Cycling through different file location options

Question

I'm essentially trying to create a function which tests the first location I give, in the form:

 myComputer.referenceLookup("/address/x/text")

and return the string in that location if it is not NULL or "None" or "" (empty).

If not, I want it to test the next possible location:

 myComputer.referenceLookup("/address/1/x/text")

Otherwise, I would like it to return an empty string ("").

I've tried looking in the Lua Manual to no avail as well as testing different forms in repl.it, but unfortunately, I can't replicate a similar example as I usually do when testing.

function firstLine(x)

if myComputer.referenceLookup("/Address/ .. (x) .. /text") != NULL or "None" or "" then

    return myComputer.referenceLookup("/Address/ .. (x) .. /text")

elseif myComputer.referenceLookup("/Address/1/ .. (x) .. /text") !=  NULL or "None" or "" then

    return myComputer.referenceLookup("/Address/1/ .. (x) .. /text")

else

    return ""

end

end

myComputer.out.firstHouseNumber = firstLine(housenumber)

It's worth noting that the usual way I would reference the fact is as follows:

myComputer.out.firstHouseNumber= myComputer.referenceLookup("/Address/housenumber/text")

or

myComputer.out.firstHouseNumber= myComputer.referenceLookup("/Address/1/housenumber/text")

The platform I'm using doesn't throw errors, it just will return blank instead of running the lua script so I am unable to debug (hence usually using repl.it).

I know this makes it a bit of an abstract question, but if anyone knows how I can do what I am describing, it would be very much appreciated.

when you say `return the string in that location` do you mean you are looking to read the text file and return the contents? or just return the validated path? -- Side note on your question: the lua example of firstLine is not valid lua. uses `NULL` rather than `nil`, `!=` rather than `~=`, and `"/Address/ .. (x) .. /text"` would not have the desired effect(it's missing some additional `"`) — Nifim, Jun 18 '19 at 17:11
@Nifim, Ideally, return the string which is at that location, my mistake about the != and the ~=, I've been converting from the DSL which the system used to Lua and confusingly, the ~= is used as approx. equal! — Gamora, Jun 19 '19 at 09:15

DarkWiiPlayer · Accepted Answer · 2019-06-19T08:32:41.580

Assumptions

Looking at your answer, I will assume that

myComputer.referenceLookup is defined somewhere else and works as intended (and not part of this question)
NULL is also defined somewhere else and represents some sort of nil-value

Answer

The line

if myComputer.referenceLookup("/Address/ .. (x) .. /text") != NULL or "None" or "" then

doesn't work, because the or operator doesn't work that way.

How Lua interprets it is

if (myComputer.referenceLookup("/Address/ .. (x) .. /text") != NULL) or "None" or ""

and since "None" is a String value and thus considered truthy, the if condition will always evaluate to true, so it will always return the first location. Also, there is no != operator in Lua; it's ~= instead.

As for a solution, you essentially need three comparisons like this:

if myComputer.referenceLookup("/Address/" .. x .. "/text") ~= NULL
and myComputer.referenceLookup("/Address/" .. x .. "/text") ~= "None"
and myComputer.referenceLookup("/Address/" .. x .. "/text") ~= "" then

Obviously calling the function three times is a bad idea, both because of performance and because it may have side effects, so it's better to save it into a variable first like so:

local result = myComputer.referenceLookup("/Address/" .. (x) .. "/text")
if result ~= NULL and result  ~= "None" and result  ~= "" then
  return result
end

Extra

If you want to make your program easier to extend, you can also use string.format to build the locations from templates. Say you have a table containing all your locations like this:

local locations = {
  "/Address/%s/text";
  "/Address/1/%s/text";
}

Then you can iterate through the entries using ipairs and build each location using string.format:

for index, template in ipairs(locations) do
  local result = myComputer.referenceLookup(template:format(x))
  if result ~= NULL and result  ~= "None" and result  ~= "" then
    return result
  end
end

Note that you can write string.format(template, x) as template:format(x) as long as template is a string. (further reading)

Thanks very much for that! My mistake about the != and the ~=, I've been converting from the DSL which the system used to Lua and confusingly, the ~= is used as approx. equal! The Extra help is also greatly appreciated. — Gamora, Jun 19 '19 at 09:16
Sorry, one more question! The location of the file is: myComputer.referenceLookup("/Address/example/text") not myComputer.referenceLookup(/Address/example/text). I.e. the quotation marks around the location. I'm wondering how that is going to work in this situation — Gamora, Jun 25 '19 at 14:22
@Bee I don't really get what you mean by that, sorry. The example without the quotation marks would not be valid Lua syntax at all. — DarkWiiPlayer, Jun 25 '19 at 15:22
That's basically what I'm saying, if you concat the bit inside the brackets as you have above, would it still return a string and therefore a valid address? I assume from your response that that is a yes, I'm fairly new to Lua so just trying to get my head around it! — Gamora, Jun 25 '19 at 15:24
@Bee short answer: `"hello" .. "world"` is the exact same as `"helloworld"`, there's absolutely no difference. Long answer: [Official Reference, 3.4.6 – Concatenation](https://www.lua.org/manual/5.3/manual.html#3.4.6) — DarkWiiPlayer, Jun 25 '19 at 17:02
Ok thanks, I was just wondering if it would work the same in a lookup, but I suppose it makes sense that it would. Thanks again! — Gamora, Jun 25 '19 at 17:06

Cycling through different file location options

1 Answers1

Assumptions

Answer

Extra