Loops & Control Flow in Lua

Learn about loops, conditionals, and program flow control in Lua

For Loops

For loops are used to iterate through a sequence of values. In Lua, there are two types of for loops: numeric for and generic for.

numeric_for.lua

-- Numeric for loop
for i = 1, 5 do
  print(i)
end

-- With step value
for i = 10, 1, -2 do  -- Count down by 2
  print(i)  -- Prints: 10, 8, 6, 4, 2
end

generic_for.lua

-- Generic for loop with ipairs (array-like iteration)
local fruits = {"apple", "banana", "orange", "grape"}

for index, value in ipairs(fruits) do
  print(index .. ": " .. value)
end

-- Generic for with pairs (table iteration)
local person = {
  name = "Aidan",
  age = 25,
  job = "Developer"
}

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

While Loops

A while loop repeats a block of code as long as a specified condition is true.

while_loop.lua

-- Basic while loop
local count = 1

while count <= 5 do
  print(count)
  count = count + 1
end

-- Using while for input validation (pseudo-code)
--[[
local input

while not isValid(input) do
  input = getInput()
  
  if not isValid(input) then
    print("Invalid input, try again")
  end
end
--]]

Repeat-Until Loops

Repeat-until loops are similar to while loops, but they execute the block at least once and continue until the condition is true.

repeat_until.lua

-- Basic repeat-until loop
local count = 1

repeat
  print(count)
  count = count + 1
until count > 5

-- Good for input validation
--[[  
local input

repeat
  input = getInput()
  
  if not isValid(input) then
    print("Invalid input, try again")
  end
until isValid(input)
--]]

Conditional Statements

Conditional statements allow your program to make decisions based on conditions.

if_statements.lua

-- Basic if statement
local playerHealth = 75

if playerHealth >= 75 then
  print("Player health is good!")
elseif playerHealth >= 25 then
  print("Player health is low!")
else
  print("Player is in danger!")
end

-- One-liner for simple conditions
if playerHealth <= 0 then return end  -- Exit early if player is defeated

-- Complex conditions with logical operators
local hasShield = true
local hasPotion = false

if playerHealth < 50 and hasPotion then
  print("Use your potion!")
elseif playerHealth < 30 and not hasShield then
  print("You are vulnerable, take cover!")
end

Breaking and Continuing Loops

Lua provides ways to control the flow of loops with break and goto statements.

loop_control.lua

-- Breaking out of a loop
for i = 1, 10 do
  if i > 5 then
    break  -- Exit the loop early
  end
  print(i)
end

-- Using goto for more complex flow control (Lua 5.2+)
for i = 1, 10 do
  if i % 2 == 0 then
    goto continue  -- Skip even numbers
  end
  print(i)  -- Only prints odd numbers
  
  ::continue::
end

Practical Example: Game Looping

Here's a practical example of using loops and control flow in a game context:

game_loop.lua

-- Simple game loop
local isGameRunning = true
local playerHealth = 100
local enemyCount = 5
local score = 0

-- Main game loop
while isGameRunning do
  -- Process player input (pseudo-code)
  local action = getPlayerAction()
  
  -- Update game state
  if action == "attack" then
    enemyCount = enemyCount - 1
    score = score + 100
    
    if enemyCount <= 0 then
      print("Level complete! Final score: " .. score)
      isGameRunning = false
    end
  elseif action == "hit" then
    playerHealth = playerHealth - 10
    
    if playerHealth <= 0 then
      print("Game over! Final score: " .. score)
      isGameRunning = false
    end
  elseif action == "quit" then
    print("Game exited. Score: " .. score)
    isGameRunning = false
  end
  
  -- Render game (pseudo-code)
  render()
  
  -- Wait for next frame (pseudo-code)
  wait(1/60)  -- 60 FPS
end

Break and Continue

Lua provides keywords to control loop execution flow.

break_continue.lua

-- Break statement exits the loop
for i = 1, 10 do
  if i > 5 then
    break  -- Exit the loop when i > 5
  end
  print(i)
end

-- Simulating continue with goto (Lua doesn't have "continue")
for i = 1, 10 do
  if i % 2 == 0 then
    goto continue  -- Skip even numbers
  end
  print(i)  -- This only prints odd numbers
  
  ::continue::
end

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