Substituting multiple inputs for a function with one function?

Question

I am trying to see if I can simplify input by using a function that produces more than one output for use with another function. Is there any way I can do this? Do I HAVE to make a function to return single variables for each input?

--here is a snippet of what im trying to do (for a game)
--Result is the same for game environment and lua demo.

en = {
box ={x=1,y=2,w=3}
}
sw = {
box = {x=1,y=2,w=3}
}

function en.getbox()
return en.box.x,en.box.y,en.box.w,en.box.w
end

function sw.getbox()
return sw.box.x,sw.box.y,sw.box.w,sw.box.w
end

function sw.getvis()
return true
end

function CheckCollision(x1,y1,w1,h1, x2,y2,w2,h2)
  return x1 < x2+w2 and
         x2 < x1+w1 and
         y1 < y2+h2 and
         y2 < y1+h1
end

if CheckCollision(en.getbox(),sw.getbox()) == true then
        if sw.getvis() == true then
            en.alive = false
        end
    end

print(tostring(en.alive))

I am expecting the enemy (en) to die (en.alive = false) but I am getting the error: input:25: attempt to perform arithmetic on a nil value (local 'w2')

Answer 1

You can find an explaination for the issue you are seeing here: Programming in Lua: 5.1 – Multiple Results

I suggest you read the whole page but here is a relevant section

A function call that is not the last element in the list always produces one result:

x,y = foo2(), 20 -- x='a', y=20

x,y = foo0(), 20, 30 -- x=nil, y=20, 30 is discarded

---

I suggest the following changes to get your code working. wrap your output from getbox into a table with clear keys that make it easy to understand.

  function en.getbox()
    return {
      x = en.box.x,
      y = en.box.y,
      w = en.box.w,
      h = en.box.w
    }
  end

  function sw.getbox()
    return {
      x = sw.box.x,
      y = sw.box.y,
      w = sw.box.w,
      h = sw.box.w
    }
  end

  function CheckCollision(o1, o2)

    return o1.x < o2.x + o2.w and
    o2.x < o1.x + o1.w and
    o1.y < o2.y + o2.h and
    o2.y < o1.y + o1.h
  end

Alternatively you can wrap the output of getbox "on the fly" like:

function CheckCollision(o1, o2)

  return o1[1] < o2[1] + o2[3] and
         o2[1] < o1[1] + o1[3] and
         o1[2] < o2[2] + o2[4] and
         o2[2] < o1[2] + o1[4]
end

if CheckCollision({en.getbox()},  {sw.getbox()}) == true then
        if sw.getvis() == true then
            en.alive = false
        end
end

I do strongly encourage the first option over the last. The last option leads to code that is harder to follow and should be accompanied by clear comments explaining it.