title: How to fix "attempt to index a nil value"
body: <p>I am having an error with my code : it keeps telling me "attempt to index a nil value (global 'sides')"</p>
  
  <p>I'm trying to learn Lua through Minecraft (OpenComputers) and found myself stuck with a nil value problem. There could be things not actually from Lua (the mod itself) but the problem concerns the "pure Lua part"</p>
  
  <pre><code>component = require("component")
  event = require("event")
  computer = require("computer")
  term = require("term")
  
  gpu = component.gpu
  
  redstone = component.redstone
  
  gpu.setResolution(160,50)
  
  while true do
      term.clear()
      term.setCursor(1,1)
      gpu.setBackground(0x5A5A5A)
  
      gpu.set(1,1," Allumer lampe    Eteindre lampe")
      term.setCursor(1,2)
  
      local _,_,x,y = event.pull("touch")
  
      if x &gt;= 2 and x &lt;= 14 and y == 1 then
          redstone.setOutput(sides.left,15)
      elseif x &gt;= 19 and x &lt;= 32 and y == 1 then
          redstone.setOutput(sides.left,0)
      else
          main()
  
      end
  
  end
  </code></pre>
  
  <p>I went on the Wiki of the mod and it says that redstone.setOutput(sides.left,15) is supposed to change the actual value of the output but it also returns the OLD value of the output (and that's where I think I'm doing it wrong)</p>