| LPC Basics |
| Written by Descartes of Borg |
| first edition: 23 april 1993 |
| second edition: 17 june 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. |
| |
| 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. |