Meet the Turtle
You’ve already seen the turtle - quietly sitting in the center of the Graphics window - patiently waiting for you to tell it what to do. Well, the waiting is over! You’re about to enter the fascinating world of turtle graphics - a special way of drawing interesting, exciting and colorful graphics on your computer screen by telling a turtle what to do. Not a real turtle, of course. The Logo turtle is a special graphics cursor that can move forward or back, rotate right or left, change color, change size, change shape and even disappear. There’s even more, but let’s do something now.
FORWARD 100 and watch what happens. The
turtle moved in the direction that its head was pointing; the line it
drew is 100 turtle steps long (that’s how turtles measure
RIGHT 90 and you’ll see the turtle has rotated like it’s peeking
around a corner. Turns are measured in degrees, but they’re not called
turtle degrees because that’s how turns are measured by just about
everything, from boats and planes to spaceships.
If you type these two instructions again and again
and again, you’ll get pretty tired of typing! Here’s a shortcut: type
FD 100 RT 90 all on the same line before you press Enter.
short for FORWARD and
RT is short for
RIGHT. Next, just press the up-arrow key to
move the cursor onto that line and then press Enter. What could be
easier? You’ll see.
There is a very simple way to repeat instructions without typing the
same thing over and over again. It’s the
REPEAT command but before you
can use it, you need to learn a little bit about a special type of Logo
data called a list. A list starts with ‘[’ and ends with ‘]’. In
between the brackets is where you put the instructions you want
repeated. Lists are used a lot in Logo but that’s enough to know for
The REPEAT command needs
a number and a list. The number tells it how many times to repeat and
the list tells it what to repeat. So, to repeat the earlier drawing,
REPEAT 4 [FD 100 RT 90]. That’s a lot easier than typing the
instructions four times.
Don’t forget about the up-arrow key - it’s still
worth using if you make a mistake or if you just want to change a
previous line of instructions. Let’s say you want to change the
we just used so that the turtle does the opposite of what it did last
time. The opposite of
BK (short for
BACK) and the opposite of
RT is ` LT `
(short for LEFT). See what happens when you
change both instructions to their opposite and then change just one at a
time. Is this a pane? It should be. Try this next:
REPEAT 3 [PRINT
If your Graphics window is getting a little messy, just type
clean it up and start drawing again. (
CS is short for
CLEARSCREEN.) You can clean up the
Listener window by typing
CT (short for
More About the Turtle
There are many Logo commands for the turtle. You can’t learn them all at once but it makes sense to learn them in pairs or groups. Commands that are opposites are easy to learn at the same time. You already know the FORWARD and BACK opposites and the RIGHT and LEFT opposites. Here are some others you can try out:
Where’s the pen? It’s right in the middle of the underside of the turtle. If the pen didn’t go up and down, you’d have a hard time making your drawings look nice. Try it with a pencil and some paper and you’ll see what I mean. Does your pencil have an eraser? The turtle’s pen does.
PE - draws in the
Erasing in the Graphics window is not exactly the same as erasing pencil
lines from paper but the effect is the same - what was there is now
gone. To see this happen quickly, try
FD 100 PE BK 100 but don’t
forget to put the pen back down when you’re done! Here’s one more pen
PX - inverts the
colors under the pen
This special pen flips colors when it passes over them. In computer speak, the color’s bits are inverted; black becomes white, red becomes yellow, and more. Try to create a drawing, set the turtle’s pen width to, say 10, and draw over your drawing to see interesting color effects!
If the turtle is moving too quickly to follow, type
SLOWTURTLE to slow it down. Type
SETSPEED 1 to restore normal speed.
The Turtle Continued
When you finish a drawing, do you leave your pencil
HT - the turtle disappears
ST - the turtle
Hiding the turtle makes drawing faster and gets the turtle out of the way. Showing the turtle helps you see which way it’s heading and where it is. If your turtle gets lost, type HOME to quickly get the turtle back to the center of the Graphics window. If you still don’t see it, type
When the point of your pencil gets dull, the lines get wider. The turtle’s pen never gets dull but you can change its width from 1 to 99. Width 1 is the default. You may not even see the turtle with a really wide pen!
SETW - changes the pen width (1-99)
With all the commands you know so far, you can make some interesting special effects. The star illustration looks like a really complicated thing to create. In fact, the star was drawn with
SETW 10 and then erased with
PD SETW 10 REPEAT 5 [FD 100 RT 144] PE SETW 4 REPEAT 5 [FD 100 RT 144]
See what you can do. Just play around.
Add Some Color
Coloring with the turtle is simple and fun!
You can choose from 16 colors for now. Let’s keep it simple.
SETPC - sets the pen color (0-137)
SETPC needs a number from 0 to 137. Not only does the pen color change, but the turtle color changes too, so you can tell which color it will use to draw. The table of color numbers is shown at the right. Try any color you want. Pick your favorite.
What about SETPC 15? Try it. When Logo starts up, the Graphics screen is white. If you draw with the same color as the background, it’s like not drawing at all. What if you want to draw in white?
SETBG - sets the background color (0-137)
SETBG needs a number from 0 to 137 just like SETPC. If you set the background color to the same color as the turtle, the turtle will disappear but you can’t get it back with
ST - you have to make sure the background and the
turtle colors are not the same.
Do you want a green square? Or, a blue star? Just change the pen color and then use the up-arrow key to move the cursor back to a previous command line and press Enter.
Can you make a red diamond? A diamond is really just a square that’s turned sideways a little.
Teaching Logo Something New
Wouldn’t it be great if you could just type
REPEAT 4 [FD 100 RT 90]? Go ahead and try it but expect an
error message because Logo doesn’t know what
SQUARE means. Yet. With
your help as the teacher, Logo can learn new commands! Then, you can
use them just like the built-in commands to save time, typing and typos.
To teach Logo how to square, type
TO SQUARE and you’ll see a special
window - think of it as your chalkboard where you’ll write today’s
lesson. May I have your attention, please? Read carefully.
There are three text boxes in the ”Define a Procedure“ window but we only need two of them for now. The ”Procedure Name“ text box is already filled in and the cursor is in the ”Commands“ text box. This is where you type the instructions that make a square, just like you did in the Listener window. Go ahead and type
REPEAT 4 [FD 100
RT 90] and then press Enter. Nothing happens.
Instructions you type in the ”Commands“ text box are remembered for later - not run immediately like they are in the Listener window. Click the Define button and you’ve just taught your first lesson! You deserve a raise!
More Teaching Fun
A square is four lines and four turns. But,
what is four
SQUARE and four turns? Type
REPEAT 4 [SQUARE RT 90] and
see what a square of squares looks like. Change the numbers for
REPEAT and RIGHT
to see what other interesting things you get. How about a pinwheel? How
many squares are in that?
Of course, all of your squares are 100 turtle steps on a side because that’s how you defined it. How can you change it? If you type
TO SQUARE again, you will see
the procedure on the chalkboard just as you left it. You could change
FD 100 to
FD 50 but then, what if you wanted bigger squares
again later? You’d have to change it back to 100 again. This would get
old in a hurry. Just remember, you’re a teacher!
Why not teach Logo how to make a smaller square? Call it BOX. Just type
TO BOX, press Enter, type
REPEAT 4 [FD 50 RT 90], click on the
Define button and there you have it! Now you can draw a big square with
SQUARE and a small square with
Try defining a procedure to draw a triangle - it’s just three lines and three turns. What about the star? Once you have your new commands defined, see what you can make. Here are some suggestions.
Your Input Matters
If you want to draw squares of a zillion different
sizes, do you need to define a zillion different procedures with a
zillion different names? That would be silly. The only difference
BOX is the number after the
FD command but how
can you replace that number with any number you want, any time you want,
without changing it over and over again in the ”Define a Procedure“
window? The answer is simple - use an input to your
so you can just type
SQUARE 100 or
SQUARE 50 or use any number as
the actual input.
What’s an actual input?
An actual input is the input you give to a command or procedure when you actually run it, like the number
An actual input is also called an actual parameter or actual
What other input is there?
A formal input is used in the definition of a procedure. It is a special container with a name chosen by you. A formal input is also called a formal parameter or formal argument. It is also called a variable because its contents can change or vary.
You will find that just the word input is often used for both types of input. Don’t let that confuse you. It’s just easier to say input.
to bring up the chalkboard again. Click in the ”List of Inputs“ text
box. Since we are going to draw squares of any size we want, let’s use
SIZE for our input to
SIZE and then press
Tab to move the cursor into the ”Commands“ text box. Replace the 100
SIZE. When you run
SQUARE later on,
be automatically replaced with the actual input you give to
Click the Define button to save your new
Now, you can draw squares of any size you want with just the
command in the same way you draw lines of any length with the
command. Just give
SQUARE an input. An actual input, that is. Add an
input to other procedures in the same way. It’s simple, now.
Terrapin Logo allows for ‘Relaxed syntax’, which does not require the colon in front of names. If you turn off this option, you will need to use
:SIZE instead of
SIZE in your
Need More Input?
Your procedures can have more than one input,
just like the REPEAT command. Suppose you
want to draw a stack of boxes. How would you do that? Of course, you
would use the
SQUARE procedure to draw the box. But then what? How
about moving the turtle to the top of the box it just drew? Then, it
would be ready to draw the next box. Now how many boxes do you want in
your stack? How about a short stack of 3 big boxes? Try
[SQUARE 40 FD 40].
If you change the 3 to 7, you get a tall stack. If
you change the 40’s to 12’s, you get smaller boxes. You can make stacks
of boxes with one procedure called
STACK and give it two inputs - one
for how many boxes and another for how big to make them. Let’s name them
TO STACK to bring up the chalkboard. Click in the ”List of Inputs“
and type the names for the two inputs. Press Tab to move into the
”Commands“ window and type in the instruction to draw a stack of any
number of boxes of any size.
- Where will you put
- Where will you put
- How about a star made from stacks?
REPEAT 5 [STACK 15 7 RT 144]