LU Lua

Lua: Control Flow

What you will learn

Lua uses if/elseif/else (note: elseif is one word!), numeric and generic for loops, and while.

Conditionals

local score = 85
if score >= 90 then
    print("A")
elseif score >= 80 then
    print("B")
else
    print("F")
end

Lua spells elseif as one word (unlike most languages that use else if). Blocks close with end.

Numeric for loop

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

for i = 1, 10, 2 do
    print(i)  -- 1, 3, 5, 7, 9 (step = 2)
end

Syntax: for var = start, stop, step do (step defaults to 1). The loop includes both start and stop.

Generic for loop (iterating over tables)

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

-- Iterate over array indices
for i, v in ipairs(fruits) do
    print(i, v)  -- 1 apple, 2 banana, 3 cherry
end

-- Iterate over all key-value pairs (including non-sequential)
for k, v in pairs(fruits) do
    print(k, v)
end

ipairs() iterates over sequential integer keys. pairs() iterates over all key-value pairs.

While loop

local i = 0
while i < 5 do
    print(i)
    i = i + 1
end

Common mistakes

  • Writing else if (two words) instead of elseif (one word) — compile error.
  • Forgetting do after for and while — for i = 1, 5 do not for i = 1, 5.
  • Using ipairs() on a dictionary table (it only returns sequential integer keys).
  • Forgetting end to close blocks.

Quick check below!

Next: Functions & Closures