CHAPTER 4: Functions
Review
By this point, you should be aware that LPC
objects consist of functions which manipulate variables. The functions
manipulate variables when they are executed, and they get executed through
calls to those functions.
The order in which the functions are placed in a file does not matter. Inside a function, the variables get manipulated. They are stored in
computer memory and used by the computer as 0's and 1's
which get translated to and from useable
output and
input through a device called data typing.
String data types tell the driver that the data should appear to you and come from you in the form of
alphanumeric characters. Variables of type
int are represented to you as whole
number values. Type status is represented to you as either 1 or 0. And finally type void has no value to you or the
machine, and is not really used with
variable data types.
What is a function?
Like math functions, LPC functions take input and return output. Languages like
Pascal distinguish between the concept of procedure and the concept of function. LPC does not, however, it is
useful to understand this distinction. What Pascal calls a procedure, LPC calls a function of type
void. In other words, a
procedure, or function of type void returns no output. What Pascal calls a
function differs in that it does return
output. In LPC, the most
trivial, correct function is:
void do_nothing() { }
This function accepts no input, performs no
instructions, and returns no value.
There are three parts to every properly written
LPC function:
- The declaration .
- The definition .
- The call .
Like with variables, functions must be
declared. This will allow the driver to know 1) what type of data the function is returning as output, and 2) how many input(s) and of what type those
input(s) are. The more common word for input is parameters.
A function declaration
therefore consists of: type name(parameter1, parameter2, ..., parameterN); The
declaration of a
function called drink_water() which accepts a string as input and an int as
output would thus look like this:
int drink_water(string str);
where str is the name of the
input as it will be used inside the function.
The function definition is the
code which describes what the function actually does with the input sent to it.
The call is any place in other functions which invokes the
execution of the function in question. For two functions write_vals() and add(), you thus might have the following bit of code:
/* First, function declarations. They usually appear at the beginning of
object code. */
void write_vals();
int add(int x, int y);
/* Next, the definition of the function write_vals(). We assume that
this function is going to be called from outside the object */
void write_vals() {
int x;
/* Now we assign x the value of the output of add() through a call */
x = add(2, 2);
write(x+"\n");
}
/* Finally, the definition of add() */
int add(int x, int y) {
return (x + y);
}
Remember, it does not matter which function
definition appears first in the code. This is because functions are not executed
consecutively. Instead, functions are executed as called. The only
requirement is that the declaration of a function appear before its definition and before the definition of any function which makes a
call to it.
Efuns
Perhaps you have heard people refer to efuns. They are externally defined functions. Namely, they are defined by the mud
driver. If you have played around at all with coding in LPC, you have
probably found some expressions you were told to use like this_player(), write(), say(), this_object(), etc. look a lot like functions. That is because they are efuns. The value of efuns is that they
are much faster than LPC functions, since they already exist in the
binary form the computer understands.
In the function write_vals() above, two functions calls were made. The first was to the functions add(), which you declared and defined. The second call, however, was to a function called
write(), and efun. The driver has already declared and defined this function for you. You needs only to make calls to it.
Efuns are created to handle
common, every day function calls, to handle input/output to the internet sockets, and other matters difficult to be dealt with in LPC. They are written in
C in the game
driver and compiled along with the driver before the mud comes up, making them much
faster in execution. But for your purposes, efun calls are just like calls made to your functions. Still, it is
important to know two things of any efun: 1) what return type does it have, and 2) what parameters of what types does it take.
Information on efuns such as input parameters and return types is often found in a directory called /doc/efun on your mud. I cannot
detail efuns here, because efuns vary from driver to driver.
However, you can often access this information using the commands "man" or "help" depending on your mudlib. For instance, the command "man write" would give you information on the write
efun. But if all else fails, "more /doc/efun/write" should work.
By looking it up, you will find write is declared as follows:
void write(string);
This tells you an appropriate call to write expects no return value and
passes a single parameter of type string.
Defining your own functions
Although ordering your functions within the file does not
matter, ordering the code which defines a function is most important. Once a function has been called, function code is executed in the
order it appears in the function
definition. In write_vals() above, the instruction:
x = add(2, 2);
Must come before the write() efun call if you want to see the appropriate value of x used in write().
With respect to values returned by function, this is done through the "return" instruction followed by a value of the same data type as the function. In add() above, the instruction is "return
(x+y);", where the value of (x+y) is the value returned to write_vals() and assigned to x. On a more general level, "return" halts the execution of a function and returns code execution to the
function which called that function. In
addition, it returns to the calling function the value of any expression that follows.
To stop the execution of a
function of type void out of order, use "return"; without any value following. Once again, remember, the data type of the value of any expression returned using "return"
MUST be the same as the data type of the function itself.
Chapter Summary
The files which define LPC objects are made of of functions. Functions, in turn, are made up of three parts:
- The declaration .
- The definition .
- The call .
Function declarations generally appear at the top of the file before any definitions, although the requirement is that the declaration must appear before the function definition and before the
definition of any function which calls it.
Function definitions may
appear in the file in any order so long as they come after their declaration. In addition, you may not define one function inside another function.
Function calls
appear inside the definition of other functions where you want the code to begin execution of your
function. They may also appear within the definition of the function itself, but this
is not recommended for new coders, as it can easily lead to infinite loops.
The function definition consists of the following in this order:
- function return type .
- function name
- opening ( followed by a parameter list and a closing )
- an opening { instructing the driver that execution begins here
- declarations of any variables to be used only in that function
- instructions, expressions, and calls to other functions as needed
- a closing } stating that the function code ends here and, if no "return" instruction has been given at this point (type void functions only), execution returns to the calling function as if a "return" instruction was given
The trivial function would thus be:
void do_nothing() {}
since this function does not accept any input, perform any instructions, or return any output.
Any function which is not of type void MUST return a value of a data type matching the
function's data type.
Each driver has a set of functions already defined for you called efuns These you need neither need to
declare nor
define since it has already been done for you. Furthermore, execution of these
functions is faster than the execution of your functions since efuns are in the driver.
In addition, each
mudlib has special functions like efuns in that they are already defined and declared for you, but different in that they are defined in the mudlib and in LPC. They are called
simul_efuns, or simulated efuns. You can find out all about each of these as they are listed in the /doc/efun directory on most muds. In addition many muds have a command called "man" or a
"help" command which allows you simply to
call up the info files on them.
Note on style:
Some drivers may not require you to declare your functions, and some may not require you to specify the return type of the function in its definition. Regardless of this fact, you should never omit
this information for the
following reasons:
- It is easier for other people (and you at later dates) to read your code and understand what is meant. This is particularly useful for debugging, where a large portion of errors (outside of
misplaced parentheses and brackets) involve problems with data types (Ever gotten "Bad arg 1 to foo() line 32"?).
- It is simply considered good coding form.
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