Activity 1.5: Making Decisions

The Idea

Conditionals in Lua read almost like English, and there are only three keywords to remember: if, then, and end. There are no parentheses around the condition and no braces anywhere.

local charge = 45

if charge >= 80 then
  print("READY")
elseif charge >= 40 then
  print("CAUTION")
else
  print("GROUNDED")
end

CAUTION

Note the spelling: elseif is one word. Writing else if is not an error, but it opens a second if that needs its own end, and the error you get points at the bottom of your file rather than at the real problem.

Comparing things

The comparisons are the usual ones with one difference: "not equal" is ~=, not !=. There is also only one equals-comparison, == — Lua has no === because it never needed one.

print(1 == 1.0, "1" == 1, 3 ~= 4)

true  false  true

Values of different types are never equal, so "1" == 1 is false with no conversion attempted. That is the strictness JavaScript needs === for, and Lua just does it by default.

The rule that catches everyone

Here is the one to tattoo somewhere. Only false and nil are false. Everything else is true.

if 0 then print("0 is true") end
if "" then print("empty string is true") end
if not nil then print("nil is false") end
if not false then print("false is false") end

0 is true
empty string is true
nil is false
false is false

Zero is true. An empty string is true. An empty table is true. If you have written Python or JavaScript, both of those treat zero and empty text as false, and code you carry over from them will take the wrong branch without ever producing an error.

It is worth seeing why this is the better design. if score then in Lua asks exactly one question — does a score exist? A player with zero points still has a score. In a language where zero is false, that same line quietly means two different things at once, and you have to write extra code to tell them apart.

and, or, not

The logical operators are words, not symbols. There is no &&, no ||, and no !.

They also do something more interesting than answering true or false. They hand back one of the actual values you gave them.

print(nil or "default")
print(false and "never")
print(5 and 10)
print(nil and 10)

default
false
10
nil

Read them as questions about which value survives. a or b gives you a unless a is false or nil, in which case you get b. That is why name or "stranger" is how Lua does default values, and you will use it constantly from Activity 1.7 onward.

Chain them and you get a stand-in for the ternary operator Lua does not have.

local hp = 0

print(hp > 0 and "alive" or "down")

down

One warning about that pattern. Because it relies on truthiness, it breaks if the middle value can itself be false or nil — you would fall through to the third value even when the condition was true. Use it for strings and numbers, and write a real if for anything else.

Build

Start a fresh main.lua. You are writing the check a docking bay would run before letting a ship in.

Task 1: Grade the charge level

Store a charge percentage, a crew count, and a hull-breach boolean.
Report READY at 80 or above, CAUTION at 40 or above, and GROUNDED otherwise.
Pick a charge value that lands in the middle branch.

1
-- main.lua - Skynest docking clearance
2

3
local charge = 62
4
local crew = 4
5
local hullBreach = false
6

7
if charge >= 80 then
8
  print("Charge: READY")
9
elseif charge >= 40 then
10
  print("Charge: CAUTION")
11
else
12
  print("Charge: GROUNDED")
13
end

Charge: CAUTION

Task 2: Combine three conditions

Clear the ship for docking only if the charge is at least 40, and there are at least 2 crew, and there is no hull breach.
Use not for the breach rather than comparing it to false. It reads better and it is what Lua programmers write.
Print CLEARED or DENIED.

1
-- main.lua - Skynest docking clearance
2

3
local charge = 62
4
local crew = 4
5
local hullBreach = false
6

7
if charge >= 80 then
8
  print("Charge: READY")
9
elseif charge >= 40 then
10
  print("Charge: CAUTION")
11
else
12
  print("Charge: GROUNDED")
13
end
14

15
if charge >= 40 and crew >= 2 and not hullBreach then
16
  print("Docking: CLEARED")
17
else
18
  print("Docking: DENIED")
19
end

Charge: CAUTION
Docking: CLEARED

Task 3: Prove the truthiness rule to yourself

Add an assignment variable set to nil and a passengers variable set to 0.
Test each one directly with if assignment then — no comparison.
Predict both results before you run it. One of the two will surprise you if you came from another language, and that surprise is the whole point of the task.

1
-- main.lua - Skynest docking clearance
2

3
local charge = 62
4
local crew = 4
5
local hullBreach = false
6

7
if charge >= 80 then
8
  print("Charge: READY")
9
elseif charge >= 40 then
10
  print("Charge: CAUTION")
11
else
12
  print("Charge: GROUNDED")
13
end
14

15
if charge >= 40 and crew >= 2 and not hullBreach then
16
  print("Docking: CLEARED")
17
else
18
  print("Docking: DENIED")
19
end
20

21
local assignment = nil
22
local passengers = 0
23

24
if assignment then
25
  print("Orders:", assignment)
26
else
27
  print("Orders: none on file")
28
end
29

30
if passengers then
31
  print("Passengers recorded:", passengers)
32
end

Challenge (Optional)

Replace the CLEARED / DENIED block with a single line using and and or.
Add a rule that a ship with zero passengers is still allowed to dock, but one with no passenger record at all is not. This is only expressible because zero and nil are different things.

Check Your Work

Your finished file should produce this.

Charge: CAUTION
Docking: CLEARED
Orders: none on file
Passengers recorded:  0

That last line is the one worth remembering. The passenger count of zero passed the test. Nothing about it looked conditional, and that is exactly how it will bite you later if you forget.

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