Operators and Logic

Written: 2026-10-14

Learning objectives

• You do arithmetic with +, -, *, /, //, %, ^, and unary -.
• You join strings with .. and measure length with #.
• You compare values and explain what a boolean result means.
• You combine conditions with and, or, and not.
• You predict truthiness and convert with tonumber and tostring.

Prerequisites

• Read First Script. You can run a Script and read output.
• Read Variables and Values. You create variables with local and print them.

Arithmetic operators

operators are symbols that compute new values. Arithmetic operators compute with numbers. Luau gives you eight: + adds, - subtracts, * multiplies, / divides, // divides and rounds down, % gives the remainder, ^ raises to a power, and a unary - in front of one value negates it.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local apples = 10 + 4
local eaten = 10 - 4
local boxes = 3 * 4
local shared = 10 / 4
print(apples)
print(eaten)
print(boxes)
print(shared)

14
6
12
2.5

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 adds 10 + 4 and stores 14 in apples.
3. Line 3 subtracts 10 - 4 and stores 6 in eaten.
4. Line 4 multiplies 3 * 4 and stores 12 in boxes.
5. Line 5 divides 10 / 4 and stores 2.5 in shared. Division keeps decimals.
6. Lines 6 to 9 print all four results in order.

Three operators surprise beginners. // is floor division: it divides, then rounds down toward negative infinity. % is the remainder after division. ^ is a power, not multiplication.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local fullTeams = 10 // 4
local leftOver = 10 % 4
local squared = 3 ^ 2
local flipped = -squared
print(fullTeams)
print(leftOver)
print(squared)
print(flipped)

2
2
9
-9

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 computes 10 // 4. Plain division gives 2.5, and flooring gives 2.
3. Line 3 computes 10 % 4. Four fits twice into ten with 2 left over.
4. Line 4 computes 3 ^ 2, which is 3 * 3, so 9.
5. Line 5 negates squared with unary -, giving -9.
6. Lines 6 to 9 print 2, 2, 9, and -9.

Join strings and measure length

The .. operator glues two strings into one. The # operator measures the length of a string in characters.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local firstName = "Maya"
local hello = "Hi, " .. firstName
local nameLength = #firstName
print(hello)
print(nameLength)

Hi, Maya
4

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 stores "Maya" in firstName.
3. Line 3 joins "Hi, " and "Maya" with .. into "Hi, Maya".
4. Line 4 counts the characters in "Maya" with # and stores 4.
5. Line 5 prints Hi, Maya.
6. Line 6 prints 4.

Notice that .. adds no space by itself. The space in the output comes from the space inside "Hi, ". When words run together unexpectedly, the missing space is almost always yours to add.

Comparisons give booleans

Comparisons ask a question and answer with a boolean: true or false. Luau has six: == (equal), ~= (not equal), <, >, <=, >=. Read ~= as "not equal to". Learn booleans here because every if statement in Conditionals and Loops runs on them.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local level = 5
local isReady = level >= 5
local isMaxed = level == 10
print(isReady)
print(isMaxed)

true
false

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 stores 5 in level.
3. Line 3 asks "is 5 >= 5?" and stores the answer true in isReady.
4. Line 4 asks "is 5 == 10?" and stores the answer false in isMaxed.
5. Lines 5 and 6 print true then false.

Use == with two equals signs to compare. One = assigns. Mixing them is a common beginner typo.

Logic with and, or, not

Logical operators combine booleans into bigger questions. and is true only when both sides are true. or is true when at least one side is true. not flips a boolean around.

In Luau, and and or return one of their operands, not always true or false. and returns its first value when the first is falsy, otherwise its second value. or returns its first value when it is truthy, otherwise its second value. Programmers use or to pick defaults, as you will see below.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local hasKey = true
local hasTicket = false
local canEnter = hasKey and hasTicket
local canTry = hasKey or hasTicket
local noKey = not hasKey
print(canEnter)
print(canTry)
print(noKey)

false
true
false

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 stores true in hasKey.
3. Line 3 stores false in hasTicket.
4. Line 4 computes true and false, which is false, and stores it in canEnter. Entry needs both.
5. Line 5 computes true or false, which is true, and stores it in canTry. Trying needs only one.
6. Line 6 flips true to false with not and stores it in noKey.
7. Lines 7 to 9 print false, true, false.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local typedName = nil
local shownName = typedName or "Guest"
print(shownName)

Guest

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 stores nil in typedName, meaning "no name typed yet".
3. Line 3 uses or to pick a fallback. Since nil is falsy, or returns "Guest".
4. Line 4 prints Guest.

Only nil and false are falsy

Every value in Luau acts as true or false inside a condition. Only two values are falsy: nil and false. Everything else is truthy. This surprises beginners twice: the number 0 is truthy, and the empty string "" is truthy. Many other languages treat those as falsy, but Luau does not.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local zeroCheck = not 0
local emptyCheck = not ""
local nilCheck = not nil
local falseCheck = not false
print(zeroCheck)
print(emptyCheck)
print(nilCheck)
print(falseCheck)

false
false
true
true

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 flips 0 with not. Since 0 is truthy, the result is false.
3. Line 3 flips "" with not. An empty string is truthy, so the result is false.
4. Line 4 flips nil with not. nil is falsy, so the result is true.
5. Line 5 flips false with not, giving true.
6. Lines 6 to 9 print false, false, true, true.

When you check "did the player type anything", never test the string directly. An empty string passes. Compare against "" instead. You will practice this in Conditionals and Loops.

Precedence and parentheses

Precedence decides which operator computes first. Luau computes in this order: ^, then unary not, -, #, then * / // %, then + -, then .., then comparisons, then and, then or. Parentheses always win. When in doubt, add parentheses. They cost nothing and remove all doubt.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local plain = 2 + 3 * 4
local grouped = (2 + 3) * 4
local powerFirst = 2 * 3 ^ 2
print(plain)
print(grouped)
print(powerFirst)

14
20
18

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 computes 3 * 4 first because * beats +, then adds 2, giving 14.
3. Line 3 computes (2 + 3) first because parentheses win, then 5 * 4 gives 20.
4. Line 4 computes 3 ^ 2 first because ^ beats *, then 2 * 9 gives 18.
5. Lines 5 to 7 print 14, 20, 18.

Convert with tonumber and tostring

Sometimes a number arrives as a string, like "7" from typed input. tonumber converts it into a real number so you can do math. tostring goes the other way and turns any value into its text form so you can join it with ... If tonumber cannot read a number in the string, it returns nil.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local typed = "7"
local asNumber = tonumber(typed)
local doubled = asNumber + asNumber
local label = "Total: " .. tostring(doubled)
print(doubled)
print(label)

14
Total: 14

Walkthrough:

1. Line 1 turns on strict mode.
2. Line 2 stores the string "7" in typed.
3. Line 3 converts "7" to the number 7 with tonumber.
4. Line 4 adds 7 + 7 and stores 14 in doubled. This works only because line 3 converted first.
5. Line 5 converts 14 back to "14" with tostring and joins it onto "Total: ".
6. Line 6 prints 14.
7. Line 7 prints Total: 14.

Common mistakes

Using = instead of ==.

Run this to see it fail: put it in a Script in ServerScriptService and press Play.

--!strict
local level = 5
local isReady = level = 5
print(isReady)

Red syntax error on line 3: the script does not run until you compare with ==.

One = assigns, two == compare. Fix it with ==. Run the fix the same way.

--!strict
local level = 5
local isReady = level == 5
print(isReady)

true

Doing math on a string.

Run this to see it fail: put it in a Script in ServerScriptService and press Play.

--!strict
local typed = "7"
local doubled = typed + typed
print(doubled)

ServerScriptService.Script:3: attempt to perform arithmetic (add) on string and number

The + operator needs numbers. Fix it by converting with tonumber first. Run the fix the same way.

--!strict
local typed = "7"
local asNumber = tonumber(typed)
local doubled = asNumber + asNumber
print(doubled)

14

Treating 0 or "" as falsy.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local coins = 0
local hasCoins = coins
if hasCoins then
	print("rich")
end

rich

This prints rich because 0 is truthy. Fix it with an explicit comparison. Run the fix the same way.

--!strict
local coins = 0
local hasCoins = coins > 0
if hasCoins then
	print("rich")
end

The fixed script prints nothing because 0 > 0 is false. An empty output is the correct result here.

Reading and and or as always true or false.

Run this: put it in a Script in ServerScriptService and press Play.

--!strict
local typedName = nil
local shownName = typedName or "Guest"
local isGuest = shownName == true
print(isGuest)

false

This prints false because shownName holds "Guest", not true. Fix it by comparing against the actual fallback value. Run the fix the same way.

--!strict
local typedName = nil
local shownName = typedName or "Guest"
local isGuest = shownName == "Guest"
print(isGuest)

true

Exercises

Run every solution below in a Script in ServerScriptService and press Play. Try each exercise yourself before you open its solution.

1. Store 17 and 5 in variables. Print their sum, difference, product, and quotient on four lines.

Solution 1

--!strict
local first = 17
local second = 5
print(first + second)
print(first - second)
print(first * second)
print(first / second)

22
12
85
3.4

2. Join "Roblox" and "Studio" with a space into one variable. Print the joined string and its length with #.

Solution 2

--!strict
local joined = "Roblox" .. " " .. "Studio"
print(joined)
print(#joined)

Roblox Studio
13

3. A ride needs height 48 or more. Store a height of 45. Store a boolean that is true when the height passes. Print it.

Solution 3

--!strict
local height = 45
local canRide = height >= 48
print(canRide)

false

4. A player typed "12" as text. Convert it with tonumber, triple it, then build the string "Score: " plus the tripled value with tostring and ... Print the final string.

Solution 4

--!strict
local typed = "12"
local asNumber = tonumber(typed)
local tripled = asNumber * 3
local label = "Score: " .. tostring(tripled)
print(label)

Score: 36

Line 3 converts the text to a number. Line 4 triples it. Line 5 converts back to text for joining.

Summary

You computed with arithmetic operators, joined strings with .., measured with #, compared values into booleans, combined conditions with and, or, and not, applied the truthiness rule that only nil and false are falsy, ordered math with precedence and parentheses, and converted with tonumber and tostring.

Next: Conditionals and Loops.