When a function fails in lua, you'll often find that it return a nil value that you can check, or it returns two values, the second of which is an error status / message that tells you what went wrong.

Here's an example:
value = tonumber(stuff)
if value == nil then
   print ("Nah!")
else
   print ("That is half of ",value * 2)
end
(full source of example)

You can reduce the code somewhat by using the built in assert function to check, generate the message, and exit with a full stack trace - example:
io.write ("How many bits: ")
val2 = tonumber(io.read())
assert(val2 >= 1,"need to have at least one piece")
(full source code of example)

There are times when a lua function will generate an error before it returns to the calling code .. but you want to trap that .. and you can do so via pcall or protected call. Example:
  goodun,eachlen = pcall(cutter,val1,val2)
This calls the cutter function with two parameters, and the result is returned into eachlen. The other return variable is a status to tell you whether or not the function worked.
(full source code of example)


Illustration - during a private Lua course at a customer's office. As well as private training courses on site, we run public Lua courses at our Melksham, Wiltshire, UK training centre. See [here].
(written 2009-08-13, updated 2010-06-19)

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U111 - Lua - Trapping and handling errors.
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Getting it right from the start - new programmers
Alpaca Case or Camel Case
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Geocoding - converting address to latitude / Longitude with PHP via Google
Error Handling in Lua with assert and pcall
A fresh example - objects the Lua way
Encapsulating shared code in a Lua module
Checking if the user has entered a number (Lua)
Passing multiple results back from a function - Lua, Python, etc
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