if I call a function foo(t[1]) through the C API, can I in any way see what table and what index is as argument, in this case t and 1?
The problem at hand is a function move_card(card, table_slots[0]) where I move a card from one slot on a game area to another one. table_slots can also be hand or player_slots. This can be solved using a metatable, stating the name of the table being accessed. But the index is impossible to solve, or is it? table_slots[0] can be a card table, or an overlay or stack (arrays of cards), or nil if it's empty, as could player_slots. But what I need to know is in fact if it's from a table_slots or player_slots.
Could I hack some code analysis? Like, get the line where the function call is made, and then grep the index through a regexp? I could send table_slots[0] as a string, also, like move_card(card, "table_slot[0]"). Not as elegant, but still working.
No, once a value reaches a function, be it C or Lua, its origins are lost.
On the other hand, when an error occurs in Lua, the runtime system tries quite a bit to reconstruct the origin of the relevant values, but it does not always succeeds.
Related
In a Roblox game I'm programming, I want to have a table of boolean values to iterate over ensuring that they're all false before making another one true, e.g.;
local bool1 = true
local bool2 = false
local bool3 = false
local bool4 = false
local tbl1 = {}
table.insert(tbl1,boolX) -- where "X" is the number above, did this in interest of shortening
for i,v in pairs(tbl1) do
if v then v = not v end
end
However, as stated in the penultimate paragraph of section 2.1 of the 5.3 manual (knowing, albeit, that Luau uses 5.1 as its base);
Tables, functions, threads, and (full) userdata values are objects: variables do not actually contain these values, only references to them. Assignment, parameter passing, and function returns always manipulate references to such values; these operations do not imply any kind of copy.
That in mind, that means that I'm not actually shoving bool1 through bool4 into the table, just their values; the table would look like {true, false, false, false}. This means that I can't simply write a loop to iterate through the table and invert any trues I find;
local bool1 = true
local tbl1 = {}
table.insert(tbl1,bool1)
tbl1[1] = false
print(bool1)
print(tbl[1])
--output:
-- true
-- false
I should point out that I very well could just shove all my variables into one giant if/else and call it a night, but that is a lot of variables to check, a lot of typing, and I can't think of any other more elgant and less tedious way other than finding a form of iteration over them all.
I want to be able to have an actual reference, in some form, to the actual variables so that I can modify them from within the table and make them iterable.
I've attempted to follow the examples given in the best answer to this question, with no successes. I would need to be able to, and cannot with these examples, substitute for any given variable at any given time, rather than just having one or two I want declated and thus returned as shown. I've attempted wrapping them in a function to provide my table and variable as arguments, but that doesn't seem to have any effect; it either outputs nothing, or nil.
The following answer after has one example that seems like it could work, but overall is pointless for what I'm trying to achieve; I don't want to re-declare or re-assign the variables I already have, I just want to return the specific value.
I've attempted using a key/value pair, making the key the potential variable name and making it equal the variable's value, but I've no way to make that key return a variable of the same name. I even attempted merging this method and the setmetatable method mentioned in the first set of examples to see if I couldn't substitute from there, to no avail.
I should point out that, while I'm not a complete newbie to Lua or Luau, I'm also not an expert in the field by any meaning of the word; I'll catch on quick, but where possible, explain like I'm 10.
The only way to iterate over local variables that are not referenced by table fields, is to use debug.getlocal in a loop.
Opinionated:
But this is a very bad approach to your problem.
You should refactor the code. If you need to update a list of variables have them in a list from the start. Don't be afraid of refactoring. Improving things usually pays off on the long term.
If your codebase is so bad that you are afraid of putting a few variables into a table you should really think over the design.
There is no way to do this in Roblox Luau.
Any potential way to do so in standard Lua is impossible to do in Luau due to how heavily sandboxed the latter is compared to the former.
Possible solutions that were recommended via comments include: making the variables needed available in _G as globals, then using them there; or using a pointer value in the table equivalent to the one you want, even though it won't technically change the value you want itself.
Another answer recommends the use of debug.getlocal() for standard Lua, but doesn't work in Luau; keep this in mind when attempting to use it, as you will get an attempt to call a nil value error.
The recommended way to do this is not to do it at all, and instead have your variables and/or values in your table from the start.
Not only does this provide ease-of-access in many cases, but it will allow you to iterate through them significantly easier than trying to shove them into tables and the like. Refactoring, if you need to do something similar to this, is highly recommended across the board.
I am creating a game in love2d (LuaJIT) and I am creating a debug window for changing values at runtime. I was able to do that, however, I also now want to be able to call functions. For example, I traverse a table and there is a function in this table called "hello" which is written like this:
self.hello = function(str, num)
print(string.format("%s: %d", str, num))
end
From the expression of type(object.hello) I only see function. If it had been a table, I could have traversed it and see the keys and values, but it is just a "function" and I have no idea how to properly call it, as I don't know what arguments does it take and how many. Is there a way to find this out at runtime in lua? Maybe this information is also stored in some table elsewhere?
it is just a "function" and I have no idea how to properly call it
Neither does Lua. As far as Lua is concerned, any Lua function can take any number of parameters and return any number of parameters. These parameters could be of any type, as could its return values.
Lua itself does not store this information. Or at least, not in any way you could retrieve without doing some decompiling of its byte-code. And since you're using LuaJIT, that "decompiling" might require decompiling assembly.
I am creating a caching system for complex shapes in Lua and I want it to be backwards compatible as a plug and play replacement for the default, non cached system, to increase performance. I've successfully rendered an object with 6.4 million points which gets turned into 3.1 million trapezoids to make a shape instead of using 6.4 million triangles, cache it and continue rendering without any performance hit beyond initial caching.
I want to set up a cached data table which uses the reference to the original table as the index. The problem is, if I use tostring, I end up with the entire table as a nicely formatted human-readable string which wouldn't be nice to use as a short index to reference the table.
I do not want to require the dev to assign an ID to the object because the default system doesn't do this, and it is an extra step which can be avoided by simply using the address which is unique to that object anyways..
I can not find a Lua implementation of lua_topointer(L, idx)... but maybe I'm missing something?
A few issues - my tostring no longer shows table: 0x12345678
I have rewritten a lot of meta-functions and __tostrings to output the data for debugging purposes. But, on Lua demo I can't find the exact function which replaces this behavior, and I didn't replace print in my latest version. So print is default, etc...
On Lua demo I have also tried getmetatable table, function m:__tostring( ) return 'blah' end
and print( { } ), print( tostring( { } ) )
The first prints the address. The second prints blah. So because print isn't using tostring, by default, what is it using? Is that accessible via Lua?
If all else fails, I'll probably have to get rid of my __tostring table metamethod and make a secondary function to output that particular data type which defaults the purpose of having a __tostring method for everything ( and the fact that print ignores it; one of the reasons I rewrote print in an old version... but I reverted to keep it default )
So, does anyone know a Lua function I can call, which is not tostring, which can get the address of a table in Lua? It must return the value so it can be used.
Clarification (sorry the question was not specific): They both try to convert the item on the stack to a lua_Number. lua_tonumber will also convert a string that represents a number. How does luaL_checknumber deal with something that's not a number?
There's also luaL_checklong and luaL_checkinteger. Are they the same as (int)luaL_checknumber and (long)luaL_checknumber respectively?
The reference manual does answer this question. I'm citing the Lua 5.2 Reference Manual, but similar text is found in the 5.1 manual as well. The manual is, however, quite terse. It is rare for any single fact to be restated in more than one sentence. Furthermore, you often need to correlate facts stated in widely separated sections to understand the deeper implications of an API function.
This is not a defect, it is by design. This is the reference manual to the language, and as such its primary goal is to completely (and correctly) describe the language.
For more information about "how" and "why" the general advice is to also read Programming in Lua. The online copy is getting rather long in the tooth as it describes Lua 5.0. The current paper edition describes Lua 5.1, and a new edition describing Lua 5.2 is in process. That said, even the first edition is a good resource, as long as you also pay attention to what has changed in the language since version 5.0.
The reference manual has a fair amount to say about the luaL_check* family of functions.
Each API entry's documentation block is accompanied by a token that describes its use of the stack, and under what conditions (if any) it will throw an error. Those tokens are described at section 4.8:
Each function has an indicator like this: [-o, +p, x]
The first field, o, is how many elements the function pops from the
stack. The second field, p, is how many elements the function pushes
onto the stack. (Any function always pushes its results after popping
its arguments.) A field in the form x|y means the function can push
(or pop) x or y elements, depending on the situation; an interrogation
mark '?' means that we cannot know how many elements the function
pops/pushes by looking only at its arguments (e.g., they may depend on
what is on the stack). The third field, x, tells whether the function
may throw errors: '-' means the function never throws any error; 'e'
means the function may throw errors; 'v' means the function may throw
an error on purpose.
At the head of Chapter 5 which documents the auxiliary library as a whole (all functions in the official API whose names begin with luaL_ rather than just lua_) we find this:
Several functions in the auxiliary library are used to check C
function arguments. Because the error message is formatted for
arguments (e.g., "bad argument #1"), you should not use these
functions for other stack values.
Functions called luaL_check* always throw an error if the check is not
satisfied.
The function luaL_checknumber is documented with the token [-0,+0,v] which means that it does not disturb the stack (it pops nothing and pushes nothing) and that it might deliberately throw an error.
The other functions that have more specific numeric types differ primarily in function signature. All are described similarly to luaL_checkint() "Checks whether the function argument arg is a number and returns this number cast to an int", varying the type named in the cast as appropriate.
The function lua_tonumber() is described with the token [-0,+0,-] meaning it has no effect on the stack and does not throw any errors. It is documented to return the numeric value from the specified stack index, or 0 if the stack index does not contain something sufficiently numeric. It is documented to use the more general function lua_tonumberx() which also provides a flag indicating whether it successfully converted a number or not.
It too has siblings named with more specific numeric types that do all the same conversions but cast their results.
Finally, one can also refer to the source code, with the understanding that the manual is describing the language as it is intended to be, while the source is a particular implementation of that language and might have bugs, or might reveal implementation details that are subject to change in future versions.
The source to luaL_checknumber() is in lauxlib.c. It can be seen to be implemented in terms of lua_tonumberx() and the internal function tagerror() which calls typerror() which is implemented with luaL_argerror() to actually throw the formatted error message.
They both try to convert the item on the stack to a lua_Number. lua_tonumber will also convert a string that represents a number. luaL_checknumber throws a (Lua) error when it fails a conversion - it long jumps and never returns from the POV of the C function. lua_tonumber merely returns 0 (which can be a valid return as well.) So you could write this code which should be faster than checking with lua_isnumber first.
double r = lua_tonumber(_L, idx);
if (r == 0 && !lua_isnumber(_L, idx))
{
// Error handling code
}
return r;
As the title says. If I have a table p in lua, is using
table.remove(p)
the same as
p[#p] = nil
if so which is quicker - I'd guess the second, but would like some reassurance.
By the 'same as' I mean does the internal array storage shrink using assignment to nil? I can't seem to find this documented anywhere. Does setting the last element in an array to nil, or the last 10 elements in an array to nil mean the array will be shrunk, or does it always keep the storage and never shrink the array again?
I've assumed the array is contiguous, i.e. it has values stored in each array entry up to #p.
Setting the last element to nil will not be a function call. So in that way, it will certainly be faster than table.remove. How much that matters is up to you.
By the 'same as' I mean does the internal array storage shrink using assignment to nil? I can't seem to find this documented anywhere.
It isn't documented; this allows the implementation to change. All that Lua promises is that setting it to nil will decrease the size returned by subsequent calls to #p. Anything more than that is up to Lua, and is certainly subject to change without warning. It's nothing you should rely on.
However, I would respectfully suggest that if you're thinking about these kinds of micro-optimizations, you probably shouldn't be using a scripting language. A scripting language should be used for code where performance is not important enough to spend a great deal on.
p[#p] = nil will be faster, and identical for the case where table.remove is the last position
As an extra note, table.remove(func_call()) may do unexpected things if the function call returns multiple values.
Going from the implementation of the Lua 5.1 VM as described in Lua Performance Tips by Roberto Ierusalimschy (chief architect of Lua), the table's allocated storage doesn't change until the next time the table is rehashed - and, as stated time and time again, you really shouldn't be thinking about this unless you have hard profiling data showing it's a significant problem.
As for the difference between table.remove(t) and t[#t] = nil, see my answer to What's the difference between `table.insert(t, i)` and `t[#t+1] = i`.
I think it is better to be consistent with adding are removing elements from array type tables. table.insert and table.remove make your code consistent and easier to read.
Re: tables Lua doesn't resize the table until you have added the first new element to it after having previously removed elements.
table.remove(p) returns the value that was removed. p[#p] = nil does not return anything.