Editing Lua (section)

=== Control flow ===
Lua has one type of [[Conditional (computer programming)|conditional]] test: <code>[[Conditional_(computer_programming)#If–then(–else)|if then end]]</code> with optional <code>else</code> and <code>elseif then</code> execution control constructs.

The generic <code>if then end</code> statement requires all three keywords:
<syntaxhighlight lang="lua">
if condition then
	--statement body
end
</syntaxhighlight>

An example of an <code>if</code> statement
<syntaxhighlight lang="lua">
if x ~= 10 then
	print(x)
end
</syntaxhighlight>

The <code>else</code> keyword may be added with an accompanying statement block to control execution when the <code>if</code> condition evaluates to <code>false</code>:
<syntaxhighlight lang="lua">
if condition then
	--statement body
else
	--statement body
end
</syntaxhighlight>

An example of an <code>if else</code> statement
<syntaxhighlight lang="lua">
if x == 10 then
	print(10)
else
	print(x)
end
</syntaxhighlight>

Execution may also be controlled according to multiple conditions using the <code>elseif then</code> keywords:
<syntaxhighlight lang="lua">
if condition then
	--statement body
elseif condition then
	--statement body
else -- optional
	--optional default statement body
end
</syntaxhighlight>
An example of an <code>if elseif else</code> statement
<syntaxhighlight lang="lua">
if x == y then
	print("x = y")
elseif x == z then
	print("x = z")
else -- optional
	print("x does not equal any other variable")
end
</syntaxhighlight>

Lua has four types of conditional loops: the [[while loop|<code>while</code> loop]], the <code>repeat</code> loop (similar to a [[do while loop|<code>do while</code> loop]]), the numeric [[for loop|<code>for</code> loop]] and the generic <code>for</code> loop.
<syntaxhighlight lang="lua">
--condition = true

while condition do
  --statements
end

repeat
  --statements
until condition

for i = first, last, delta do  --delta may be negative, allowing the for loop to count down or up
  --statements
  --example: print(i)
end
</syntaxhighlight>

This generic <code>for</code> loop would iterate over the table <code>_G</code> using the standard iterator function <code>pairs</code>, until it returns <code>nil</code>:
<syntaxhighlight lang="lua">
for key, value in pairs(_G) do
  print(key, value)
end
</syntaxhighlight>

Loops can also be [[Nesting (programming)|nested]] (put inside of another loop).
<syntaxhighlight lang="lua">
local grid = {
  { 11, 12, 13 },
  { 21, 22, 23 },
  { 31, 32, 33 }
}

for y, row in pairs(grid) do
  for x, value in pairs(row) do
    print(x, y, value)
  end
end
</syntaxhighlight>