New to Lua - Table operation - lua

Can someone explain this to me? I have figured it out through this tutorial that this is known as a table. Coming from a C/C++ background, can someone explain how this works (I am trying to understand some existing Lua code)?
config = {
devices = {
C56 = "/dev/ttyS2",
ELTRA = "/dev/ttyS3",
-- MICORE = "/dev/ttyS4",
HID = "/dev/ttyS1",
KEYCARD = {
-- [6] = { tty="/dev/ttyS1", speed=9600 },
[7] = { tty="/dev/ttyS4", speed=9600 },
},
},
}
Is it a config table, consisting of a device table but then there is a KEYCARD table? What are C56 and ELTRA called in Lua? Are they fields?

A table in Lua is just an untyped map, like Javascript objects or Python dictionaries. The table associates one value (like "devices" or 6) with another value (like "/dev/ttyS2"). The value could be another table. Tables are used to create objects, maps, and arrays.
In your example, the config variable references a table. That table has one element, "devices", whose value is another table. That table has 5 elements. Four of those elements ("C56", "ELTRA", "MICORE", and "HID") have strings as their values. The fifth element ("KEYCARD") has a table as its value. That table has two elements (6, 7) whose values are other tables (each of two elements).

You have a config table two subtables within it, devices and Keycard, which is a subtable of devices. It's been a while since I used Lua, but to access, for example ELTRA, you'd type Config.devices.ELTRA and to access the 7 keycard you type Config.devices.KEYCARD[7]
To get at the speed of the keycard, you could do
speed = Config.devices.KEYCARD[7].speed

Related

How do I get a value by index from a nested table in lua?

I've been making a game with the LOVE2D game engine, and I've stumbled across an issue. I want to access a variable inside a nested table, but I don't know how.
Here's my code right now:
local roomNum = 1
local rooms = { r1 = { complete = false, name = "Room 1" }
if rooms[roomNum].complete == true then --problematic line
--do stuff
end
If I replace rooms[roomNum].complete with rooms.r1.complete then it works.
Any help would be appreciated!
'http://lua-users.org/wiki/TablesTutorial'
The provided link gives easy to understand examples on tables in Lua, so it may prove a useful resource in the future.
As for the why the replacement code worked, a dictionary is just sets of key/value pairs (kvp) . In examples from other languages, these pairs are normally shown as something like KeyValuePair.
In your case, you are using a variation on how dictionaries are used. As you have seen, you can use numbered indexes like room[1], or you can use a string like room["kitchen"]. It gets interesting when you provide a set of data to initialize the dictionary.
Building off of the provided data, you have the following:
local rooms = { r1 = { complete = false, name = "Room 1" }
r1 is equivalent to using rooms["r1"] without the dataset. In providing the dataset, any "named" Key can be referenced like it is a property of the dictionary (think of classes with public getter/setter). For the named keys of a dataset, you can provide a key as numbers as well.
local rooms = { [1] = { complete = false, name = "Room 1" }
This indexing fits the direction you were headed on providing a room index. So, you could either swap the dataset to use integers instead of r1, r2 and so on, or you could concatenate r and the index numbering. That is pretty much up to you. Keep in mind as you go further down nesting the same rules apply. So, complete could look like rooms[1].complete, rooms["r1" ].complete, or rooms.r1.complete.

Lua 5.0 - iterations over tables ignore duplicate keys even though values are different

Reading the injected comments in the Code Snippet should give enough context.
--| Table |--
QuestData = {
["QuestName"]={
["Quest Descrip"]={8,1686192712},
["Quest Descrip"]={32,1686193248},
["Quest Descrip"]={0,2965579272},
},
}
--| Code Snippet |--
--| gets QuestName then does below |--
if QuestName then
-- (K = QuestName) and (V = the 3 entries below it in the table)
for k,v in pairs(QuestData) do
-- Checks to make sure the external function that obtained the QuestName matches what is in the table before cont
if strlower(k) == strlower(QuestName) then
local index = 0
-- Iterates over the first two pairs - Quest Descrip key and values
for kk,vv in pairs(v) do
index = index + 1
end
-- Iterates over the second two pairs of values
if index == 1 then
for kk,vv in pairs(v) do
-- Sends the 10 digit hash number to the function
Quest:Function(vv[2])
end
end
end
end
end
The issue I'm running into is that Lua will only pick up one of the numbers and ignore the rest. I need all the possible hash numbers regardless of duplicates. The QuestData table ("database") has well over 10,000 entries. I'm not going to go through all of them and remove the duplicates. Besides, the duplicates are there because the same quest can be picked up in more than one location in the game. It's not a duplicate quest but it has a different hash number.
Key is always unique. It is the point of the key, that the key is pointing to unique value and you can't have more keys with same name to point different values. It is by definition by Lua tables.
It is like if you would want to have two variables with same name and different content. It does not make sense ...
The table type implements associative arrays. [...]
Like global variables, table fields evaluate to nil if they are not initialized. Also like global variables, you can assign nil to a table field to delete it. That is not a coincidence: Lua stores global variables in ordinary tables.
Quote from Lua Tables
Hashing in Lua
Based on comments, I update the answer to give some idea about hashing.
You are using hashing usually in low-level languages like C. In Lua, the associative arrays are already hashed somehow in the background, so it will be overkill (especially using SHA or so).
Instead of linked lists commonly used in C, you should just construct more levels of tables to handle collisions (there is nothing "better" in Lua).
And if you want to have it fancy set up some metatables to make it somehow transparent. But from your question, it is really not clear how your data look like and what you really want.
Basically you don't need more than this:
QuestData = {
["QuestName"]={
["Quest Descrip"]={
{8,1686192712},
{32,1686193248},
{0,2965579272},
},
},
}
As Jakuje already mentioned table keys are unique.
But you can store both as a table member like:
QuestData = {
-- "QuestName" must be unique! Of course you can put it into a table member as well
["QuestName"]={
{hash = "Quest Descrip", values = {8,1686192712} },
{hash = "Quest Descrip", values = {32,1686193248} },
{hash = "Quest Descrip", values = {0,2965579272} }
}
}
I'm sure you can organize this in a better way. It looks like a rather confusing concept to me.
You've said you can't "rewrite the database", but the problem is the QuestData table doesn't hold what you think it holds.
Here's your table:
QuestData = {
["QuestName"]={
["Quest Descrip"]={8,1686192712},
["Quest Descrip"]={32,1686193248},
["Quest Descrip"]={0,2965579272},
},
}
But, this is actually like writing...
QuestData["Quest Descrip"] = {8,1686192712}
QuestData["Quest Descrip"] = {32,1686193248}
QuestData["Quest Descrip"] = {0,2965579272}
So the second (and then, third) values overwrite the first. The problem is not that you can't access the table, but that the table doesn't contain the values any more.
You need to find a different way of representing your data.

How to get the program and feature in the lua table

from the below
Control Panel\Programs\Programs and Features
Need the List view in the Lua table
Suppose the Name of the Program installed , its version and size etc.
Issue is Lua seems to doesn't takes two argument like two dimension array
Using Lua table you can store List of string
But how to store a two dimension array ?
table = {}
for i = 1, 10 do
table[i] = {}
for j = 1, 10
table[i][j] = i+j
end
end
A multidimensional array in Lua is simply multiple tables in another :), one for each row/col.
Oh and another option is faking a multidimensional array through some simple arithmatic, something like: table[x+rowlen*y]

Lua table C api

I know of:
http://lua-users.org/wiki/SimpleLuaApiExample
It shows me how to build up a table (key, value) pair entry by entry.
Suppose instead, I want to build a gigantic table (say something a 1000 entry table, where both key & value are strings), is there a fast way to do this in lua (rather than 4 func calls per entry:
push
key
value
rawset
What you have written is the fast way to solve this problem. Lua tables are brilliantly engineered, and fast enough that there is no need for some kind of bogus "hint" to say "I expect this table to grow to contain 1000 elements."
For string keys, you can use lua_setfield.
Unfortunately, for associative tables (string keys, non-consecutive-integer keys), no, there is not.
For array-type tables (where the regular 1...N integer indexing is being used), there are some performance-optimized functions, lua_rawgeti and lua_rawseti: http://www.lua.org/pil/27.1.html
You can use createtable to create a table that already has the required number of slots. However, after that, there is no way to do it faster other than
for(int i = 0; i < 1000; i++) {
lua_push... // key
lua_push... // value
lua_rawset(L, tableindex);
}

erlang - how can I match tuple contents with qlc and mnesia?

I have a mnesia table for this record.
-record(peer, {
peer_key, %% key is the tuple {FileId, PeerId}
last_seen,
last_event,
uploaded = 0,
downloaded = 0,
left = 0,
ip_port,
key
}).
Peer_key is a tuple {FileId, ClientId}, now I need to extract the ip_port field from all peers that have a specific FileId.
I came up with a workable solution, but I'm not sure if this is a good approach:
qlc:q([IpPort || #peer{peer_key={FileId,_}, ip_port=IpPort} <- mnesia:table(peer), FileId=:=RequiredFileId])
Thanks.
Using on ordered_set table type with a tuple primary key like { FileId, PeerId } and then partially binding a prefix of the tuple like { RequiredFileId, _ } will be very efficient as only the range of keys with that prefix will be examined, not a full table scan. You can use qlc:info/1 to examine the query plan and ensure that any selects that are occurring are binding the key prefix.
Your query time will grow linearly with the table size, as it requires scanning through all rows. So benchmark it with realistic table data to see if it really is workable.
If you need to speed it up you should focus on being able to quickly find all peers that carry the file id. This could be done with a table of bag-type with [fileid, peerid] as attributes. Given a file-id you would get all peers ids. With that you could construct your peer table keys to look up.
Of course, you would also need to maintain that bag-type table inside every transaction that change the peer-table.
Another option would be to repeat fileid and add a mnesia index on that column. I am just not that into mnesia's own secondary indexes.

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