Activity 1.11

Asking Two Things at Once

Key Concepts

and, or, not

Fixing the Stuck Ball

Which One You Actually Meant

A Promise From Session 9

A ball that starts outside the window gets stuck vibrating at the wall, forever.

A Promise From Session 9

A ball that starts outside the window gets stuck vibrating at the wall, forever.

Two decisions undoing each other, sixty times a second, and no error at all.

Today you fix it, and the fix is four words.

Three New Words

print(true and false)
print(true or false)
print(not true)

Three New Words

print(true and false)
print(true or false)
print(not true)

false
true
false

They take booleans and produce a boolean. Same kind of value in, same kind out.

What Each One Means

and

True only when both sides are true.

or

True when either side is true, including both.

not

Flips one value. true becomes false.

or in a program is not the or in "soup or salad". Both is allowed.

Every Case, on One Slide

a	b	a and b	a or b
true	true	true	true
true	false	false	true
false	true	false	true
false	false	false	false

Four rows. That is the entire definition, and there is nothing else to learn about them.

The Sides Are Usually Comparisons

print(5 > 3 and 2 > 1)
print(5 > 3 or 2 > 9)
print(not (5 > 3))

true
true
false

Each comparison produces a boolean first. Then and looks at the two answers.

Now the Stuck Ball

if x > 800 - radius then
    speedX = -speedX
end

This asks is it past the wall? A ball that starts past the wall answers yes on every single frame.

Now the Stuck Ball

if x > 800 - radius and speedX > 0 then
    speedX = -speedX
end

This asks is it past the wall and still heading that way?

Once it has turned around, the second half is false, so it stops flipping. The ball flies back in.

What You Are Building

Same position, different direction. The and disagrees with the or.

Today's Objectives

• Use and, or and not
• Read a truth table
• Fix a bug that a single condition cannot fix
• Tell which of and or or a problem needs

Key Terms

Logical operator

and, or and not. They combine booleans.

Compound condition

A condition built from more than one comparison.

Truth table

Every combination of inputs, with the answer for each.

'F' → Fullscreen

Build

Task 1: Fill In the Table

Copy template-console to 1-11-logic. Fill in the truth table on your sheet first, then write the twelve lines that check it:

1
print(true and true)
2
print(true and false)
3
print(false and true)
4
print(false and false)

Then the same four for or, and print(not true) and print(not false).

Ten lines, ten answers, and you should already know all of them. Mark any you got wrong.

Task 2: Fix the Stuck Ball

Copy 1-09-bounce to 1-11-bounce. First reproduce the bug: set the starting x to 900 and run it. Nothing appears.

Now add four words:

1
function love.update(dt)
2
    x = x + speedX * dt
3

4
    if x > 800 - radius and speedX > 0 then
5
        speedX = -speedX
6
    end
7

8
    if x < radius and speedX < 0 then
9
        speedX = -speedX
10
    end
11
end

Run it again. The ball flies in from off-screen and bounces normally forever.

Task 3: Watch All Three

Put the starting x back to 100 and replace love.draw with a readout:

1
function love.draw()
2
    if x > 400 and speedX > 0 then
3
        love.graphics.setColor(0.2, 0.8, 0.3)
4
    else
5
        love.graphics.setColor(0.6, 0.6, 0.6)
6
    end
7
    love.graphics.circle("fill", x, y, radius)
8

9
    love.graphics.setColor(1, 1, 1)
10
    love.graphics.setFont(font)
11
    love.graphics.print("x > 400              " .. tostring(x > 400), 40, 340)
12
    love.graphics.print("speedX > 0           " .. tostring(speedX > 0), 40, 380)
13
    love.graphics.print("both (and)           " .. tostring(x > 400 and speedX > 0), 40, 430)
14
    love.graphics.print("either (or)          " .. tostring(x > 400 or speedX > 0), 40, 470)
15
    love.graphics.print("not past middle      " .. tostring(not (x > 400)), 40, 510)
16
end

Watch it for a full round trip. Find the moment when the top two lines disagree, and write down what each of the bottom three says at that moment.

Task 4: Swap the and for an or

Change the color condition, and only that one:

if x > 400 or speedX > 0 then

The ball is now green for three quarters of its round trip. Work out which quarter it is still gray for, and write down why.

or is a much weaker demand than and. Most of the time, one of two things is true.

Challenge (Optional): In the Box

Give the ball a speedY so it bounces around the whole window, then turn it a different color only when it is in the middle square — between 300 and 500 across and between 200 and 400 down.

That is four comparisons joined by three ands. Write it on your sheet before you type it.

Challenge (Optional): Say It With not

Rewrite your middle-square condition so it says the same thing using not and or instead of and.

Test that both versions behave identically, then say which one you would rather read in six months.

Same Idea, Exam Notation

Three words, in capitals, and they work exactly as they do in Lua.

if lives > 0 and score >= 100 then
    print("Next level")
end

if lives == 0 or timeLeft == 0 then
    print("Game over")
end

if not paused then
    print("Running")
end

IF(lives > 0 AND score ≥ 100)
{
  DISPLAY("Next level")
}

IF(lives = 0 OR timeLeft = 0)
{
  DISPLAY("Game over")
}

IF(NOT paused)
{
  DISPLAY("Running")
}

The truth tables are identical, so anything you worked out today is true on the exam as well. The differences are all in the pieces you already know — = for equality and ≥ for greater-or-equal inside the conditions.

Checkpoint

• The truth table on your sheet was filled in before running anything
• 1-11-bounce recovers from a start position of 900
• The readout shows all three operators changing as the ball moves
• You can say when and is true but or is not — and when it is the other way round

Reflection

Answer the following questions before submitting your work.

1. The stuck ball could not be fixed by adding another if. Explain what the old condition was actually asking, what the new one asks, and why only the second one works.
2. Swapping and for or in one line changed almost nothing on screen for most of the trip. Explain why a wrong logical operator is harder to notice than a wrong comparison.
3. Describe a rule from outside programming — a school rule, a game rule, an entry requirement — that needs and, and one that needs or. Say how you can tell them apart from the wording.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete