Chapter 4 Em

7/31/2019 Chapter 4 Em

1/10

A First Procedure: Teaching theTurtle to Draw a Square


2/10

Chapter 4 A First Procedure: Teaching the Turtleto Draw a Square

Using the commands f o r w a r d and r i g h t or

l e f t we can make the turtle draw a square. For

convenience we will often use the short names for

commands.

FD 3 0

RT 9 0

FD 3 0

RT 9 0

FD 3 0

RT 9 0

FD 3 0

RT 9 0

SQUARE

If we used 50 instead of 30, the turtle would draw

a bigger square. We could use any number.

Lets define a new command in Logo to get the

turtle to draw a square. New commands in Logo

are programs or procedures written by you. Each

time you want a square you can use your new pro

cedure rather than retyping the individual in

structions.

To define a new command you should first choose

a name. Lets use s q u a r e because that seems

natural. But any name will do. You could then use

the Logo command t o and define s q u a r e in thesame way as you did g r e e t .

That is, you could type t o s q u a r e , then the in

structions, then e n d . This method is good if you


3/10

are careful about typing and you know just what

you want to type in advance.

Introducing the Logo Editor

There is another way to define a procedure. You

could use the Logo editor. Then, if you make typ

ing mistakes you can remove them easily. When

you are using the editor, Logo carries out only ed

iting actions.

There is a disadvantage to using the editor. Yourturtle drawings will be replaced by the editing

screen and the picture will be lost.

On the other hand, the disadvantage of using t o

to define your new procedure is that you will not

be able to fix typing errors except on the line

where the I (cursor) rests.

EDIT

e d i t or e d signals Logo that you want to edit.

Follow it by the name of the procedure you want

to edit. You must prefix the name with a "(quote

mark). Do not type a space between " and the

name of the procedure.

E D I T " S Q U A R E Remember the " (quote mark).

After you press the r e t u r n key you will be using

the Logo editor and only editing actions will be

carried out.

Bug BoxIf you do not remember to prefix s q u a r e with

a " (quote mark) and type

ED IT SQUARE


4/10

v

I

r

Logo will respond

I DON'T KNOW HOW TO SQUARE

LOGO FfllTflR

If you had turtle drawings on the screen they will

go away when you start editing.

When the editor starts up, the title line of the pro

cedure will appear at the top of the screen.

TO SQUARE This is the title line.

t o informs Logo that the followingtext is part of a proceduredefinition.

s q u a r e is the name of theprocedure. You are free to chooseanother name.

The! (cursor) is at the end of the title line.

Notice that Logo does not print ? or any other

prompt symbol while you are using the editor.

Since you do not want to change the title line

press the r e t u r n key.

Now type in the commands that make up

s q u a r e . They are the commands you used

previously.

FD 30

RT 90

FD 30

RT 90

FD 30

RT 90

FD 30

RT 90

Think of everything you type in the editor as a

stream of characters. If you want to add some

thing to the stream, move the I (cursor) to that

place.

29


5/10

If you want to move the I (cursor) backwards in

the stream, use c t r l - b . Y ou can move all the way

to the beginning of the stream.

If you want to move the I (cursor) forward, use

CTRL-F.

Then type the characters you want and they will

become part of the stream.

Thus, if you have made a typing error in a previous line, use c t r l - b to move the I (cursor) back

wards to where the bug is. When the I (cursor)

passes over the characters they remain un

changed.

If you want to erase a character the - key will

erase the one to the left of the I (cursor) and will

move the I (cursor) to that position.

Notice if th e! (cursor) is at the beginning of a text

line, pressing the - key will move it along with

the entire line of text to the end of the previous

text line. Thus

FORWARD 30

R IGHT 90

becomes

FORWARD 30RIGHT 90

Press the r e t u r n key to separate the lines again.

FORWARD 30

R IGHT 90

If you had pressed space instead of the r e t u r n

key then you would see

FORWARD 30 RI GH T 90

30


6/10

You can move the I (cursor) from the last charac

ter typed to the t in t o on the title line by typing

c t r l - b several times.

SQUARE

REPEAT 3 [RT 45 SQUARE]

REPEAT 3 [RT 45 SQUARE]

Type c t r l - c when you have completed your edit

ing. If you do not type e n d , Logo will insert the

word when you type c t r l - c .

Logo will now end the procedure definition and

carry out the t o command by defining s q u a r e .

Logo will type

SQUARE DEF INED

Using the New Command

Try your new command. Type

SQUARE

Again, type

SQUARE

This time the turtle just retraced its path.

If you turn the turtle left or right and then type

s q u a r e again, a new drawing will appear. For ex

ample, tell the turtle

RIGHT 45

and now type

SQUARE

Continue to repeat these two commands ( r t 45

and s q u a r e ) . To do this you could use the Logo

command, r e p e a t , r e p e a t requires two inputs.

For example,

REP EAT 3 [RT 45 SQUARE]

31


7/10

The first input indicates how many times to re

peat the enveloped instructions. The second inputis a list of instructions. The instructions must be

enclosed in brackets. Think of the brackets asmaking an envelope. Complete the design bytyping

RE PE AT 3 CRT 45 SQUARE ]

Now l e t s m a k e a p r o c e d u r e fo r th i s d e s i g n a n d Call it SQUARE STAR

E D I T " S Q U A R E S T A R

You will now be using the editor and the title linewill be displayed on the screen.

TO SQUARESTAR

Notice that th e! (cursor) is at the end of the title

line. Since you do not want to change the title linepress the r e t u r n key and type

REPEAT 8 [SQUARE RT 45]

END

Dont forget, you type c t r l -c when you are fin

ished editing.

It is always a good idea to try out your new proce

dure. Put the turtle in its startup state in the cen

ter of the screen facing straight up. Rememberthat CS will do this. Type

CS

and thenSQUARESTAR

32


8/10

Bug Box

If your squares look like rectangles the bug is in

your TV, and not in Logo. The Logo command

s e t s c r u n c h allows you to change the aspect ra

tio of the screen. If you want to know what the

current setting is type

PR SCRUNCH

Logo prints a number which is the current set

ting. Now try

SETSCRUNCH .8

Then type

CS SQUARE

If your square looks worse try

SETSCRUNCH 1

Try other settings until you are satisfied.

Other Uses o f s q u a r e

Once you have defined a procedure, you can use it

as you would any Logo primitive such as f d , b k ,

l t , r t , etc. Thus, a procedure you define can be

used as part of the definition of other procedures.This is one of the powerful features of Logo. For

example, there are many designs which use

s q u a r e . Some more examples are:

cu

FLAG CROSS FLAGS MANYFLAGS


9/10

FLAG

CROSS

FLAGBACK

FLAGS

Both f l a g and f l a g b a c k make the turtle draw

the same design, but they leave the turtle in dif

ferent states. Both procedures leave the turtlewith the same heading as they found it, but f l a g

leaves the turtle in a different position from the

one it started in.

f l a g b a c k , on the other hand, leaves the turtle in

the same position on the screen as it was found.

We can see the effect of these differences in c r o s s

and f l a g s , c r o s s runs f l a g four times while

f l a g s runs f l a g b a c k four times.

TO FLAG

FD 30

SQUARE

END

TO CROSSR E P E A T 4

END

[FLAG RT 90 ]

TO FLAGBACK

FLAG

BK 30

END

TO FLAGS

R E P E A T 4

END

[F L A GBA C K R T 90 ]

MANYFLAGS Uses FLAGS.

TO MANYFLAGS

FLAGS

RT 45

FLAGSEND

34


10/10

Logo Vocabulary

Full Name

REPEAT

SETSCRUNCH

SCRUNCH

CLEARSCREEN

SHOWTURTLE

FORWARD

RIGHT

BACK

LEFT

TO

END

EDIT

PRINT

Special Keys

CTRL-G

CTRL-B

CTRL-F

RETURN

Short Name

cs

ST

FD

RT

BK

LT

ED

PR

CTRL-C

Chapter 4 Em

Documents

Transcript of Chapter 4 Em