x the value of the expression (y+2) which uses the addition operator to come up with a value which is the sum of the value of y and the value of the constant expression 2. Sound like a lot of hot air?

In another manner of speaking, operators can be used to form complex expressions. In the above example, there are two expressions in the one instruction x = y + 2;:

As stated before, all expressions have a value. The expression y+2 has the value of the sum of y and 2 (here, 7); The expression x = y + 2 *also* has the value of 7. So operators have to important tasks:

6.5 Complex expressions

As you may have noticed above, the expression x = 5 *itself* has a value of 5. In fact, since LPC operators themselves have value as expressions, they cal allow you to write some really convoluted looking nonsense like: 
    i = ( (x=sizeof(tmp=users())) ? --x : sizeof(tmp=children("/std/monster"))-1)
which says basically: Would you ever use the above statement? I doubt it. However you might see or use expressions similar to it, since the ability to consolidate so much information into one single line helps to speed up the execution of your code. A more often used version of this property of LPC operators would be something like: 
    x = sizeof(tmp = users());
    while(i--) write((string)tmp[i]->query_name()+"\n");
instead of writing something like: 
    tmp = users();
    x = sizeof(tmp);
    for(i=0; iquery_name()+"\n");
Things like for(), while(), arrays and such will be explained later. But the first bit of code is more concise and it executed faster.

NOTE: A detailed description of all basic LPC operators follows the chapter summary.

6.6 Chapter Summary

You now know how to declare variables and understand the difference between declaring and using them globally or locally. Once you become familiar with your driver's efuns, you can display those values in many different ways. In addition, through the LPC operators, you know how to change and evaluate the values contained in variables. This is useful of course in that it allows you to do something like count how many apples have been picked from a tree, so that once all apples have been picked, no players can pick more. Unfortunately, you do not know how to have code executed in anything other than a linera fashion. In other words, hold off on that apple until the next chapter, cause you do not know how to check if the apples picked is equal to the number of apples in the tree. You also do not know about the special function init() where you give new commands to players. But you are almost ready to code a nice, fairly complex area.

6.7 LPC operators

This section contains a detailed listing of the simpler LPC operators, including what they do to the values they use (if anything) and the value that they have.

The operators described here are: 

=    +    -    *    /    %    +=    -=    *=    /=    %=
--    ++    ==    !=    >    <    >=    <=    !    &&    ||
->    ? :

Those operators are all described in a rather dry manner below, but it is best to at least look at each one, since some may not behave *exactly* as you think. But it should make a rather good reference guide.

A pair of more complicated ones that are here just for the sake of being here. Do not worry if they utterly confuse you.

A note on equality: A very nasty error people make that is VERY difficult to debug is the error of placing = where you mean ==. Since operators return values, they both make sense when being evaluated. In other words, no error occurs. But they have very different values. For example: 
  if(x == 5)    if(x = 5)
The value of x == 5 is true if the value of x is 5, false othewise. The value of x = 5 is 5 (and therefore always true). The if statement is looking for the expression in () to be either true or false, so if you had = and meant ==, you would end up with an expression that is always true. And you would pull your hair out trying to figure out why things were not happening like they should :)

Chapter 7