Im getting a error message in my computercraft computer

Question

I'm using this code from How do I detect if MFSU has half below energy, from an answer about half way down the page:

mfe = peripheral.wrap("left")
while true do
  euPercent = mfe.getEUStored() / mfe.getEUCapacity()
  if euPercent==1 then
    redstone.setOutput("right", false)
  elseif euPercent<=.5 then
    redstone.setOutput("right", true)
  end
  sleep(1)
end

This is so that I can have my computer turn off my nuclear reactor when it's full of power and on when it's not full, but when I run the program it says

reactor:3: attempt to index ? (a nil value)

Answer 1

periphera.wrap() is returning nil and storing that in mfe. That's a problem, and why it fails to be useful when you try to access it on line 3.

It's hard to figure out why from a distance. Three reasons I can think of are:

1. That the "side" argument to wrap() is incorrect and there's nothing on that side, which would make the API deliberately return nil to alert you to the lack of peripheral.

2. CC does not understand IC2 blocks natively, and requires a mod to make it able to use IC2 blocks as peripherals — if you don't have one installed, the MFE would count as "nothing" to wrap() and it will return nil. OpenCCSensors is one such mod.

3. Another reason might be that somewhere before this code runs peripheral has been overwritten in some way, making the call to wrap() return nil.

Answer 2

Where you got your code from, it says zo replace "side" with the side you have mfe/reactor on.