print("T-minus 5")
print("T-minus 4")
print("T-minus 3")
print("T-minus 2")
print("T-minus 1")
print("Liftoff")

print("T-minus 5")
print("T-minus 4")
print("T-minus 3")
print("T-minus 2")
print("T-minus 1")
print("Liftoff")

while condition do

end

local count = 5

while count > 0 do
    print("T-minus " .. count)
    count = count - 1
end

print("Liftoff")

local count = 5

while count > 0 do
    print("T-minus " .. count)
    count = count - 1
end

print("Liftoff")

T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1
Liftoff

local count = 5

while count > 0 do
    print("T-minus " .. count)
end

local count = 5

while count > 0 do
    print("T-minus " .. count)
end

T-minus 5
T-minus 5
T-minus 5
T-minus 5
T-minus 5
...

repeat

until condition

local m = 100

repeat
    print("runs once anyway, m is " .. m)
until m > 0

local m = 100

repeat
    print("runs once anyway, m is " .. m)
until m > 0

runs once anyway, m is 100

'F' → Fullscreen

Build

Back to the terminal today. Copy template-console, not template-game.

Task 1: Count Down

Copy template-console to 1-15-countdown and write it:

1
local count = 5
2

3
while count > 0 do
4
    print("T-minus " .. count)
5
    count = count - 1
6
end
7

8
print("Liftoff")

T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1
Liftoff

Then change it to count down from 20, and then to count up to 20. The second one needs all three parts changed — work out which three before you type.

Task 2: Break It on Purpose (Required)

Delete the line that changes count:

1
local count = 5
2

3
while count > 0 do
4
    print("T-minus " .. count)
5
end

T-minus 5
T-minus 5
T-minus 5
T-minus 5
T-minus 5
...

Write down what the terminal did, and how long you let it run before you stopped it. Then put the line back.

Task 3: Repeat Until

Copy the template again to 1-15-guess:

1
local secret = 7
2
local guess = 0
3

4
repeat
5
    io.write("Guess my number: ")
6
    guess = tonumber(io.read())
7
until guess == secret
8

9
print("You got it.")

Guess my number: 3 [Enter]
Guess my number: 9 [Enter]
Guess my number: 7 [Enter]
You got it.

Try to rewrite this with while and see what goes wrong. You have to invent a first guess before the loop starts, and the person has not guessed anything yet.

The local guess = 0 line is there for a reason. Work out what breaks without it.

Task 4: Count the Guesses

Add a tries variable that starts at 0, goes up by one inside the loop, and gets reported at the end: "You got it in " .. tries .. " tries."

This is the same shape as last session's hits. It is also the shape of every score in every program you will write this year.

Challenge (Optional): Hotter and Colder

Inside the loop, tell the guesser whether they were too high or too low. Two decisions inside a loop, which is the first time you have nested one of these boxes inside another.

Challenge (Optional): Let the Computer Pick

Replace local secret = 7 with a random number between 1 and 20:

math.randomseed(os.time())
local secret = math.random(1, 20)

math.randomseed(os.time()) has to be there, and it has to be first. Take it out and run the program three times — write down what you notice.

Same Idea, Exam Notation

This is the panel to pay attention to this quarter. The exam has two loops, and one of them is not while.

local count = 5

while count > 0 do
    print(count)
    count = count - 1
end

count ← 5

REPEAT UNTIL(count = 0)
{
  DISPLAY(count)
  count ← count - 1
}

There is no WHILE on the exam sheet

The two loops are REPEAT n TIMES and REPEAT UNTIL(condition). That is all of them.

So the exact Lua translation of REPEAT UNTIL(c) is while not c do — not repeat ... until c.

The two differ only when the condition is already true at the start. Then the exam's loop does nothing and Lua's repeat runs once anyway — which is exactly the case a trace question likes to ask about.

To translate a while loop, flip the condition. while count > 0 becomes REPEAT UNTIL(count = 0). Keeping the condition the same is the single most common mistake on these questions.

REPEAT n TIMES is the other one, and it is next session — along with the fact that it does not give you a counter.

Checkpoint

• 1-15-countdown counts down from 20 and up to 20
• You made a loop run forever and stopped it yourself, and you know which button does it
• 1-15-guess keeps asking until the guess is right
• It reports how many tries it took
• You can say why this one is repeat and not while

Reflection

Answer the following questions before submitting your work.

1. Name the three parts every loop needs, and say which one you deleted in task 2. Explain why deleting it produced no error message.
2. The guessing loop must ask at least once, so it uses repeat. Describe a situation where a loop should be able to run zero times, and say which loop you would use for it.
3. while count > 0 becomes REPEAT UNTIL(count = 0) in exam notation. Explain why the condition had to change, and what would happen to a program that kept it the same.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete