Writing your own scripting language

Introduction

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Well, this article is about creating a scripting language for your game. But what exactly is a scripting language? A scripting language is that you type in commands in, for example, a text-file and the program reads the text-file and knows what to do from the commands it reads from the text-file. I'll give an example:

OPENBOX
DISPLAYTEXT Hi! This text is written in a box
WAITKEY
CLOSEBOX
END
        

That are some simple instructions you can make a program respond on. Like, with OPENBOX it opens a box, then displays the text, waits for a key to be pressed, closes the box and ends. How do you make this stuff with QBasic? Well, that's why I'm writing this, to explain that to you!

A simple scripting language

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Ok, first we'll start with the basics like calling a PRINT command from an external file. Ok, lets say we want to call the command to PRINT a text: DISPLAYTEXT. Create a new file in Notepad and write this in it:

CLEARSCREEN
DISPLAYTEXT Wow! This is my first scripting language!!
DISPLAYTEXT Press any key to end...
WAITFORKEY
DISPLAYTEXT You pressed a key and now the program ends!
ENDSCRIPT
DISPLAYTEXT This action will NOT be executed because there
DISPLAYTEXT is an ENDSCRIPT command before it!

        

Save it as script.dat. So now we have some commands in an external file. But QBasic can't do anything with that so we need to make a program that reads the script-file and ports it so QBasic can understand it. A good way is to read the script-file line by line, then check if the line contains commands and what they are, if there are any expressions after the command store them in a temporarily variable and then execute the command. For example to let the program execute the commands we just used you could use this code:

OPEN "script.dat" FOR INPUT AS #1
 DO
  INPUT #1, ReadLine$
  ReadLine$ = LTRIM$(RTRIM$(ReadLine$)
  IF LEFT$(ReadLine$, 11) = "DISPLAYTEXT" THEN
   Temp1$ = RIGHT$(ReadLine$, LEN(ReadLine$) - 12)
   PRINT Temp1$
  END IF
  IF LEFT$(ReadLine$, 7) = "WAITKEY" THEN
   WHILE INKEY$ = "": WEND
  END IF
  IF LEFT$(ReadLine$, 9) = "ENDSCRIPT" THEN
   EXIT DO
  END IF
  LOOP UNTIL EOF(1)
CLOSE #1
        

I'll discuss the code line by line now:

OPEN "script.dat" FOR INPUT AS #1
        

Opens our scripting file 'script.dat' for reading as file #1

DO
        

Start the Do...Loop event that runs until the file ends

INPUT #1, ReadLine$
        

Reads a single line from the file.

ReadLine$ = LTRIM$(RTRIM$(ReadLine$)
        

Removes all the spaces from the left and right of the line.

IF LEFT$(ReadLine$, 11) = "DISPLAYTEXT" THEN
        

If the first 11 characters from the line we just read form the word DISPLAYTEXT the DISPLAYTEXT command is run.

Temp1$ = RIGHT$(ReadLine$, LEN(ReadLine$) - 12)
        

The DISPLAYTEXT command contains one expression so we just read the right part of the line we took minus twelve character (the DISPLAYTEXT command + the space).

PRINT Temp1$
        

We PRINT the expression we just took.

END IF
        

End the IF...THEN command

IF LEFT$(ReadLine$, 7) = "WAITKEY" THEN
        

If the first 7 characters from the line we read are WAITKEY we run it.

WHILE INKEY$ = "": WEND
        

We wait until a key is pressed

END IF
        

End the IF...THEN command IF LEFT$(ReadLine$, 9) = "ENDSCRIPT" THEN If the first 9 characters from the line we read are ENDSCRIPT we run the command.

EXIT DO
        

We exit the DO...LOOP event.

END IF
        

End the IF...THEN command

LOOP UNTIL EOF(1)
        

Loop back to the DO

CLOSE #1
        

We close the file.

Now you have your first (small) scripting language! Now onto the harder things!

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Adding comments

Well, this isn't exactly hard but I still wanted to include it. I'll now explain on how to add QBasic like comments (the little ' thing). Add a line with a command somewhere in the 'script.dat' file before the ENDSCRIPT command.

Ok, now add this 2 lines somewhere in the do loop:

IF LEFT$(ReadLine$,1) = "'" THEN
END IF
        

This checks if the first command is a ' and if it is, it just does nothing because a comment does nothing. Good, now you can have comments in your scripts!

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Next time

Next time I'll talk about variables and IF...THEN commands. Maybe also about DO...LOOPs and FOR...NEXTs. Until then, enjoy the rest of the issue!

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