Loops in Lua

Beginner ~25 min read

Loops allow you to repeat code multiple times, making your programs more efficient and powerful. Lua provides several types of loops: while, repeat-until, and for (both numeric and generic). Each has its own use case and advantages. Let's explore them all!

The while Loop

The while loop repeats code as long as a condition is true:

local count = 1

while count <= 5 do
    print("Count:", count)
    count = count + 1
end

-- Output:
-- Count: 1
-- Count: 2
-- Count: 3
-- Count: 4
-- Count: 5

Syntax:

Start with while followed by a condition
Use do after the condition
Write the code to repeat
End with end

-- While Loops in Lua

-- Basic while loop
print("=== Basic While Loop ===")
local count = 1

while count <= 5 do
    print("Count:", count)
    count = count + 1
end

-- While loop with condition
print("\n=== While with Condition ===")
local sum = 0
local num = 1

while sum < 100 do
    sum = sum + num
    num = num + 1
end

print("Sum:", sum)
print("Numbers added:", num - 1)

-- While loop with break
print("\n=== While with Break ===")
local i = 1

while true do
    if i > 5 then
        break
    end
    print("i =", i)
    i = i + 1
end

-- Repeat-Until Loop (condition checked AFTER)
print("\n=== Repeat-Until Loop ===")
local counter = 1

repeat
    print("Counter:", counter)
    counter = counter + 1
until counter > 5

-- Repeat-until guarantees at least one execution
print("\n=== Repeat-Until (at least once) ===")
local x = 10

repeat
    print("This runs at least once, x =", x)
    x = x + 1
until x > 5  -- Condition is false, but loop ran once!

-- Infinite loop with break condition
print("\n=== Infinite Loop with Break ===")
local attempts = 0

while true do
    attempts = attempts + 1
    print("Attempt:", attempts)
    
    if attempts >= 3 then
        print("Max attempts reached!")
        break
    end
end

Output

Click Run to execute your code

Warning: Make sure your loop condition eventually becomes false, or you'll create an infinite loop! Always update the loop variable inside the loop.

The repeat-until Loop

The repeat-until loop executes at least once, then checks the condition:

local count = 1

repeat
    print("Count:", count)
    count = count + 1
until count > 5

-- Output is the same as while loop
-- But the code runs at least once!

Tip: Use repeat-until when you need the loop body to execute at least once, like validating user input or menu systems.

-- Repeat-Until Loops

-- Basic repeat-until
print("=== Basic Repeat-Until ===")
local count = 1

repeat
    print("Count:", count)
    count = count + 1
until count > 5

-- Key difference: condition checked AFTER
print("\n=== Condition Checked After ===")
local x = 10

repeat
    print("x =", x, "(runs even though x > 5)")
    x = x + 1
until x > 5

-- Menu-driven program pattern
print("\n=== Menu Pattern ===")
local choice = 0
local attempts = 0

repeat
    attempts = attempts + 1
    print("\nMenu:")
    print("1. Option A")
    print("2. Option B")
    print("3. Exit")
    
    -- Simulate user input
    if attempts == 1 then choice = 1
    elseif attempts == 2 then choice = 2
    else choice = 3 end
    
    print("You chose:", choice)
    
    if choice == 1 then
        print("Executing Option A")
    elseif choice == 2 then
        print("Executing Option B")
    elseif choice == 3 then
        print("Exiting...")
    end
until choice == 3

-- Input validation pattern
print("\n=== Input Validation ===")
local number
local tries = 0

repeat
    tries = tries + 1
    -- Simulate getting input
    number = tries * 10
    print("Attempt", tries, "- Number:", number)
until number >= 50

print("Valid number found:", number)

-- Repeat-until with break
print("\n=== Repeat-Until with Break ===")
local counter = 0

repeat
    counter = counter + 1
    print("Counter:", counter)
    
    if counter == 3 then
        print("Breaking early!")
        break
    end
until counter >= 10

print("Final counter:", counter)

Output

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while vs repeat-until

Feature	while	repeat-until
Condition check	Before execution	After execution
Minimum executions	0 (may not run)	1 (always runs once)
Condition	Continue while true	Stop when true

The Numeric for Loop

The numeric for loop is perfect for counting:

-- Count from 1 to 5
for i = 1, 5 do
    print(i)
end

-- Count from 1 to 10 by 2s
for i = 1, 10, 2 do
    print(i)  -- 1, 3, 5, 7, 9
end

-- Count backwards
for i = 5, 1, -1 do
    print(i)  -- 5, 4, 3, 2, 1
end

Syntax: for variable = start, stop, step do

start: Starting value
stop: Ending value (inclusive)
step: Increment (default is 1)

-- For Loops in Lua

-- Numeric for loop (basic)
print("=== Basic For Loop ===")
for i = 1, 5 do
    print("i =", i)
end

-- For loop with step
print("\n=== For Loop with Step ===")
for i = 0, 10, 2 do  -- start, end, step
    print("Even:", i)
end

-- Counting backwards
print("\n=== Counting Backwards ===")
for i = 5, 1, -1 do
    print("Countdown:", i)
end
print("Blast off!")

-- For loop with calculations
print("\n=== Multiplication Table ===")
local number = 5

for i = 1, 10 do
    print(number, "x", i, "=", number * i)
end

-- Nested for loops
print("\n=== Nested Loops (Grid) ===")
for row = 1, 3 do
    for col = 1, 3 do
        io.write("(" .. row .. "," .. col .. ") ")
    end
    print()  -- New line
end

-- Sum with for loop
print("\n=== Sum 1 to 100 ===")
local sum = 0

for i = 1, 100 do
    sum = sum + i
end

print("Sum:", sum)

-- For loop with break
print("\n=== For Loop with Break ===")
for i = 1, 10 do
    if i == 6 then
        print("Breaking at", i)
        break
    end
    print("i =", i)
end

-- Factorial calculation
print("\n=== Factorial ===")
local n = 5
local factorial = 1

for i = 1, n do
    factorial = factorial * i
end

print(n .. "! =", factorial)

Output

Click Run to execute your code

Best Practice: The loop variable (i) is local to the loop and cannot be modified inside the loop body. This prevents common errors!

The Generic for Loop

The generic for loop iterates over collections using iterators:

Iterating Over Arrays

local fruits = {"apple", "banana", "cherry"}

-- Using ipairs for arrays
for index, value in ipairs(fruits) do
    print(index, value)
end

-- Output:
-- 1    apple
-- 2    banana
-- 3    cherry

Iterating Over Tables

local person = {
    name = "Alice",
    age = 25,
    city = "New York"
}

-- Using pairs for tables
for key, value in pairs(person) do
    print(key, value)
end

-- Output:
-- name    Alice
-- age     25
-- city    New York

-- Generic For Loops with pairs and ipairs

-- Arrays in Lua (1-based indexing!)
local fruits = {"apple", "banana", "cherry", "date"}

-- ipairs for arrays (integer pairs)
print("=== ipairs (for arrays) ===")
for index, value in ipairs(fruits) do
    print(index, value)
end

-- pairs for tables (all key-value pairs)
print("\n=== pairs (for tables) ===")
local person = {
    name = "Alice",
    age = 25,
    city = "New York"
}

for key, value in pairs(person) do
    print(key, "=", value)
end

-- pairs vs ipairs difference
print("\n=== pairs vs ipairs ===")
local mixed = {
    [1] = "first",
    [2] = "second",
    [5] = "fifth",  -- Gap in indices!
    name = "test"
}

print("With ipairs (stops at first nil):")
for i, v in ipairs(mixed) do
    print(i, v)
end

print("\nWith pairs (gets all):")
for k, v in pairs(mixed) do
    print(k, v)
end

-- Iterating over array of tables
print("\n=== Array of Tables ===")
local users = {
    {name = "Alice", age = 25},
    {name = "Bob", age = 30},
    {name = "Charlie", age = 35}
}

for i, user in ipairs(users) do
    print(i .. ".", user.name, "-", user.age, "years old")
end

-- Nested table iteration
print("\n=== Nested Tables ===")
local matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
}

for i, row in ipairs(matrix) do
    for j, value in ipairs(row) do
        io.write(value .. " ")
    end
    print()
end

-- Counting and filtering
print("\n=== Filtering ===")
local numbers = {10, 25, 30, 15, 40, 5}
local count = 0

for i, num in ipairs(numbers) do
    if num > 20 then
        print("Found:", num, "at index", i)
        count = count + 1
    end
end

print("Total found:", count)

-- Building new table from iteration
print("\n=== Transform ===")
local original = {1, 2, 3, 4, 5}
local doubled = {}

for i, value in ipairs(original) do
    doubled[i] = value * 2
end

print("Doubled:", table.concat(doubled, ", "))

Output

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ipairs vs pairs

Iterator	Use For	Order
`ipairs`	Arrays (sequential tables)	Guaranteed (1, 2, 3...)
`pairs`	All tables (any keys)	Not guaranteed

Tip: Use ipairs for arrays and pairs for hash tables. ipairs stops at the first nil value!

The break Statement

Use break to exit a loop early:

-- Find first even number
for i = 1, 10 do
    if i % 2 == 0 then
        print("First even number:", i)
        break  -- Exit the loop
    end
end

-- Search in array
local numbers = {1, 3, 5, 8, 9, 11}
for i, num in ipairs(numbers) do
    if num % 2 == 0 then
        print("Found even number at index", i)
        break
    end
end

-- Repeat-Until Loops

-- Basic repeat-until
print("=== Basic Repeat-Until ===")
local count = 1

repeat
    print("Count:", count)
    count = count + 1
until count > 5

-- Key difference: condition checked AFTER
print("\n=== Condition Checked After ===")
local x = 10

repeat
    print("x =", x, "(runs even though x > 5)")
    x = x + 1
until x > 5

-- Menu-driven program pattern
print("\n=== Menu Pattern ===")
local choice = 0
local attempts = 0

-- Input validation pattern
print("\n=== Input Validation ===")
local number
local tries = 0

repeat
    tries = tries + 1
    -- Simulate getting input
    number = tries * 10
    print("Attempt", tries, "- Number:", number)
until number >= 50

print("Valid number found:", number)

-- Repeat-until with break
print("\n=== Repeat-Until with Break ===")
local counter = 0

repeat
    counter = counter + 1
    print("Counter:", counter)
    
    if counter == 3 then
        print("Breaking early!")
        break
    end
until counter >= 10

print("Final counter:", counter)

Output

Click Run to execute your code

Note: Lua does NOT have a continue statement. To skip to the next iteration, use if statements or restructure your code.

Nested Loops

You can put loops inside other loops:

-- Multiplication table
for i = 1, 5 do
    for j = 1, 5 do
        print(i .. " x " .. j .. " = " .. (i * j))
    end
end

-- 2D grid
for row = 1, 3 do
    for col = 1, 3 do
        print("(" .. row .. "," .. col .. ")")
    end
end

-- For Loops in Lua

-- Numeric for loop (basic)
print("=== Basic For Loop ===")
for i = 1, 5 do
    print("i =", i)
end

-- For loop with step
print("\n=== For Loop with Step ===")
for i = 0, 10, 2 do  -- start, end, step
    print("Even:", i)
end

-- Counting backwards
print("\n=== Counting Backwards ===")
for i = 5, 1, -1 do
    print("Countdown:", i)
end
print("Blast off!")

-- For loop with calculations
print("\n=== Multiplication Table ===")
local number = 5

for i = 1, 10 do
    print(number, "x", i, "=", number * i)
end

-- Nested for loops
print("\n=== Nested Loops (Grid) ===")
for row = 1, 3 do
    for col = 1, 3 do
        io.write("(" .. row .. "," .. col .. ") ")
    end
    print()  -- New line
end

-- Sum with for loop
print("\n=== Sum 1 to 100 ===")
local sum = 0

for i = 1, 100 do
    sum = sum + i
end

print("Sum:", sum)

-- For loop with break
print("\n=== For Loop with Break ===")
for i = 1, 10 do
    if i == 6 then
        print("Breaking at", i)
        break
    end
    print("i =", i)
end

-- Factorial calculation
print("\n=== Factorial ===")
local n = 5
local factorial = 1

for i = 1, n do
    factorial = factorial * i
end

print(n .. "! =", factorial)

Output

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Common Loop Patterns

Sum of Numbers

local sum = 0
for i = 1, 10 do
    sum = sum + i
end
print("Sum:", sum)  -- 55

Finding Maximum

local numbers = {5, 2, 9, 1, 7}
local max = numbers[1]

for i = 2, #numbers do
    if numbers[i] > max then
        max = numbers[i]
    end
end
print("Maximum:", max)  -- 9

Filtering

local numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
local evens = {}

for i, num in ipairs(numbers) do
    if num % 2 == 0 then
        table.insert(evens, num)
    end
end
-- evens = {2, 4, 6, 8, 10}

Practice Exercise

Try these loop challenges:

-- Exercise: Loops
-- TODO: Complete the following exercises

-- 1. Print numbers 1 to 10 using a for loop
-- your code here

-- 2. Calculate the sum of numbers from 1 to 50
local sum = 0
-- your code here
print("Sum 1-50:", sum)

-- 3. Print even numbers from 2 to 20
-- your code here

-- 4. Create a multiplication table for 7 (7x1 to 7x10)
-- your code here

-- 5. Count down from 10 to 1 and print "Liftoff!"
-- your code here

-- 6. Find the factorial of 6 (6! = 6*5*4*3*2*1)
local n = 6
local factorial = 1
-- your code here
print("6! =", factorial)

-- 7. Iterate over this array and print each fruit
local fruits = {"apple", "banana", "cherry", "date"}
-- your code here (use ipairs)

-- 8. Sum all numbers in this array
local numbers = {10, 20, 30, 40, 50}
local total = 0
-- your code here
print("Total:", total)

-- 9. Print a 5x5 grid of asterisks
-- your code here (nested loops)

-- 10. Find the largest number in this array
local values = {23, 45, 12, 67, 34, 89, 15}
local largest = values[1]
-- your code here
print("Largest:", largest)

Output

Click Run to execute your code

Summary

In this lesson, you learned:

while loops: repeat while condition is true
repeat-until loops: execute at least once
Numeric for loops: count from start to stop
Generic for loops: iterate with ipairs and pairs
break statement to exit loops early
Nested loops for multi-dimensional iteration
Common patterns: sum, max, filtering

What's Next?

Congratulations on completing Module 2! You now understand operators, strings, conditionals, and loops—the building blocks of programming. Next, we'll dive into functions, one of Lua's most powerful features. You'll learn how to write reusable code, work with closures, and master functional programming concepts. Let's continue! 🚀

Previous Conditionals

Next Functions

The while Loop

The repeat-until Loop

while vs repeat-until

The Numeric for Loop

The Generic for Loop

Iterating Over Arrays

Iterating Over Tables

ipairs vs pairs

The break Statement

Nested Loops

Common Loop Patterns

Sum of Numbers

Finding Maximum

Filtering

Practice Exercise

Summary

What's Next?

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