Lua Gotchas

Lua is one of my favorite programming languages. Lua is easy to learn, but not entirely without gotchas if you’re used to other languages. One gotcha is related to for-loops. What do you think the following piece of code will print?

for i = 1, 5 do
    print(i)
    if i == 3 then i = i + 1 end
end

The numbers from 1 to 5, skipping 4? Because that’s what we expect from an equivalent loop in, say, C++:

for (int i = 1; i <= 5; i++) {
    std::cout << i << "\n";
    if (i == 3) i++;
}

Except it’s not equivalent. Lua’s for-loops are implemented in terms of while. Above example is translated to:

do
    local _i, _end, _step = 1, 5, 1
    while _i <= _end do
        local i = _i
        print(i)
        if i == 3 then i = i + 1 end
        _i = _i + _step
    end
end

Quoting from Programming in Lua:

The for loop has some subtleties that you should learn in order to make good use of it. First, all three expressions are evaluated once, before the loop starts. […] Second, the control variable is a local variable automatically declared by the for statement and is visible only inside the loop. […] Third, you should never change the value of the control variable.