Web Turtle Commands

How To Use The Turtle!

You instruct the web turtle to move, turn and draw things on the screen by placing commands in the text field and then clicking the "Draw It!" button. The screen will be updated with the new image, and your program will be in the text field again.

Place one command per line in the text field and the turtle will follow your commands one by one. (Automatic looping and subroutines are available, explained later).

Note: you can have blank lines and can place spaces before commands (indenting them) to make your program more readable.

Examples:

The easiest way is to learn by example. A number of pre-written programs are available on the Examples page. When you begin with an example, the drawing will appear, and the pre-written program will be in the text field. Change things around and then click "Draw It" to see the results. If you made a mistake, you can always change it back or click your "BACK" button.

Movement:

General movement in a turtle-based drawing environment uses the idea of direction and motion. The turtle is facing a certain direction and you can tell it to turn around or move forward or backward.

MOVE pixels	Move the turtle forward pixels pixels in the direction it's pointing. Example: MOVE 100
RIGHT degrees	Rotate the turtle degrees degrees to the right Example: RIGHT 90
LEFT degrees	Rotate the turtle degrees degrees to the left Example: LEFT 45

Additional commands in Web Turtle allow you to point the turtle to a certain angle, regardless of which way it's pointing, return it to it's "home" starting position, and remember and return to where it was.

POINT degrees	Rotate the turtle so it's facing degrees degrees. (0 is right, 90 is up, -90 or 270 is down, 180 is left.) Example: POINT 33
HOME	Return the turtle to the center ("home") position and face it 90 degrees (up).
REMEMBER	This tells the turtle to make a note of its current location and direction. You can remember more than one place- the latest place remembered will be used when using "GOBACK" (below), followed by the next latest place, and so on.
GOBACK	This transports the turtle to its most recently "REMEMBERed" spot. Example: REMEMBER DRAW 100 GOBACK (Draw it)
FORGET	This is similar to the "GOBACK" command, in that the most recently "REMEMBERed" spot is discarded; however, the turtle does not move.

Drawing:

Web Turtle drawing gives you features like pen color, pen thickness and even transparency control (how dark to color over the existing image).

DRAW pixels	Move the turtle forward pixels pixels, leaving a trail (line) on the screen. Example: DRAW 55
COLOR color	This lets you set the color that the turtle draws with. You can also specify "+1", "+2", etc. or "-1", "-2", etc. to change the color relative to what it is now. Examples: COLOR BLACK COLOR 5 See the Color Table.
THICK pixels	Set the thickness of the lines the turtle draws. You can also specify "+1", "+2", etc. or "-1", "-2", etc. to increase or decrease the thickness relative to what it is now. Example: THICK 2
TRANSPARENT percentage	Set the transparency of the lines the turtle draws. 0 transparency means the line is drawn dark (any lines you draw over are replaced). 100 means the line you draw is invisible. You can also specify "+1", "+2", etc. or "-1", "-2", etc. to increase or decrease the transparency. Example: TRANSPARENT 50

Text:

You can even draw text with Web Turtle! Standard (upright) text as well as angled text rendering is available.

PRINT text	Use this to draw upright text at the position of the turtle. The turtle is not moved. Example: PRINT Hello World!
TURTLEPRINT text	This draws text at the angle the turtle is pointing, and it moves the turtle forward as it draws. It's like drawing lines out of words! Example: TURTLEPRINT Hello RIGHT 90 TURTLEPRINT World!

To print variables (numeric variables A through Z and strings $A through $Z), use the new "PRINTVAR" command. (There's currently no support for TURTLEPRINT-style variable printing; sorry!)

Looping:

It's extremely useful to be able to repeat a set of commands a number of times. To draw a square, for example, you repeat the set of commands: "draw a line and turn right 90 degrees" four times.

REPEAT iterations	This tells the turtle to repeat all of the commands between here and the command "NEXT" iterations times. You can even "nest" other "REPEAT...NEXT" loops within a "REPEAT...NEXT" loop!
NEXT	This tells the turtle to go back to the "REPEAT" and repeat the commands, unless it's already done that iterations times.

An example of using "REPEAT."

Subroutines:

You can create complicated drawing routines (like one to draw a square, for example) and access them many different times in your program by turning them into a "subroutine." Subroutines are like small programs within a program.

# label	This tells the turtle that the subroutine called label begins here.
GO label	Use "GO" to tell the turtle to "jump" into the subroutine called label and execute the commands in there.
RETURN	This tells the turtle to "jump" back out of the current subroutine and continue what it was doing.
END	This stops the turtle and shows you the image.

An example of creating subroutines.

Branching:

As with other languages, you can have WebTurtle do different things depending on the state of your program. For example, if the variable "L" is less than 2, you may want to change the color... or if the variable "A" is greater than 10, you might want to keep it at 10.

The "IF...ELSE...ENDIF" structure lets you do this depending on that evaluation of expressions...

IF v1 operation v2	If v1, when compared using the operation operation to v2 is true, then do the following commands, otherwise jump to the next matching "ELSE" or "ENDIF" below.
ELSE	If the expression evaluated in the matching "IF" above was false, the program continues here. Otherwise, when this command is met, the program jumps to the next macthing "ENDIF" below.
ENDIF	If the expression evaluated in the matching "IF" above was false, and this structure contains no "ELSE", the program continues here. Otherwise, when this command is met, it is ignored.

An example of using the "IF...ELSE...ENDIF" structure.

Valid operators:

= (or ==)	Equal to
!= (or <>)	Not equal to
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to

Comments:

It's also useful to add comments to your code, explaining what part of it does, or temporarily "commenting out" lines of code instead of deleting them. Comments are ignored by Web Turtle.

; comment

The text comment is ignored by the turtle when it sees the ";" (semicolon). Example: ; the next part of the program draws a face

User Input:

The little text type-in fields below the program and the "Draw It!" button can be used to read input from the user.

Whatever the user types can be referred to by using the special "^1" through "^9" variables. For example, if you want to print out the exact text they typed into the first field, you could go:

PRINT ^1

Or, as a short-cut, just use "^" to get the value of the "Input 1" field:

PRINT ^

You can also read user input as numbers (any non-digit input from the user will be seen as "0"):

LET I ^2

You can even work with a pair. To add the first two inputs together, you could do:

LET X ^1+^2

Debugging:

Sometimes you may want to know where your turtle is and which way it's pointing.

SHOWTURTLE

If this command ever gets executed during your program, an image of a turtle will appear on the screen at the turtle's final location, rotated to point in the direction that the turtle is facing.

Variables:

Like other programming languages, Web Turtle lets you use "variables." These are like boxes with numbers in them. Just like the way subroutines let you write some code once and use it many times, variables let you state a number and use it many times. (Say you had a picture where you draw a line of 100 pixels, and then back up 100 pixels to get back to where you where. If you wanted to change the length the line, you can either change both "100"s, or instead, use a variable and change its value.)

LET variable value

You have 26 variables available to you, lettered "A" through "Z." The value after the variable can be a number (like "100"), another variable (like "X") or even a simple equation (like "10*X")! Example: LET A 10

How to use variables and equations.

Strings:

Web Turtle also gives you "strings" - boxes with words in them. You would use these in the commands that can accept text: "PRINT," "TURTLEPRINT," and "COLOR."

You can also convert numbers and variables into strings by preceding them with a period ("."). For example: "PRINT .X" will print the number stored in "X" as if you had entered it manually as text.

LET string value

You have 26 strings available to you, lettered "$A" through "$Z." The value after the variable can be plain text (like "Hello there"), another string (like "$B"), or a number or variable, if preceeded by a "." (like ".123" or ".X"). Example: LET $M Test Message

Recursion:

Since WebTurtle is a procedural language (not a functional one), and since it has no "local" variables (that is, variables that are only affected within a subroutine), it's difficult to create recursive functions.

Recursive functions are those that call themselves. The "factorial" (or "!") mathematical function is an excellent example... 5! is actually 5*4*3*2*1. 6! is 6*5*4*3*2*1 (or, 6*5!).

To simulate what happens when a function calls itself (its local variables are recorded, the function call is executed, and then local variables are restored), WebTurtle provides a "stack."

A stack is like a large box that contains many values. You can place values into it and take them out. If you stick the number "5" in, you can pull the number "5" out. If you stick the number "1" in, and then stick the number "8" on top of it, you can pull out the number "8" and then the number "1." This is called "FILO", or "First In, Last Out."

PUSH value	The value value is placed into the stack.
POP variable	The value at the top of the stack is removed and placed into the variable variable.

Factorial in WebTurtle