Control Flow

if / elseif / else

local score = 72

if score >= 90 then
    print("A")
elseif score >= 70 then
    print("B")
else
    print("C or lower")
end

Comparisons are == and ~= (not !=). Logical operators are spelled out: and, or, not. Both and and or short-circuit, which makes x = x or default a common idiom for filling in a default value.

while and repeat

repeat ... until checks its condition after the body runs, so the body always executes at least once, and unlike while, locals declared inside the body are still visible in the until condition.

local n = 5
while n > 0 do
    print(n)
    n = n - 1
end

repeat
    local line = read_next()
until line == nil

Numeric for

for var = start, stop[, step] do. The step defaults to 1 and can be negative to count down. The loop variable is local to the loop; it does not leak.

for i = 1, 5 do
    print(i)
end

for i = 10, 0, -2 do
    print(i) -- 10, 8, 6, 4, 2, 0
end

Generic for

For iterating a table, reach for pairs (every key) or ipairs (the 1..n array part, in order, stopping at the first hole). More on why the distinction matters in Tables.

local colors = { "red", "green", "blue" }

for index, value in ipairs(colors) do
    print(index, value)
end

break, and no continue

break exits the innermost loop. Lua has no continue keyword; the common workaround is wrapping the rest of the loop body in an if, or using goto with a label at the end of the loop body.