Logo Lesson 3

TBE 540

Fall 2004

Farah Fisher

Prerequisites for Lesson 3 Before beginning this lesson, the student

must be able to… Use simple Logo commands to make a shape. Create and edit a procedure. Use the following commands appropriately

POTS SAVE LOAD

Objectives for Lesson 3 After completing this lesson, the student will

be able to… Move the turtle to any screen position. Determine the turtle’s screen position. Explain the term variable. Create procedures with variable input, using

MAKE and RANDOM.

Logo Screen Positions The position of the Logo turtle on the

screen is described by two numbers - a horizontal and a vertical coordinate.

The center of the screen is the turtle’s home. Its designation is [0 0], and all other coordinates are measured from there.

It is very much like graphing in algebra.

Logo Screen Positions The first of the two numbers describes

the horizontal distance in turtle steps right or left of the center.

For example, in the position [10 15], the 10 represents 10 turtle steps to the right of the center.

In the position [-25 10], the -25 represents 25 turtle steps to the left of the center.

Logo Screen Positions The second of the two numbers

describes the vertical distance in turtle steps above or below the center.

For example, in the position [10 15], the 15 represents 15 turtle steps above the center.

In the position [25 -10], the -10 represents 10 turtle steps below the center.

Guess the Positions Make your best guess about the

positions. Click to see the answers.

1

3

2

4

Guess the Positions Possible screen positions (depending

on Logo version)

1

3

2

4

[-100 50]

[ 150 75]

[0 -50]

[ 25 -25]

Position Commands PR POS is the command used to print (in the

command window) the current position of the turtle.

SETPOS [number number] or SETXY (number number) sets the position of the turtle.

SETX number changes only the first number SETY number changes only the second

number HOME returns the turtle to [ 0 0 ]

Challenge What shape do you think would result

from these commands (assume turtle starts at [0 0])? Click to see the answer. SETPOS [ 0 25 ] SETPOS [ 25 25 ] SETPOS [ 25 0 ] SETPOS [ 0 0 ] or HOME

Challenge The commands make a square!

[0 25] [25 25]

[25 0][0 0]

Logo Variables A variable is a letter or word that

represents a number that can vary (thus the term variable).

In Logo, variables often have a colon in front of the letter or word.

Sample Logo variables - :L2 :WIDTH :NAME :X

Logo Variables Variables must be assigned a value. The Logo command MAKE is often

used to “fill up” a variable. For example, the command below puts

the number 17 into the variable called :NUM (notice the quote and lack of colon) MAKE “NUM 17

Logo Variables See if you can determine the final value

of :Z after the commands below. (NOTE: * means multiply, / means divide)

MAKE “Z 25 MAKE “Z :Z * 2 MAKE “Z :Z + 10 MAKE “Z :Z / 2

Logo Variables :Z becomes 30 after several

calculations. MAKE “Z 25 (:Z starts at 25)

MAKE “Z :Z * 2 (:Z is multiplied by 2, :Z = 50)

MAKE “Z :Z + 10 (10 is added to :Z, :Z = 60)

MAKE “Z :Z / 2 (:Z is divided by 2, :Z = 30)

Logo Variables After you define the value of a variable,

you can use it in Logo commands in place of any number.

Examples: FD :X RT :D2 REPEAT 4 [ FD :SIZE RT 90 ]

RANDOM Logo variables can also be filled with random

numbers. The RANDOM command returns a number

from 0 to a set limit. Example: RANDOM 10 returns a number

between 0 and 9. Example: 1 + RANDOM 100 returns a

number between 1 and 100.

RANDOM Examples of MAKE with RANDOM MAKE “Z RANDOM 15

:Z becomes a number between 0 and 14 MAKE “H23 1 + RANDOM 10

:H23 becomes a number between 1 and 10 MAKE “LENGTH 10 + RANDOM 50

:LENGTH becomes a number between 10 and 59 (10 + 0 to 10 + 49)

Samples of Procedures with Variables (MAKE/RANDOM) A variable size SQUARE

TO SQUARE MAKE “SIZE 10 + RANDOM 100 REPEAT 4 [FD :SIZE RT 90] END

Samples of Procedures with Variables (MAKE/RANDOM) A variable size TRIANGLE

TO TRIANGLE MAKE “SIZE 25 + RANDOM 50 REPEAT 3 [FD :SIZE RT 120] END

Samples of Procedures with Variables (MAKE/RANDOM) A variable size RECTANGLE

TO RECTANGLE MAKE “S1 25 + RANDOM 50 MAKE “S2 25 + RANDOM 100 REPEAT 2 [FD :S1 RT 90 FD :S2 RT 90] END

Another Way to Fill a Variable You can also fill up a variable by

defining it when you “run” a procedure. Look at this procedure:

TO SQUARE :S REPEAT 4 [FD :S RT 90] END

Somehow, the :S needs a value.

Another Way to Fill a Variable To start this procedure…

TO SQUARE :S REPEAT 4 [FD :S RT 90] END

…you would type SQUARE 50 or SQUARE 100 or SQUARE 87

The number you type after the procedure name fills the variable.

Another Way to Fill a Variable Another example:

TO TRIANGLE :F REPEAT 3 [FD :F RT 120] END

To draw a triangle with sides of 50, type TRIANGLE 50 to start the procedure.

Another Way to Fill a Variable Consider this example:

TO RECTANGLE :L :W REPEAT 2 [FD :L RT 90 FD :W RT 90] END

You would need two numbers after the name of the procedure (the first becomes :L and the second becomes :W): RECTANGLE 50 100 RECTANGLE 40 40

Self Check Lesson 3 In the screen below, what is the best

estimate of the turtle’s position? [-50 50] [50 -50] [50 50] [-50 -50]

Self Check Lesson 3 In the screen below, what is the best

estimate of the turtle’s position? [-50 50] [50 -50] [50 50] [-50 -50]

-50

50

Self Check Lesson 3 Which of the following could not be the

name of a Logo variable? :Z :The Size :SIZE :B123

Self Check Lesson 3 Which of the following could not be the

name of a Logo variable? :Z :The Size {no spaces allowed} :SIZE :B123

Self Check Lesson 3 What will be the value of :BIG after the

Logo commands below? MAKE “BIG 5 MAKE “BIG :BIG + 25 MAKE “BIG :BIG - 7

Self Check Lesson 3 What will be the value of :BIG after the

Logo commands below? MAKE “BIG 5 {:BIG=5} MAKE “BIG :BIG + 25

{:BIG=30} MAKE “BIG :BIG - 7

{:BIG=23}

Self Check Lesson 3 Suppose you want a randomly chosen

number between 1 and 10. Which of the following would generate that number? RANDOM 10 RANDOM 1 + 10 RANDOM 10 + 1

Self Check Lesson 3 Suppose you want a randomly chosen

number between 1 and 10. Which of the following would generate that number? RANDOM 10 {0 TO 9} RANDOM 1 + 10 {10.0 TO 10.999} RANDOM 10 + 1

Self Check Lesson 3 Suppose you have entered the procedure

below. What would you type to use it to draw a SQUARE with sides of 50? TO SQUARE :X REPEAT 4 [FD :X RT 90] END

SQUARE SQUARE 50 TO SQUARE 50

Self Check Lesson 3 Suppose you have entered the procedure

below. What would you type to use it to draw a SQUARE with sides of 50? TO SQUARE :X REPEAT 4 [FD :X RT 90] END

SQUARE SQUARE 50 TO SQUARE 50

Try Some Exercises Practice with procedures at

http://www.csudh.edu/fisher/tbe540/LW3A.htm Check out screen positions at

http://www.csudh.edu/fisher/tbe540/LW3.htm Try the hand-on exercise at

http://www.csudh.edu/fisher/tbe540/LEX3.htm