This is the program for the sixth RetroBattlestations BASIC challenge. In here you'll find a simple BASIC program that uses a stack based command interpreter to do "turtle graphics". The program has been ported to several platforms already, but you're welcome to add more!
Commands are put into a string called CM$ with values and commands separated by a space. Numeric values will be pushed onto a stack, a command will pop off a value and then execute. There is very little error checking being done. There are four commands supported:
- D - lower the pen
- U - raise the pen
- M - move
- T - turn
Looping is done by putting a value on the stack representing the number of iterations and then placing commands inside of ( ). There must be a space separating the ( ) characters from the commands and values inside.
I did my best to minimize "dirty tricks" although I did use some bit fields for the Cohen-Sutherland line clipping algorithm. If you want to know how the algorithm works, see here.
For more details about the challenge, check out the post here:
If you are working on a port, choose the version to start with carefully. Some BASIC interpreters support bitwise operators, some do not. If you are working on a platform that does support bitwise operators then you do not want to use the Apple II source as a reference!
See List of Computers With On-Board BASIC
When typing the program in you can leave off any lines which begin with REM, they are not needed for the program to run. On many platforms you can leave out the whitespace between keywords and operators. IBM BASIC is not one of those however.
Note: On the TRS-80 Color Computer and BBC Micro you need to include the spaces around any IF, AND, OR, or THEN statement.
If you make a mistake and don't want to retype the entire line, most of the BASICs have a way to make corrections.
On Amstrad CPC, use AUTO to start typing commands. Use the arrow keys to move about the line. Exit the AUTO mode py pressing Escape. You can also start on a specific line by entering AUTO 100 (for line 100).
To go to the start or the end of the line use CONTROL+Arrow keys. You can also use SHIFT+Arrow keys to use the copy cursor. This is a second cursor that you move independendly, and will copy whatever is under it to the main cursor when you press COPY.
On an Apple II+ use LIST to print the line with an error, then use ESC followed by A/B/C/D to move the cursor one step at a time. Position the cursor at the beginning of the line, then use the right arrow to move over the line and fix the error. Be sure to arrow all the way to the end of the entire line before you hit RETURN!
On an enhanced Apple IIe, Apple IIc, or Apple IIgs you can also use ESC with the arrow keys. In 80 column mode (enter with PR#3) the cursor will change to a white block with a + in it, push ESC to drop out of movement mode.
Use LIST to print the line with a mistake, then use the arrow keys to move up to the beginning of the line. Each press of the copy key will type in the character under the cursor. Make any necessary edits by just typing on the keyboard and using copy to avoid retyping everything.
Like the others, use LIST to display the line with problems, then use the arrow keys to move up and make any corrections. By pressing shift-INST you can insert a blank character if you missed something. Unlike the Apple II you don't need to arrow to the end of the line before pushing RETURN.
This has to be the best built-in BASIC editor I've seen so far! Just type EDIT and the entire BASIC program will be loaded into the built-in word processor where you can make any changes you want. Press F8 to exit the editor and go back to BASIC.
Type EDIT and it will print the line on the screen and put your cursor at the beginning of the line. Arrow left/right and you can use Insert & Delete to make corrections. Like Commodore BASIC, you don't need to arrow to the end of the line before pushing RETURN.
Type the line number and then arrow up (FCTN+E) and it will enter edit mode with that line loaded. You can move within the line with arrow left (FCTN+S) and arrow right (FCTN+D) and move to the previous or next line with arrow up (FCTN+E) or arrow down (FCTN+X). You can delete the character under the cursor with DEL (FCTN+1) or turn on insert mode to insert extra characters with INS (FCTN+2). You do not need to move to the end of the line before pressing ENTER.