Append elements to a Lua table from variables / Clearing a Lua table - lua

Question

I make a function to parsing path name and file name from a computer directory using Lua Cheat Engine, next I want store the results in to a Lua table.
My function :
function addSongList()
load_dialog = createOpenDialog(self)
load_dialog.InitalDir = os.getenv('%USERPROFILE%')
load_dialog.Filter = 'MP3 files|*.mp3|*'
load_dialog.execute()
file = load_dialog.FileName
if file then
--- parsing path and filename
local pathN = file:match("(.*[\\/])")
local path, name = file:match('(.*\\)(.*)%.mp3')
--- test to open E:\MyMP3\mysong.mp3
print(pathN) --- result : E:\MyMP3\
print(name) --- result : mysong.mp3
end
end
local mp3Table = {}
table.insert(mp3Table,{pathN,name})
Is this correct way and correct syntax using table.insert(mp3Table,{pathN,name})
How to check if elements already added to the table by print out them?
How to clearing / empty the table ?
Thank you

Answer 1

1 - Inserting to the table:
table.insert(tb, value) inserts the value to de table tb. Using table.insert(mp3Table,{pathN,name}) you are dynamically creating a (sub)table and then appending to the main one.
2 - Printing the table.
As already pointed out you can just traverse the table using pairs or ipairs in order to get the elements.
I prefer ipairs in this case because the table is numerically indexed and order is guaranteed in accordance to table.insert.
The inner table must be indexed by numbers because you created it usign numeric indices.
for k, v in ipairs(mp3Table) do
print(v[1], v[2])
end
But you can also opt for a metatable which will also give you the possibility to generate a string representation for the table:
mp3Table_mt =
{
__tostring = function(self)
local ret = {}
for k, v in ipairs(self) do
table.insert(ret, v[1] .. "\t" .. v[2])
end
return table.concat(ret, "\n")
end
}
When initializing mp3Table you have to assign the metatable
local mp3Table = setmetatable({}, mp3Table_mt)
Then you can just tell Lua to print the table:
print(mp3Table)
3 - Empty/Delete the table:
Well there are two different things here. One is empty another is delete.
Lua uses garbage collection so actual deleting only occurs when there are no more references to a particular table. What you can do to tell Lua you no longer need a variable is assing nil to it. If there is no other reference to the value your variable was pointing to, the GC will clean it when it runs.
But you can empty the table without deleting it.
It may be tempting to say that mp3Table = {} "empties the table". But it does not.
What you are doing in this case is assigning a fresh new table to mp3Table variable. And if any other variable is still pointing to the old table it will no get collected and the inner values will remain untouched. If there's no such other variable, the table will be garbage collected just as if you assigned nil to mp3Table variable.
So to effectivelly empty a table you have to traverse it and set all its variables to nil.
function clearTable(tb)
for i, v in pairs(tb) do
tb[i] = nil
end
end
Specifically in the case asked, just assigning a new table to mp3Table may be enough because there are no more references to the same table. Assign nil afterwards is not necessary. What matters is if there are variables pointing to the same value. If you know what you are doing and the consequenses then no problem go ahead.
Putting it all together:
mp3Table_mt =
{
__tostring = function(self)
local ret = {}
for k, v in ipairs(self) do
table.insert(ret, v[1] .. "\t" .. v[2])
end
return table.concat(ret, "\n")
end
}
function addSongList(mp3Table)
local load_dialog = createOpenDialog(self)
load_dialog.InitalDir = os.getenv('%USERPROFILE%')
load_dialog.Filter = 'MP3 files|*.mp3|*'
load_dialog.execute()
file = load_dialog.FileName
if file then
--- parsing path and filename
local pathN = file:match("(.*[\\/])")
local path, name = file:match('(.*\\)(.*)%.mp3')
--- test to open E:\MyMP3\mysong.mp3
print(pathN) --- result : E:\MyMP3\
print(name) --- result : mysong.mp3
table.insert(mp3Table,{pathN,name})
end
return mp3Table
end
function clearTable(tb)
for i, v in pairs(tb) do
tb[i] = nil
end
end
local mp3Table = setmetatable({}, mp3Table_mt)
print(addSongList(mp3Table))
clearTable(mp3Table) -- I'm not assigning a new one. Just clearing the fields.
print(mp3Table) -- Must print nothing

Answer 2

1)yes
2)printing table in cycle:
for k,v in pairs(mp3Table) do
print( v.pathN, v.name)
end
3)empty table
mp3Table = {} -- clean
mp3Table = nil -- delete