Logo Lesson 3
TBE 540
Fall 2004
Farah Fisher
Prerequisites for Lesson 3
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Before beginning this lesson, the student
must be able to…
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Use simple Logo commands to make a shape.
Create and edit a procedure.
Use the following commands appropriately
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POTS
SAVE
LOAD
Objectives for Lesson 3
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After completing this lesson, the student will
be able to…
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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
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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
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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
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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
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Make your best guess about the
positions. Click to see the answers.
2
1
4
3
Guess the Positions
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Possible screen positions (depending
on Logo version)
[-100 50]
2
1
[ 150 75]
4
3
[0 -50]
[ 25 -25]
Position Commands
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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
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What shape do you think would result
from these commands (assume turtle
starts at [0 0])? Click to see the answer.
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SETPOS [ 0 25 ]
SETPOS [ 25 25 ]
SETPOS [ 25 0 ]
SETPOS [ 0 0 ] or HOME
Challenge
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The commands make a square!
[0 25]
[0 0]
[25 25]
[25 0]
Logo Variables
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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
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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
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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
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: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
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After you define the value of a variable,
you can use it in Logo commands in
place of any number.
Examples:
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FD :X
RT :D2
REPEAT 4 [ FD :SIZE RT 90 ]
RANDOM
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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
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Examples of MAKE with RANDOM
MAKE “Z RANDOM 15
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MAKE “H23 1 + RANDOM 10
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:Z becomes a number between 0 and 14
:H23 becomes a number between 1 and 10
MAKE “LENGTH 10 + RANDOM 50
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:LENGTH becomes a number between 10 and 59
(10 + 0 to 10 + 49)
Samples of Procedures with
Variables (MAKE/RANDOM)
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A variable size SQUARE
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TO SQUARE
MAKE “SIZE 10 + RANDOM 100
REPEAT 4 [FD :SIZE RT 90]
END
Samples of Procedures with
Variables (MAKE/RANDOM)
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A variable size TRIANGLE
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TO TRIANGLE
MAKE “SIZE 25 + RANDOM 50
REPEAT 3 [FD :SIZE RT 120]
END
Samples of Procedures with
Variables (MAKE/RANDOM)
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A variable size RECTANGLE
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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
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You can also fill up a variable by
defining it when you “run” a procedure.
Look at this procedure:
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TO SQUARE :S
REPEAT 4 [FD :S RT 90]
END
Somehow, the :S needs a value.
Another Way to Fill a Variable
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To start this procedure…
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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
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Another example:
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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
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Consider this example:
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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):
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RECTANGLE 50 100
RECTANGLE 40 40
Self Check Lesson 3
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In the screen below, what is the best
estimate of the turtle’s position?
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[-50 50]
[50 -50]
[50 50]
[-50 -50]
Self Check Lesson 3
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In the screen below, what is the best
estimate of the turtle’s position?
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[-50 50]
[50 -50]
[50 50]
[-50 -50]
50
-50
Self Check Lesson 3
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Which of the following could not be the
name of a Logo variable?
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:Z
:The Size
:SIZE
:B123
Self Check Lesson 3
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Which of the following could not be the
name of a Logo variable?
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:Z
:The Size
:SIZE
:B123
{no spaces allowed}
Self Check Lesson 3
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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
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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
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Suppose you want a randomly chosen
number between 1 and 10. Which of the
following would generate that number?
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RANDOM 10
RANDOM 1 + 10
RANDOM 10 + 1
Self Check Lesson 3
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Suppose you want a randomly chosen
number between 1 and 10. Which of the
following would generate that number?
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RANDOM 10
RANDOM 1 + 10
RANDOM 10 + 1
{0 TO 9}
{10.0 TO 10.999}
Self Check Lesson 3
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Suppose you have entered the procedure
below. What would you type to use it to draw
a SQUARE with sides of 50?
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TO SQUARE :X
REPEAT 4 [FD :X RT 90]
END
SQUARE
SQUARE 50
TO SQUARE 50
Self Check Lesson 3
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Suppose you have entered the procedure
below. What would you type to use it to draw
a SQUARE with sides of 50?
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TO SQUARE :X
REPEAT 4 [FD :X RT 90]
END
SQUARE
SQUARE 50
TO SQUARE 50
Try Some Exercises
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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