[Lua] [Error] Attempted to index ? (a nil value)

Hello,
Im working on a simple program for my nuclear reactor and since i don´t know how to make a good software, i have 1 computer for each reactor. But when i was finished with my code i got this error:
startup:2: attempted to index ? (a nil value)

Here is my code

term.clear()
Term.setCursorPos (1, 1)
Print ("Command prompt for Reactor 1")
if input == "start" then
redstone.setoutput ("back", false )
Print ("Command accepted, starting reactor 1")
if input == "stop" then
rs.setoutput ("back", true) else
print ("Invalid command")
os.reboot()
end
end



I am a newbie and i know it. But help will be much appreciated. Thanks

Term.setCursorPos (1, 1)

should be term.setCursorPos(1,1)

caps matter.

also you're calling for it to read the input variable bust not taking input anywhere,

local function start()
term.clear()
term.setCursorPos (1, 1)
Print ("Command prompt for Reactor 1")
input = read()
   if input == "start" then
	  redstone.setOutput ("back", false )
	  Print ("Command accepted, starting reactor 1")
   elseif input == "stop" then
	  redstone.setOutput ("back", true) else
	  print ("Invalid command")
	  os.reboot()
   else
	  print("invalid input")
	  sleep(2)
	  start()
   end
end

start()

by making it a function you can easily avoid errors from invalid input by restarting the function on invalid input as I did above.

Also, I formatted it for you :D/>/>

Ohh, Big thanks dude! You are the best.

ltstingray, on 09 September 2012 - 05:53 PM said:

Term.setCursorPos (1, 1)

should be term.setCursorPos(1,1)

caps matter.

also you're calling for it to read the input variable bust not taking input anywhere,

local function start()
term.clear()
term.setCursorPos (1, 1)
Print ("Command prompt for Reactor 1")
input = read()
   if input == "start" then
	  redstone.setOutput ("back", false )
	  Print ("Command accepted, starting reactor 1")
   elseif input == "stop" then
	  redstone.setOutput ("back", true) else
	  print ("Invalid command")
	  os.reboot()
   else
	  print("invalid input")
	  sleep(2)
	  start()
   end
end

start()

by making it a function you can easily avoid errors from invalid input by restarting the function on invalid input as I did above.

Also, I formatted it for you :D/>/>

Sorry for bothering you but your code doesnt work either :/
Error:
bios:206: [string "startup"] :13: end expected (to close if at line 6)

Try this:

while true do
    term.clear()
    term.setCursorPos (1, 1)
    print("Command prompt for Reactor 1")
    input = read()
    if input == "start" then
        redstone.setOutput ("back", false )
        print("Starting Reactor 1")
        sleep(1)
    elseif input == "stop" then
        redstone.setOutput ("back", true)
        print("Stopping Reactor 1")
        sleep(1)
    elseif input == "exit" then
        break
    else
        print("Invalid command")
        sleep(1)
    end
end

The code Itstingray wrote was bad because it was calling a function from within itself to create a loop, which is bad. He also missed an else, which caused the error when the program encountered another else. Commands are now start, stop and exit.

Lyqyd, on 09 September 2012 - 07:08 PM said:

Try this:

while true do
	term.clear()
	term.setCursorPos (1, 1)
	print("Command prompt for Reactor 1")
	input = read()
	if input == "start" then
		redstone.setOutput ("back", false )
		print("Starting Reactor 1")
		sleep(1)
	elseif input == "stop" then
		redstone.setOutput ("back", true)
		print("Stopping Reactor 1")
		sleep(1)
	elseif input == "exit" then
		break
	else
		print("Invalid command")
		sleep(1)
	end
end

The code Itstingray wrote was bad because it was calling a function from within itself to create a loop, which is bad. He also missed an else, which caused the error when the program encountered another else. Commands are now start, stop and exit.

There is no problem in using recursive functions if you make sure that it does tail calls.
if you change the recursive call in itstingray program from start() to return start() it wil not overflow
the stack

Magus, on 09 September 2012 - 07:36 PM said:

Lyqyd, on 09 September 2012 - 07:08 PM said:

Try this:

while true do
	term.clear()
	term.setCursorPos (1, 1)
	print("Command prompt for Reactor 1")
	input = read()
	if input == "start" then
		redstone.setOutput ("back", false )
		print("Starting Reactor 1")
		sleep(1)
	elseif input == "stop" then
		redstone.setOutput ("back", true)
		print("Stopping Reactor 1")
		sleep(1)
	elseif input == "exit" then
		break
	else
		print("Invalid command")
		sleep(1)
	end
end

The code Itstingray wrote was bad because it was calling a function from within itself to create a loop, which is bad. He also missed an else, which caused the error when the program encountered another else. Commands are now start, stop and exit.

There is no problem in using recursive functions if you make sure that it does tail calls.
if you change the recursive call in itstingray program from start() to return start() it wil not overflow
the stack

Technically true (in Lua), but still terrible practice when trying to create a loop. Recursion has its place, but it is no replacement for a good while loop. It's bad programming practice to use them where they don't belong, regardless of whether it would "work" or not.