BFOIT Introduction to Programming Predicates   (Conditional Execution)

Introduction

Think about how the TG performs your programs.  TO commands introduce new procedures that are remembered for execution at some later point in time. 

Instructions not surrounded by "TO" and "end (end lines) are executed from left to right on each line.  Lines are executed one after another down the page.  When an instruction to be performed is a procedure invocation, say to some procedure that you wrote, jLogo recalls your definition and performs the instructions in it, then picks up where it left off.  All of the instructions in your program get executed.

What if you only want some instructions to be interpreted when some condition is true?  As an example, what if you want to do something when an up-arrow key is pressed and something else when the down-arrow key is pressed?

In this lesson, you will learn

1. about another kind of thing called a boolean value, which can have two values: the symbol true or the symbol false
2. about primitive (built-in) operators in Logo which output a boolean value, called predicates
3. about the if command which expects two inputs, a predicate and a list of instructions.  The list of instructions is only executed if the predicate outputs a true value.

Boolean Values

Boolean values are used in computer programs to control what the program does.

Let's look at a few primitive (built-in) operators that Logo has and some examples of how you can use them.

Predicates - Operators Which Output Boolean Values

      predicate n. 1 a: something that is affirmed or
      denied of the subject in a proposition of logic
      b: a term designating a property or relation
 
                - Webster's New Collegiate Dictionary

So, in other words, a predicate is something that it either true or false - the boolean values.  In Logo, predicates are operators that output a boolean value.  Here are the first three predicates you'll need.

Procedure	Inputs	Description	Example
EQUAL? EQUALP	value1  value2	Outputs "true if value1 is equal to value2, "false otherwise.	EQUAL? 15 0
GREATER? GREATERP	value1  value2	Outputs "true if value1 is greater than value2, "false otherwise.	GREATER? 10 5
LESS? LESSP	value1  value2	Outputs "true if value1 is less than value2, "false otherwise.	LESS? 1 4

IF - a Command Which Optionally Executes a List of Instruction

if

<predicate>

<List-of-Instructions>

1. the command name: "if"
2. a <predicate>, and
3. a <List-of-Instructions>

And, the syntax of a <List-of-Instructions> is:

[

<Instructions>

]

1. an open square bracket,
2. <Instructions>, and
3. a close square bracket

Use of IF - Directing Turtle Through a Maze

Now, checkout the key numbers in Table 9.1 for the arrow keys.

Key Number	Keyboard Key
128	Down-Arrow
129	Left-Arrow
130	Right-Arrow
131	Up-Arrow
Table 9.1

Change your keyPressed procedure such that

• When the Up-Arrow key is pressed, the turtle moves forward.
• When the Down-Arrow key is pressed, the turtle moves backward.
• When the Left-Arrow key is pressed, the turtle rotates 90 degrees to the left.
• When the Right-Arrow key is pressed, the turtle rotates 90 degrees to the right.

Use of IF in our Graphical Programs

Figure 10.1

The program draws four sets of rays projecting out from a center point.  It starts out with a few very thick rays of random length but guaranteed to be the longest.  Each other set progressively draws more, but shorter and thinner, rays.  Here's the core part of the program.  It's missing the definitions of the randomColor, randomRange, and ray procedures.  randomColor outputs a random number in the legal range for color numbers.  randomInRange is the procedure you wrote as an exercise back in the Output Command lesson.  The ray procedure has an input that is the length of a line to draw; it just moves the turtle forward this amount and then back the amount.

  to randomRay :minLen :maxLen
    setc randomColor
    ray randomInRange :minLen :maxLen
    end

  to splat :nRay :minLen :maxLen :width
    setpensize :width
    repeat :nRay [randomRay :minLen :maxLen right quotient 360 :nRay]
    end

  to main
    hideturtle
    splat 10 110 150 40
    splat 20 80 110 20
    splat 40 50 80 7
    splat 80 1 0 50
    end

Look closely at the images.  What's ruining them is that the background color, white, is getting chosen.  In the first image, the large, thick rays are missing at 180 degrees, 252 degrees, and 324 degrees.  What we need is a random color, excluding the background color - white.

I can use an if command to check for a random output that is equal to the color number for white.  When the comparison is true, I can substitute another color I like.  Here is a procedure, an operator, which accepts two color numbers as inputs.  The first is a random color number.  The second input is a color number to substitute for white.  It outputs the input color as long as it's not white.  If the first input, the random color number matches white, it outputs the second input.

  to white
    output 7
    end

  to randomColorXWhite :color1 :color2
    if equal? :color1 white [ output :color2 ]
    output :color1
    end

Here is the TG applet, type in a version of splat that generates patterns that do not include white.

alt="Your browser understands the <APPLET> tag but isn't running the applet, for some reason." Your browser is completely ignoring the <APPLET> tag!

TurtleGraphics Applet

What if you don't really want to favor one particular color?  We could just scrap the idea of a substitution and invoke randomColor again to get a different random color number.  Here's that code:

    to randomColorXWhite :color
      if equal? :color white [ output randomColor ]
      output :color
      end
  

    if equal? :color white [ output randomColorXWhite randomColor ]
  

Does this work?  If not, why not?  If it does work, what's going on?

Figure 10.2

Our first pass at the interface consists of a strip of six colored boxes, a big empty frame, and a rectangle with the label "Clear Box" in it.  The first thing our program will do is draw all of these things.  Doing stuff like this is commonly called initialization of the program.

The program then waits for mouseClicked events.  When the mouse is clicked within one of the colored boxes, the program fills in the big frame with the color clicked-on.  Figure 10.3 shows what happens when the mouse is clicked within the red box.

Figure 10.3

For these lessons, I'm always going to use the name "init" for the procedure that initializes a program - that gets it ready for servicing events.  For this exercise, init needs to draw a strip of colored boxes, a "Clear Box" button, and a big empty frame.

Program Consistency/Symmetry Helps Writing Correct Programs

mouseClicked needs to check to see if the location where the mouse was clicked is in any of the colored boxes or the "Clear Box" button.  If so, the large frame is filled in with the appropriate color.  So, what's a common operation that's needed?  How about writing an operation named "mouseInRect?" which returns true if the mouse was clicked in a specified area of TurtleSpace? 

One of the most important things in writing this program is to make sure the procedure you write to draw a box or a rectangle is similar to the procedure that checks to see if the mouse is in it.  What do I mean?

 It will be much easier to get the program right if your procedure which draws a frame is defined with the same inputs as your procedure which draws a solid colored box, which is defined with the same inputs as your mouseInRect? procedure.  Here are my suggested procedure skeletons.

  to drawFrame :x :y :width :height
    ;draw a rectangle with lower left corner at :x, :y
    end

  to drawSolidRect :x :y :width :height
    ;draw a colored-in rectangle with lower left corner at :x, :y
    end

  to mouseInRect? :x :y :width :height
    ;output true if the mouse is within the rectangle who's lower
    ;left corner is at :x, :y or false if it's outside the rectangle
    end

By doing this, when you draw a red box, you use the same inputs when you invoke drawSolidRect as you do when you invoke mouseInRect? to check to see if the mouse is in the red box.

Determining If mouseClicked Is In An Interesting Area

  ;if the mouse's X coordinate is equal to or greater-than the :x input
  ;and the mouse's Y coordinate is equal to or greater-than the :y input
  ;and the mouse's X coordinate is less-than the sum of :x and :width
  ;and the mouse's Y coordinate is less-than the sum of :y and :height
  ;output "true

  to mouseInRect? :x :y :width :height
    if less? mousex :x [output "false]
    if less? mousey :y [output "false]
    ;if for mouse being to the right of the rectangle - outputs false
    ;if for mouse being above the rectangle - outputs false
    output "true
    end

  to numberInRange? :number :min :max
    if greater? :number :min [output "false]
    if less? :number :min [output "false]
    output "true
    end

to mouseInRect? :x :y :width :height
  ;if and numberinRange? mousex :x sum :x difference :width 1
  ;       numberInRange? mousey :y sum :y difference :height 1
  ;   output true
  output "false
  end

to mousexInWidth? :x :width
  if numberInRange? mousex :x sum :x difference :width 1 [output "true]
  output "false
  end

to mouseyInHeight? :y :height
  if numberInRange? mousey :y sum :y difference :height 1 [output "true]
  output "false
  end

to mouseInRect? :x :y :width :height
  if and mousexInWidth? :x :width mouseyInHeight? :y :height [output "true]
  output "false
  end

Operator	Inputs	Description	Example
AND	trueFalse1  trueFalse2	Outputs true if trueFalse1 AND trueFalse2 are true, false otherwise.  Input 1   Input 2   Output  true true true true false false false true false false false false	AND true true
NOT	trueFalse	Outputs false if trueFalse is true, else true if trueFalse is false - the opposite trueFalse value.	NOT true
OR	trueFalse1  trueFalse2	Outputs true if trueFalse1 OR trueFalse2 is true, false otherwise.  Input 1   Input 2   Output  true true true true false true false true true false false false	OR false false

Write a program that draws the X and Y axes and, when the mouse is clicked on your image, displays the name of the quadrant where the mouse was when it was clicked.  As an example, if the mouse is clicked above the X axis and to the right of the Y axis, you should display "Quadrant I"

Summary

New Logo Procedures Used In This Lesson
Name	Input	Description	Example
AND	trueFalse1  trueFalse2	Outputs true if trueFalse1 AND trueFalse2 are true, false otherwise.  Input 1   Input 2   Output  true true true true false false false true false false false false	AND true true
EQUAL? EQUALP	value1  value2	Outputs "true if value1 is equal to value2, "false otherwise.	EQUAL? 15 0
GREATER?  GREATERP	value1  value2	Outputs "true if value1 is greater than value2, "false otherwise.	GREATER? 10 5
IF	trueFalse instructionList	If the trueFalse input is true, the instructionList input is performed. If trueFalse is false, nothing is done.	IF LESS? MOUSEX 0 [ ... ]
LESS? LESSP	value1  value2	Outputs "true if value1 is less than value2, "false otherwise.	LESS? 1 4
NOT	trueFalse	Outputs false if trueFalse is true, else true if trueFalse is false - the opposite trueFalse value.	NOT true
OR	trueFalse1  trueFalse2	Outputs true if trueFalse1 OR trueFalse2 is true, false otherwise.  Input 1   Input 2   Output  true true true true false true false true true false false false	OR false false