13. Logo (Part 2)

B.A. (Mahayana Studies)000-209 Introduction to Computer Science

November 2005 - March 2006

13. Logo (Part 2)

More complex procedures using parameters, arguments, and recursion. Local variables and conditional statements.

000-209 Intro to CS. 13/logo2 2

Overview

1. Shorter Commands 2. Parameters 3. Rectangle 4. Fun with Squares 5. Fun with Triangles 6. Printing

continued


7. Local Variables 8. Procedures that Output 9. Conditional Statements 10. Stopping Recursive Calls 11. Fractals 12. More Logo Commands


1. Shorter Commands

Most commands have 2-letter short forms. Some of them:

FD = FORWARD BK = BACK RT = RIGHT LT = LEFT PU = PENUP PD = PENDOWN

I'll start using the short forms from now, since it means less typing.


2. Parameters

The numbers following command names are called parameters.

Examples: fd 50 setpencolor 15 setpos [ 50 25]

parameters


Parameters and Procedures

Parameters can be passed into a procedure and used by its commands.

The parameters are stored in arguments.

Example: to sized_forward :size

forward :size end

command usingthe argument

argument holding a parameter


An Argument is a Memory Box

An argument is a memory box inside the procedure.

Any commands inside the procedure can use the memory box value. sized_forward

:size

forward :size

valuethe name of the box

use :size's valuecontinued


A memory box is a location in RAM. It has a name and may contain a value A command accesses the box's value by using the

box's name.

The memory box only exists until the end of the procedure when the procedure finishes, the memory box is deleted


Using sized_forward

Two calls to the sized_forwardprocedure with different parameters.


Parameters and Memory Boxes

A procedure's parameter is copied into the memory box for the argument the parameter becomes the box's value

sized_forward

:size

forward :size

sized_forward 100100

use :size's value, 100


Many Arguments

Procedures can have many arguments. Draw any size of rectangle:

to rectangle :width :height fd :height right 90 fd :width right 90 fd :height right 90 fd :width end

two arguments, forthe width and height

of the rectangle

rectangle

:width

code using box values

:height


3. Rectangle


Rectangles Parameters

For the first call to rectangle:

For the second call to rectangle:

rectangle

:width

code using box values:heightrectangle 100 50

rectangle

:width

code using box values:heightrectangle 10 100


rectangle is called twice

At the end of the first call to rectangle, its width and height memory boxes (arguments) are deleted.

They are newly created again when rectangle is called the second time.

continued


4. Fun with Squares A square procedure that has a size argument:


A Squares4 Procedure


Procedure Calls

squares4

:size

square :sizesquare :size + 20square :size + 40square :size + 60

squares4 50

50square

:size

repeat command70

There are two :size parameters: one in squares4, the other in square.

square is called4 times


A Tables Procedure


A Mirror Procedure


A Mirrors Procedure


Procedure Calls

mirrors mirror calls

tables

squares4 :size

square :size


5. Fun with Triangles


A Hexagon Procedure


Procedure Calls

hexagon

:sz

repeat 6 [ triangle :sz rt 60 ]

hexagon 90

90triangle

:size

repeat command90

triangle is called6 times


A Spider Web Procedure (1)


Procedure Calls

hexagon

:sz

repeat 6 [ triangle :sz rt 60 ]

spiderWeb 50

70

triangle

:size

repeat command

70

spiderWeb

:sz

hexagon :szhexagon :sz+20hexagon :sz+40

hexagon is called3 times


A Spider Web Procedure (2)


spiderWeb 30 35:sz

hexagon :szhexagon :sz + :stephexagon :sz + (:step*2)

:step

spiderWeb

Procedure Call


Too Many Spider Webs

a problem continued


I had to press the "Halt" button to stop the execution spiderWebR will never stop on it's own

The spiderWebR procedure is recursive it calls itself

Recursion is a great technique, but I've used it incorrectly I didn't include code in spiderWebR to tell it how to stop


Recursion in Picture Form

spiderWebR 20

spiderWebR 40

draws hexagon and calls

spiderWebR 60


spiderWebR 80


spiderWebR 100


forever...


6. Printing

print <number or word> print the number or text in the command window. e.g. print 60

print 60 in the command window

A word must start with a quotation mark e.g. print "Andrew print Andrew in the command window

continued


For several words, put them in square brackets e.g. print [My name is Andrew]


7. Local Variables

A local variable is a memory box inside a procedure for storing numbers (or text).

Each variable has a name

:foo 90

name value

a variable

A local variable is just like a procedure argument, but can be created anywhere inside the procedure.


Creating a Local Variable

You create a local variable by giving it a name and a value: to proc

localmake "foo 90 :end

This creates a local variable called :foo, with the value 90, inside the procedure proc

proc

:foo 90


Using a Local Variable

A variable must be created before it can be used.


Procedure Call

:x :y

add2

:z

add2 7 13

localmake "z :x + :y

7 13


8. Procedures that Output

A procedure can output (return) an answer to the procedure that called it by using the output command.


Procedure Calls

:x

:x + :y

:y

adder

:sum

:a :badder3 :c :ab

adder3 6 7 2

output :sum

localmake "ab (adder :a :b)

6 7 2


9. Conditional Statements

A conditional statement is a command (or commands) which is only carried out if a test succeeds.

Examples: if (it is raining) then put up your umbrella if (7 > 5) then print a "bigger" message

The test part (e.g. 7 > 5) is called a condition. The part after the "then" is called the then part.


Conditional Statements

if ( <test>) [ <command(s)> ] Example:


The Else Part

You often want to do something if the test fails. Examples:

if (it is raining) then put up your umbrella else put on your sunglasses

if (:foo > :bar) then print a "bigger" messageelse print a "smaller" message

The part after the "else" is called the else part. It's carried out if the test fails.


More Conditionals

ifelse ( <test> ) [ <command(s)> ] [ <command(s)> ] the second […] is the else part

Example ifelse ( :foo > :bar ) [ print [foo is bigger] ]

[ print [foo is not bigger] ]

The conditional must be on one line.

continued



Better Formatting with ~

A long Logo line can be split across multiple lines by using ~'s this is useful for conditional statements

The previous example with ~'s: ifelse ( :foo > :bar ) ~

[ print [foo is bigger] ] ~ [ print [foo is not bigger] ]

Indent the two parts of the ifelse.


'If' Questions

1. Write a procedure that prints a message if two numbers are equal.

2.Write a procedure that prints "true" if the first number is less than the second, and "false" otherwise.

3.Write a procedure that prints one message if the first number is greater than the second, and a different message if it is not.


Answers

to equal :num1 :num2 if ( :num1 = :num2 ) ~ [ print [the numbers were equal] ] end

to lessThan :a :b ifelse ( :a < :b ) ~ [ print "true ] ~ [ print "false ] end

continued


Use (print …) to print arguments


10. Stopping Recursive Calls

The problem with my first version of spiderWebR was that it couldn't stop.

This can be fixed by putting the recusive call into a conditional statement.


Lots of Spider Webs



spiderWebR 130

spiderWebR 150


spiderWebR 170


spiderWebR 190


spiderWebR 210


STOPS since 210 > 200


Amazing



maze 5

maze 10

draws line, rotates and calls

maze 15

maze 200

STOPS since 200 = 200



many more maze calls...


Polygons



polygon 9 80 1

polygon 10 80 1


polygon 11 80 1

polygon 201 80 1

STOPS since 201 > 200



many more polygon calls...


Other Polygons

Try these: polygon 1 123 3

polygon 1 90 5polygon 5 144 5polygon 1 172 3

Try to create these with the polygon procedure.


11. Fractals

Fractals are curves created using recursion. Complex looking 2D and 3D shapes can be gener

ated using very small (and fast) code.

Fractals have been used in many computer-generated movies (e.g. Shrek, Toy Story) to create realistic looking backgrounds trees, clouds, water, fire, etc.


The C Curve

Now try cCurve 2 15 make sure to reset and zoom out


Procedure Calls

c 5 10

c 5 8 r c 5 8 l

c 5 9 r c 5 9 l

c 5 8 r c 5 8 l

c 5 7 r c 5 7 l c 5 7 r c 5 7 l c 5 7 r c 5 7 l c 5 7 r c 5 7 l

many, many more calls, until level = 0

c = cCurver = right 90l = left 90


The Dragon

Now try dragon 4 9 make sure to reset first


Procedure Calls

ld 10 10

ld 10 8 l rd 10 8

ld 10 9 l rd 10 9

ld 10 8 r rd 10 8

ld 10 7 l rd 10 7 ld 10 7 r rd 10 7 ld 10 7 l rd 10 7 rd 10 7 r rd 10 7

many, many more calls, until level = 0

d = dragonld = leftDragonrd = rightDragonr = right 90l = left 90

d 10 10


12. More Logo Commands

setpensize [ <width> <height> ] sets the width and height of the “pen” used by the turtl

e e.g. setpensize [10 10]

MSW Logo only uses the width value

continued


setscreencolor <colour> changes the colour of the display e.g. setscreencolor 14

setpos [<X> <Y> ] moves the turtle to the (X,Y) coordinate in the display e.g. setpos [300 300]


Example


Random

random <number> makes a random number between 0 and number e.g. random 50

makes a random number between 0 and 50

You can use random in many ways: setpencolor random 20 setscreencolor random 10 fd random 300

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13. Logo (Part 2)

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