local function shout(text)
    print(text .. "!")
end

shout("hi")

local function shout(text)
    print(text .. "!")
end

shout("hi")

local function double(n)
    return n * 2
end

print(double(21))

local function double(n)
    return n * 2
end

print(double(21))

42

local function shout(text)
    print(text .. "!")
end

local result = shout("hi")
print("result is " .. tostring(result))

local function shout(text)
    print(text .. "!")
end

local result = shout("hi")
print("result is " .. tostring(result))

hi!
result is nil

local function makeIt()
    local hidden = 99
    return hidden
end

print(makeIt())
print("outside: " .. tostring(hidden))

local function makeIt()
    local hidden = 99
    return hidden
end

print(makeIt())
print("outside: " .. tostring(hidden))

99
outside: nil

local score = 10

local function bump(score)
    score = score + 5
    return score
end

print(bump(score), score)

local score = 10

local function bump(score)
    score = score + 5
    return score
end

print(bump(score), score)

15  10

local function minMax(a, b)
    if a < b then
        return a, b
    else
        return b, a
    end
end

local lo, hi = minMax(9, 4)
print(lo, hi)

local function minMax(a, b)
    if a < b then
        return a, b
    else
        return b, a
    end
end

local lo, hi = minMax(9, 4)
print(lo, hi)

4  9

'F' → Fullscreen

Build

Terminal today. Copy template-console.

Task 1: Two Functions, Two Jobs

Copy template-console to 1-22-return and write both:

1
local function shout(text)
2
    print(text .. "!")
3
end
4

5
local result = shout("hi")
6
print("result is " .. tostring(result))

hi!
result is nil

Then write double from the slides and store its answer in a variable. Write down what each of the two functions gave back.

Task 2: Use the Answer

1
local function area(w, h)
2
    return w * h
3
end
4

5
local kitchen = area(3, 4)
6
local hall = area(2, 6)
7

8
print(kitchen + hall)

24

The two calls became numbers and then got added together. Now call area inside another call — try print(area(area(2, 3), 4)) and work out the answer before you run it.

Task 3: Write Three of Your Own

Each one takes something and hands something back:

• celsius(f) — turns Fahrenheit into Celsius
• isEven(n) — hands back true or false
• biggest(a, b, c) — hands back the largest of three

isEven can be written with an if, and it can also be written in one line with no if at all. Find the second way if you can.

Task 4: Scope

Run both of these and record the output:

1
local function makeIt()
2
    local hidden = 99
3
    return hidden
4
end
5

6
print(makeIt())
7
print("outside: " .. tostring(hidden))

1
local score = 10
2

3
local function bump(score)
4
    score = score + 5
5
    return score
6
end
7

8
print(bump(score), score)

The second one is the surprising one. The function changed score and the outer score did not move. Write down both numbers and explain them to the person next to you before you read on.

Task 5: Break It on Purpose (Required)

Write a function that computes something and forgets to return it:

local function noReturn(n)
    n * 2
end

Error: Syntax error: main.lua:2: '=' expected near '*'

This one will not even start. Lua reads n * 2 on its own and expects it to be the left side of an assignment. Copy the message down.

Then make the forgetful version that does run: put local answer = n * 2 in the function with no return, and use it. That one has no error at all, and it is the version you will actually write by accident.

Challenge (Optional): Two Answers

Write minMax(a, b) from the slides, then write stats(t) that takes a table of numbers and hands back three things: the total, the count, and the average.

It is a traversal from session 20 with a return at the end.

Same Idea, Exam Notation

The exam's procedures return values too, and the capture is written with the arrow you already know.

local function area(w, h)
    return w * h
end

local kitchen = area(3, 4)

PROCEDURE area(w, h)
{
  RETURN(w * h)
}

kitchen ← area(3, 4)

RETURN takes parentheses and Lua's return does not. It also behaves the same way in both: it can appear anywhere, and it ends the procedure immediately.

Checkpoint

• You recorded what a function with no return hands back
• Two calls to area were added together
• All three of your own functions return values
• Both scope results are written down, including the two numbers from the shadowing one
• Both versions of the forgotten return were run

Reflection

Answer the following questions before submitting your work.

1. A function that prints and a function that returns look similar and are not. Explain the difference, and describe what goes wrong if you try to use the answer from the printing one.
2. bump(score) returned 15 and the outer score stayed 10. Explain why, and say what you would have to write to actually change the outer one.
3. One forgotten return gave a syntax error and the other gave no error at all. Explain the difference between the two versions, and say which one you would rather write by mistake.

Submit

Submit the required files to the appropriate dropbox.

Activity Complete