Writing your own scripting language

Introduction


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A lot of people responded positive on my last article so I'm going to continue this series! First I would like to point out that I'm not gonna tell you how to make a full working scripting language that you can put in your program. I'm gonna teach you techniques on how to make your own scripting language. Hence, this article is called Writing your own scripting language!

Secondly, in the last article I made an error. I said to put WAITFORKEY in the script.dat file but in the script.bas file we called it WAITKEY. Change WAITFORKEY in script.dat to WAITKEY.

Third, in this issue I'm gonna talk about variables and how to use them. You need the script.bas program and the script.dat file we created in issue #1 to use this article, if you don't have it I suggest you read the article in issue #1.

Variables


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Ok, first you'll need to know what variables are. If you don't know what they are I suggest to find a beginners tutorial which explains variables.

Unlike QBasic we're gonna have two types of variables, text and numeric. We're gonna modify our DISPLAYTEXT command so that it can handle variables and equations. First you need to add this two lines at the beginning of your script.bas file.

DIM SHARED Variables$(100,2)
DIM SHARED VariableCount%
        

What we've done is that we've made one arrays which hold the variable name, type and the value. VariableCount% controls how many variables we allready have in memory. Add this at the beginning of our script.dat file:

$text = Hello! This is Nightwolf's Scripting language article!!
%a = 5
%b = 10
DISPLAYTEXT $text
DISPLAYTEXT (%a * %b)

        

This creates one text variable ($text) with the value of 'Hello! This is Nightwolf's Scripting language article!!' and two numeric values (%a and %b) with the value of 5 and 10. Then it displays the value of $text and displays the equation %a times %b. First we're gonna add variable detection and storage. Add this at the beginning of the DO...LOOP command just behind the reading of the line (just after INPUT #1, ReadLine$):

IF LEFT$(ReadLine$, 1) = "$" THEN
 IF VariableCount% <= 100 THEN
  VariableCount% = VariableCount% + 1
  Variables$(VariableCount%, 0) = LEFT$(ReadLine$, INSTR(ReadLine$, " ") - 1)
  Variables$(VariableCount%, 1) = "1"
  Variables$(VariableCount%, 2) = RIGHT$(ReadLine$, LEN(ReadLine$) - INSTR(ReadLine$, "=") - 1)
 END IF
END IF
IF LEFT$(ReadLine$, 1) = "%" THEN
 IF VariableCount% <= 100 THEN
  VariableCount% = VariableCount% + 1
  Variables$(VariableCount%, 0) = LEFT$(ReadLine$, INSTR(ReadLine$, " ") - 1)
  Variables$(VariableCount%, 1) = "2"
  Variables$(VariableCount%, 2) = RIGHT$(ReadLine$, LEN(ReadLine$) - INSTR(ReadLine$, "=") - 1)
 END IF
END IF

        

I'm gonna explain the code line by line now.

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

If the first character of ReadLine$ is a $ then it's a text variable and we start the IF.

IF VariableCount% <= 100 THEN

If we've less than 100 variables in memory we continue.

VariableCount% = VariableCount% + 1

Increases the number of variables in memory by 1.

Variables$(VariableCount%, 0) = LEFT$(ReadLine$, INSTR(ReadLine$, " ") - 1)

Puts the name of the variable in Variables$. (name = all characters to the first space)

Variables$(VariableCount%, 1) = "1"

Set the type of variable to 1 (text).

Variables$(VariableCount%, 2) = RIGHT$(ReadLine$, LEN(ReadLine$) - INSTR(ReadLine$, "=") - 1)

Set the value of the variable to all the characters behind the = and the space following the =.

END IF

End the IF

END IF

End the IF

IF LEFT$(ReadLine$, 1) = "%" THEN

If the first character of ReadLine$ is a % then it's a numeric variable and we start the IF.

IF VariableCount% <= 100 THEN

VariableCount% = VariableCount% + 1

If we've less than 100 variables in memory we continue.

Variables$(VariableCount%, 0) = LEFT$(ReadLine$, INSTR(ReadLine$, " ") - 1)

Puts the name of the variable in Variables$. (name = all characters to the first space)

Variables$(VariableCount%, 1) = "2"

Set the variable type to 2 (numeric).

Variables$(VariableCount%, 2) = RIGHT$(ReadLine$, LEN(ReadLine$) - INSTR(ReadLine$, "=") - 1)

Set the value of the variable to all the characters behind the = and the space following the =.

END IF

End the IF.

END IF

End the IF.

Displaying variables and equations


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Now we have variable detection and storage but I'm sure you want to do something with the variables, we're gonna implent variable display and equations now. Search in the script.bas file for the following code:

IF LEFT$(ReadLine$, 11) = "DISPLAYTEXT" THEN
 Temp1$ = RIGHT$(ReadLine$, LEN(ReadLine$) - 12)
 PRINT Temp1$
END IF
        

And then replace it with this:

IF LEFT$(ReadLine$, 11) = "DISPLAYTEXT" THEN
 Temp1$ = RIGHT$(ReadLine$, LEN(ReadLine$) - 12)
 IF LEFT$(Temp1$, 1) = "$" OR LEFT$(Temp1$, 1) = "%" OR LEFT$(Temp1$, 1) = "(" THEN
  IF LEFT$(Temp1$, 1) = "(" THEN
   SELECT CASE MID$(Temp1$, INSTR(Temp1$, " ") + 1, 1)
    CASE "+"
     Action% = 1
    CASE "-"
     Action% = 2
    CASE "*"
     Action% = 3
    CASE "/"
     Action% = 4
   END SELECT
   Temp2$ = MID$(Temp1$, 2, INSTR(Temp1$, " ") - 1)
   Temp3$ = RIGHT$(Temp1$, LEN(Temp1$) - INSTR(INSTR(Temp1$, " ") + 1, Temp1$, " "))
   Temp3$ = LEFT$(Temp3$, LEN(Temp3$) - 1)
   Temp2$ = LTRIM$(RTRIM$(Temp2$))
   Temp3$ = LTRIM$(RTRIM$(Temp3$))
   FOR i = 1 TO VariableCount%
    IF Variables$(i, 0) = Temp2$ THEN Temp1% = VAL(Variables$(i, 2))
    IF Variables$(i, 0) = Temp3$ THEN Temp2% = VAL(Variables$(i, 2))
   NEXT i
   SELECT CASE Action%
    CASE 1
     PRINT Temp1% + Temp2%
    CASE 2
     PRINT Temp1% - Temp2%
    CASE 3
     PRINT Temp1% * Temp2%
    CASE 4
     PRINT Temp1% / Temp2%
   END SELECT
  ELSE
   FOR i = 1 TO VariableCount%
    IF Variables$(i, 0) = Temp1$ AND Variables$(i, 1) = "1" THEN PRINT Variables$(i, 2)
    IF Variables$(i, 0) = Temp1$ AND Variables$(i, 1) = "2" THEN PRINT VAL(Variables$(i, 2))
   NEXT i
  END IF
 ELSE
  PRINT Temp1$
 END IF
END IF

        

I'll explain the code line by line now:

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

If the first 11 characters of the ReadLine$ variable are DISPLAYTEXT execute the DISPLAYTEXT command.

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

This gets the text, variable or equation to display from the ReadLine$ variable and stores it in Temp1$.

IF LEFT$(Temp1$, 1) = "$" OR LEFT$(Temp1$, 1) = "%" OR LEFT$(Temp1$, 1) = "(" THEN

If the first character of Temp1$ is a $, % or ( then we start the variable/equation IF.

IF LEFT$(Temp1$, 1) = "(" THEN

If the first character of Temp1$ is a (, start the equation IF.

SELECT CASE MID$(Temp1$, INSTR(Temp1$, " ") + 1, 1)

Start a SELECT CASE command for the first character behind the first space in Temp1$. This is the equation method (+, -, *, /).

CASE "+"
Action% = 1

If it's a + (plus) then set Action% to 1.

CASE "-"
Action% = 2

If it's a - (minus) then set Action% to 2.

CASE "*"
Action% = 3

If it's a * (times) then set Action% to 3.

CASE "/"
Action% = 4

If it's a / (divide by) then set Action% to 4.

END SELECT

End the SELECT CASE command.

Temp2$ = MID$(Temp1$, 2, INSTR(Temp1$, " ") - 1)

The first variable name.

Temp3$ = RIGHT$(Temp1$, LEN(Temp1$) - INSTR(INSTR(Temp1$, " ") + 1, Temp1$, " "))

Get the second variable name.

Temp3$ = LEFT$(Temp3$, LEN(Temp3$) - 1)

Remove the last character from the second variable name because when we got it in the last step there still was the ) of an equation

Temp2$ = LTRIM$(RTRIM$(Temp2$))

Remove all the spaces left and right from the first variable name.

Temp3$ = LTRIM$(RTRIM$(Temp3$))

Remove all the spaces left and right from the second variable name.

FOR i = 1 TO VariableCount%

Start a FOR...NEXT from 1 to the number of varialbes we have in memory.

IF Variables$(i, 0) = Temp2$ THEN Temp1% = VAL(Variables$(i, 2))

If the variable names are the same make Temp1% equal to the value of the variable.

IF Variables$(i, 0) = Temp3$ THEN Temp2% = VAL(Variables$(i, 2))

If the variable names are the same make Temp2% equal to the value of the variable.

NEXT i

Return to the FOR

SELECT CASE Action%

Start a SELECT CASE of the value of Action%

CASE 1
PRINT Temp1% + Temp2%

If Action% is a 1 (plus) then do the equation.

CASE 2
PRINT Temp1% - Temp2%

If Action% is a 2 (minus) then do the equation.

CASE 3
PRINT Temp1% * Temp2%

If Action% is a 3 (times) then do the equation.

CASE 4
PRINT Temp1% / Temp2%

If Action% is a 4 (divide) then do the equation.

END SELECT

End the SELECT CASE.

ELSE

If it's not an equation then:

FOR i = 1 TO VariableCount%

Start a FOR...NEXT loop from 1 to the number of variables we have in memory.

IF Variables$(i, 0) = Temp1$ AND Variables$(i, 1) = "1" THEN PRINT Variables$(i, 2)

If the variable names are the same and the type is 1 (text) print the value of that variable.

IF Variables$(i, 0) = Temp1$ AND Variables$(i, 1) = "2" THEN PRINT VAL(Variables$(i, 2))

If the variable names are the same and the type is 2 (numeric) print the numeric value of that variable.

NEXT i

Go back to the FOR

END IF

End the if

ELSE

If it's not an equation or variable display:

PRINT Temp1$

Just print the expression that was behind the DISPLAYTEXT command

END IF

End the If

END IF

End the If

Conclusion


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After this issue you should now know how to do things like variable handling, displaying and equations. I told some people I would have IF...THENs in this issue but because the article is allready rather big I decided to put IFs in the next issue. At the end of this article your script.bas program should look like this:

DIM SHARED Variables$(100, 3)
DIM SHARED VariableCount%

OPEN "script.dat" FOR INPUT AS #1
 DO
  INPUT #1, ReadLine$
IF LEFT$(ReadLine$, 1) = "$" THEN
 IF VariableCount% <= 100 THEN
  VariableCount% = VariableCount% + 1
  Variables$(VariableCount%, 0) = LEFT$(ReadLine$, INSTR(ReadLine$, " ") - 1)
  Variables$(VariableCount%, 1) = "1"
  Variables$(VariableCount%, 2) = RIGHT$(ReadLine$, LEN(ReadLine$) - INSTR(ReadLine$, "=") - 1)
 END IF
END IF
IF LEFT$(ReadLine$, 1) = "%" THEN
 IF VariableCount% <= 100 THEN
  VariableCount% = VariableCount% + 1
  Variables$(VariableCount%, 0) = LEFT$(ReadLine$, INSTR(ReadLine$, " ") - 1)
  Variables$(VariableCount%, 1) = "2"
  Variables$(VariableCount%, 2) = RIGHT$(ReadLine$, LEN(ReadLine$) - INSTR(ReadLine$, "=") - 1)
 END IF
END IF

  ReadLine$ = LTRIM$(RTRIM$(ReadLine$))
  IF LEFT$(ReadLine$, 11) = "DISPLAYTEXT" THEN
   Temp1$ = RIGHT$(ReadLine$, LEN(ReadLine$) - 12)
   IF LEFT$(Temp1$, 1) = "$" OR LEFT$(Temp1$, 1) = "%" OR LEFT$(Temp1$, 1) = "(" THEN
    IF LEFT$(Temp1$, 1) = "(" THEN
      
       SELECT CASE MID$(Temp1$, INSTR(Temp1$, " ") + 1, 1)
        CASE "+"
         Action% = 1
        CASE "-"
         Action% = 2
        CASE "*"
         Action% = 3
        CASE "/"
         Action% = 4
       END SELECT
       Temp2$ = MID$(Temp1$, 2, INSTR(Temp1$, " ") - 1)
       Temp3$ = RIGHT$(Temp1$, LEN(Temp1$) - INSTR(INSTR(Temp1$, " ") + 1, Temp1$, " "))
       Temp3$ = LEFT$(Temp3$, LEN(Temp3$) - 1)
       Temp2$ = LTRIM$(RTRIM$(Temp2$))
       Temp3$ = LTRIM$(RTRIM$(Temp3$))
       FOR i = 1 TO VariableCount%
        IF Variables$(i, 0) = Temp2$ THEN Temp1% = VAL(Variables$(i, 2))
        IF Variables$(i, 0) = Temp3$ THEN Temp2% = VAL(Variables$(i, 2))
       NEXT i
        
       SELECT CASE Action%
        CASE 1
         PRINT Temp1% + Temp2%
        CASE 2
         PRINT Temp1% - Temp2%
        CASE 3
         PRINT Temp1% * Temp2%
        CASE 4
         PRINT Temp1% / Temp2%
        END SELECT
        ELSE
       FOR i = 1 TO VariableCount%
        IF Variables$(i, 0) = Temp1$ AND Variables$(i, 1) = "1" THEN PRINT Variables$(i, 2)
        IF Variables$(i, 0) = Temp1$ AND Variables$(i, 1) = "2" THEN PRINT VAL(Variables$(i, 2))
       NEXT i

    END IF
   ELSE
    PRINT Temp1$
  END IF
 END IF
 
IF LEFT$(ReadLine$, 7) = "WAITKEY" THEN WHILE INKEY$ = "": WEND
IF LEFT$(ReadLine$, 9) = "ENDSCRIPT" THEN
 EXIT DO
END IF

LOOP UNTIL EOF(1)
CLOSE #1
        

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