Friday, January 08, 2016

Tiny BASIC part 4 - Adding a PSET statement

Okay, now we are ready to add a PSET statement for drawing pixels.  Today we will simply add the ability to draw a single black pixel at a time.  Next time we will add color!

Here is the code to accomplish this.  This is just another SELECT CASE block to add after the one we added for the GRAPHICWIN command.
 CASE "pset"
    E$ = "PSET error - Graphic window is not open"
    GOTO [Ready]
  GOSUB [GetExpression]
  IF E$<>"" THEN [Ready]
  GOSUB [GetChar]
  IF C$ <> "," THEN
   E$= "Comma expected after x parameter"
   GOTO [Ready]
  C = C + 1
  GOSUB [GetExpression]
  IF E$<>"" THEN [Ready]
  #GWIN "down ; set "; PSETX; " "; PSETY
  GOTO [FinishStatement]
This was a bit tricky to write because there isn't really any documentation with the original Tiny BASIC source code, but by looking at the code for the other statements I think I figured it out correctly.  It does seem to work.

The syntax for the new statement is PSET x, y

Let me explain what it does.
  • First check to see if the graphic window is open.  If it isn't then set E$ to be an error string.  Then GOTO [Ready].  This will display the error.
  • Then get the next expression using GOSUB [GetExpression].  This unleashes the expression parser which is easily the largest and most complex part of the Tiny BASIC source code.  Then it checks for an error using IF E$<>"".  If E$ does contain an error, then GOTO [Ready].
  • Okay so got this far, so set PSETX to be what was in N, which is the result of the call to [GetExpression].
  • Now get the next character, which we expect to be a comma to separate the x and y values.  If the next character is not a comma, set E$ to be an error and GOTO [Ready].
  • Now advance C one character by adding 1 to it.  We do this because we found the expected comma, and now we need to skip over that so that we can get the next expression for our y value.
  • Get the next expression using GOSUB [GetExpression].  Test E$ for an error and GOTO [Ready] if there is one.
  • Get the value of N and put it into the variable PSETY.
  • Finally, draw the pixel in the graphics window!
Here is the sample Tiny BASIC program that uses the PSET statement:

10 graphicwin
20 pset x, y
30 x = x + 1
40 y = y + 2
50 if x < 200 then goto 20

And here is the output of the program!