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:
int x;
int query_x() { return x; }
void set_x(int y) { x = y; }
void set_x(int y) {
int x;
x = y;
write("x is set to x"+x+" and will now be forgotten.\n");
}
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.x = 5;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;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;:
1) the expression y+2
2) the expression 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:
1) They *may* act upon input like a function
2) They evaluate as having a value themselves.
Now, not all operators do what 1 does. The = operators does act upon
the value of 7 on its right by assigning that value to x. The operator
+ however does nothing. They both, however, have their own values.
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.
= assignment operator:
x = 5
+ addition operator:
x + 7
- subtraction operator:
x - 7
* multiplication operator:
x * 7
/ division operator:
x / 7
+= additive assignment operator:
x += 5
-= subtraction assignment operator:
x -= 7
*= multiplicative assignment operator:
x *= 7
/= division assignment operator:
x /= 7
++ post/pre-increment operators
i++ or ++i
i++ has the value of i
++i has the value of i+1
-- post/pre-decrement operators:
i-- or --i
== equality operator
x == 5
!=inequality operator
x != 5
> greater than operator:
x > 5
< less than operator,
>= greater than or equal to operator,
<= less than or equal to operator:
x < y x >= y x <= y
&& logical and operator,
|| logical or operator:
x && y x || y
! negation operator:
-> the call other operator:
this_player()->query_name()
? : conditional operator:
x ? y : z
if(x == 5) if(x = 5)The value of
x == 5 is true if the value of x is 5, false otherwise.
The value of x = 5 is 5 (and therefore always true).() 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 :)
mud@wl.mud.de