Tiny BASIC part 5 - Adding color to PSET

It's much more fun to draw in color than with only a black pen, so let's add a third parameter to the PSET statement for color, for example:

PSET x, y, "color"

To do this we need to add some code to our  case "pset"  block:

 CASE "pset"
  IF GWINOPEN = 0 THEN
    E$ = "PSET error - Graphic window is not open"
    GOTO [Ready]
  END IF
  GOSUB [GetExpression]
  IF E$<>"" THEN [Ready]
  PSETX = N
  GOSUB [GetChar]
  IF C$ <> "," THEN
   E$= "Comma expected after x parameter"
   GOTO [Ready]
  END IF
  C = C + 1
  GOSUB [GetExpression]
  IF E$<>"" THEN [Ready]
  PSETY = N  PSETCOLOR$ = "black"
  GOSUB [GetChar]
  IF C$ = "," THEN
   C = C + 1
   GOSUB [GetStringLiteral]
   IF E$ <> "" THEN [Ready]
   PSETCOLOR$ = B$
  END IF
  #GWIN "color "; PSETCOLOR$
  #GWIN "down ; set "; PSETX; " "; PSETY
  GOTO [FinishStatement]

The way this works is that it will look for a comma and a string expression after it sets PSETX and PSETY.  If there is no comma it will skip over the part that gets a color parameter, so the default color will be black in that case.

The string will be the name of a valid Liberty BASIC color, for example red, blue, green, black, etc.    The following code is stolen from the routine that parses for the PRINT statement.  In a later post we will incorporate this into the PRINT code by calling it as a subroutine so that we won't have the same code twice.

[GetStringLiteral]
  GOSUB [SkipSpace]
  GOSUB [GetChar]
  IF C$=G$ THEN
   B$=""
[NextStringMember]
   C = C + 1 : C$=MID$(A$,C,1)
   IF C$="" THEN
    E$="Unterminated string"
    RETURN
   ELSE
   IF C$<>G$ THEN
    B$=B$+C$
    GOTO [NextStringMember]
   END IF
  END IF
  C = C + 1 : C$=MID$(A$,C,1)
  IF C$=G$ THEN
   B$=B$+C$
   GOTO [NextStringMember]
  END IF
 END IF
 RETURN

So we call the [GetStringLiteral] subroutine and check E$ for an error.  If there is none then we set PSETCOLOR$ to the value of B$ as shown here.

   GOSUB [GetStringLiteral]
   IF E$ <> "" THEN [Ready]
   PSETCOLOR$ = B$
  END IF
  #GWIN "color "; PSETCOLOR$

Then we add a drawing command like so to set the color:

  #GWIN "color "; PSETCOLOR$

Here is a sample that uses PSET with color!

5 graphicwin
10 pset x, y, "red"
20 pset x + 10, y, "blue"
30 pset x + 20, y, "green"
40 x = x + 1
50 y = y + 2
60 if x < 100 then goto 10

And here is a screenshot!

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"
  IF GWINOPEN = 0 THEN
    E$ = "PSET error - Graphic window is not open"
    GOTO [Ready]
  END IF
  GOSUB [GetExpression]
  IF E$<>"" THEN [Ready]
  PSETX = N
  GOSUB [GetChar]
  IF C$ <> "," THEN
   E$= "Comma expected after x parameter"
   GOTO [Ready]
  END IF
  C = C + 1
  GOSUB [GetExpression]
  IF E$<>"" THEN [Ready]
  PSETY = N
  #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!