local scores = {90, 85, 78}

print(#scores)

local scores = {90, 85, 78}

print(#scores)

3

local scores = {90, 85, 78}

print(scores[#scores])

local scores = {90, 85, 78}

print(scores[#scores])

78

local scores = {90, 85, 78}

print(scores[4])

local scores = {90, 85, 78}

print(scores[4])

nil

local scores = {90, 85, 78}
local fourth = scores[4]

print("Fourth score: " .. fourth)

local scores = {90, 85, 78}
local fourth = scores[4]

print("Fourth score: " .. fourth)

Error: main.lua:4: attempt to concatenate local 'fourth' (a nil value)

  main.lua:4: in main chunk

local scores = {90, 85, 78}
local backup = scores

backup[1] = 0

print("backup[1] is " .. backup[1])
print("scores[1] is " .. scores[1])

local scores = {90, 85, 78}
local backup = scores

backup[1] = 0

print("backup[1] is " .. backup[1])
print("scores[1] is " .. scores[1])

backup[1] is 0
scores[1] is 0

'F' → Fullscreen

Build

No window today — everything here is easier to see as text. Copy template-console, rename it 1-19-lists, and work in main.lua.

Task 1: How Many, and the Last One

1
local scores = {90, 85, 78}
2

3
print("There are " .. #scores .. " scores")
4
print("The last one is " .. scores[#scores])

There are 3 scores
The last one is 78

scores[#scores] looks strange and reads simply: the element at the position "how many there are." Add a fourth score to the table and run it again without changing anything else. Both lines follow along.

Task 2: Break It on Purpose, Then Fix It

First, make it fail. Type this and run it:

1
local scores = {90, 85, 78}
2
local fourth = scores[4]
3

4
print("Fourth score: " .. fourth)

Error: main.lua:4: attempt to concatenate local 'fourth' (a nil value)

  main.lua:4: in main chunk

Write down which line the error names, and which line the mistake is on. They are not the same line, and that is the point of the whole session.

Now fix it, by checking before you use it:

1
local scores = {90, 85, 78}
2
local wanted = 4
3

4
if scores[wanted] == nil then
5
    print("There is no score number " .. wanted)
6
else
7
    print("Score number " .. wanted .. " is " .. scores[wanted])
8
end

There is no score number 4

Change wanted to 2 and run it again. The same program now takes the other branch.

Task 3: The Backup That Is Not a Backup

Predict first, on your sheet. Then run it.

1
local scores = {90, 85, 78}
2
local backup = scores
3

4
backup[1] = 0
5

6
print("backup[1] is " .. backup[1])
7
print("scores[1] is " .. scores[1])

Write down what you expected and what happened. If you were surprised, say so — that is worth marks and it is worth more than being right.

Task 4: Make a Real Copy

One table cannot become two by giving it another name. To get a real copy you have to move the elements across one at a time:

1
local scores = {90, 85, 78}
2
local copy = {}
3

4
for i = 1, #scores do
5
    copy[i] = scores[i]
6
end
7

8
copy[1] = 0
9

10
print("copy[1] is   " .. copy[1])
11
print("scores[1] is " .. scores[1])

copy[1] is   0
scores[1] is 90

scores[1] is still 90. This time there really are two tables, and the loop is what made the second one.

Challenge (Optional): Guard the Whole Thing

Write a program that asks for a score number with io.read(), and answers sensibly for any number typed — including 0, including 99, including a word.

Remember what session 4 taught you about what io.read() hands back, and what tonumber does with something that is not a number.

Same Idea, Exam Notation

Last session the exam agreed with Lua. This session it disagrees twice, and these two are the ones most likely to cost you points.

local scores = {90, 85, 78}
local backup = scores
print(#scores)
print(scores[4])

scores ← [90, 85, 78]
backup ← scores
DISPLAY(LENGTH(scores))
DISPLAY(scores[4])

1. Assignment makes a copy on the exam

The reference sheet says aList ← bList assigns a copy of the list. So on the exam, backup ← scores gives you two separate lists, and changing one leaves the other alone.

In Lua it does not. You proved that in task 3.

2. An out-of-range index ends the program on the exam

The sheet is explicit: for any list operation, an index below 1 or above the length produces an error message and the program terminates. There is no nil, and nothing after that line runs.

In Lua you get nil and the program keeps going — which is why task 2's error pointed at the wrong line.

Checkpoint

• Task 1 printing the count and the last element
• The two line numbers from task 2 written down — the one the error named, and the one the mistake was on
• A working safe check that answers for a missing score
• Your task 3 prediction, written before you ran it
• A real copy in task 4, proved by scores[1] still being 90

Reflection

Answer the following questions before submitting your work.

1. In task 2 the error message named a different line than the mistake. Explain why, and say what you would look for next time you see an error about nil.
2. You changed backup and scores changed too. Explain what local backup = scores actually did.
3. On the exam, the same line makes a real copy. Describe a question you could get wrong by answering from Lua habit, and say which answer the exam wants.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete