What is the difference between tables and metatables in Corona? What are the types of metatables? How and where can I use them? What is the main purpose of using tables and metatables?
Tables in Lua are the main data type you can use to create dynamic, structured data. Other languages have arrays, lists, dictionaries (key-value storage), in Lua you only have tables. The only operations you can do with a basic table is indexing and storing a value using the tab[key] syntax, i.e.:
local tab = {}
tab['key1'] = 'Hello' -- storing a value using a string key
tab.key2 = 'World' -- this is syntax sugar, equivalent to previous
print(tab.key1, tab['key2']) -- indexing, the syntax is interchangable
You cannot do anything else with basic tables, for example adding them:
local v1={x=0,y=0}
local v2={x=1,y=1}
print(v1+v2)
--> stdin:1: attempt to perform arithmetic on local 'v1' (a table value)
A metatable allows you to modify the behavior of tables, to specify what should be done when tables are added, multiplied, concatenated (..), etc. A metatable is just a table, which contains functions with special keys, also called metamethods. You can assign a metatable to a table using setmetatable(). For example:
local Vector = {} -- this will be the metatable for vectors
function Vector.__add(v1, v2) -- what to do when vectors are added
-- create a new table and assign it a Vector metatable
return setmetatable({x=v1.x+v2.x, y=v1.y+v2.y}, Vector)
end
function Vector.__tostring(v) -- how a vector should be displayed
-- this is used by tostring() and print()
return '{x=' .. v.x .. ',y=' .. v.y .. '}'
end
local v1 = setmetatable({x=1, y=2}, Vector)
local v2 = setmetatable({x=3, y=4}, Vector)
-- vectors are added and the resulting vector is printed
print(v1 + v2) --> {x=4,y=6}
If you want to understand metatables better, you should definitely read the Programming in Lua chapter on metatables.
Lua (which is the language that Corona is based on) uses metatables for different purposes.
The relevant entry in the manual is Section 2.8.
A nice tutorial can be found here or here.
A metatable is just a table like any other, but is set as metatable on another table (which I'll call a base table further on, to make a difference between the 2 tables).
A metatable can contain anything, but the special keys (starting with a double underscore) are the interesting ones. The values set to this keys in this table will be called on special occasions. Which occasion depends on which key. The most interesting are:
__index: Will be used whenever a key in the base table is looked up, but does not exist. This can either contain table, in which the key will be looked up instead, or a function, which will be passed the original table and the key. This can be used for implementing methods on tables (OOP style), for redirection, fall through cases, setting defaults, etc etc
__newindex: Will be used whenever a new key is to be assigned in a table (which was previously nil). If it's a table, the key will be assigned in that table. If it's a function, that function will be passed the original table, key and value. This can be used for controlling access to a table, preprocessing data, redirection of assignments.
__call: enables you to set a function to be called if you use eg. table().
__add,__sub,__mul,__div,__mod are used to implement binary operations,
__unm is used to implement unary operations,
__concat is used for implementing concatenation (the .. operator)
__len is used for implementing the length operator (#)
__eq,__lt,__le are used for implementing comparisons
A small thing to know when using __index & co.: in those methods, you should use rawget and rawset in order to prevent calling the metamethod each time again, causing a loop.
As a small example:
t={1,2,3} -- basetable
mt={} -- metatable
mt.__index=function(t,k)
print("__index event from "..tostring(t).." key "..k)
return "currently unavailable"
end
mt.__newindex=function(t,k,v)
print("__newindex event from "..tostring(t).." key: "..k.." value: "..v)
if type(k)=="string" then
rawset(t,k,v:reverse())
else
rawset(t,k,v)
end
end
mt.__call=function(t,...)
print("call to table "..tostring(t).." with arguments: ".. table.concat({...},','))
print("All elements of the table:")
for k,v in pairs(t) do print(k,v) end
end
setmetatable(t,mt)
t[4]="foo" -- this will run the __newindex method
print(t[5]) -- this will run the __index method
t("foo","bar")
-- Multiple fall through example:
t={}
mt={}
mt2={}
setmetatable(t,mt) -- metatable on base table
setmetatable(mt,mt2) -- second layer of metatable
mt.__index=function(t,k) print('key '..k..' not found in '..namelookup[t]) return getmetatable(t)[k] end -- tries looking nonexistant indexes up in mt.
mt2.__index=mt.__index -- function was written portably, reuse it.
t[1]='A'
mt[2]='B'
mt2[3]='C'
namelookup={[t]="t",[mt]="mt",[mt2]="mt2"}
print(t[1],t[2],t[3],t[4])
Now these are but silly examples, you can do much more complex stuff. Take a look at the examples, take a look at the relevant chapters in Programming in Lua, and experiment. And try not to get confused ;)
Related
I am really newbie in lua. I have this lua code
local gun_info = {
g_sword={rate=0.5;spd=0;dmg=1;ammo=1;};
g_pistol={rate=0.5;spd=5;dmg=1;ammo=40;};
g_knife={rate=0.8;spd=5;dmg=1;ammo=1;};
g_shuriken={rate=0.3;spd=5;dmg=1;ammo=40;};
g_bomb={rate=0.8;spd=5;dmg=1;ammo=20;};
};
I just want get values of every ammo. Other properties are no needed.
for k, v in pairs(gun_info) do
print(k, v[1], v[2], v[3], v[4], v[5])
end
this prints out whole tables but I need just value of ammos
Use comma between table variables rather than semicolon. Using semicolon is not syntactically wrong but optional in Lua. Semicolon is usually used to separate multiple statements written in single line.
You can directly access the variable ammo by indexing the key of the table
for k, v in pairs(gun_info) do
print(k, v.ammo)
end
v.ammo and v[ammo] are not same in Lua.
Note: The order in which the elements appear in traversal will not be the same as you defined and can produce different order each time. This is due to the way tables are implemented in Lua.
From the Lua 5.3 doc:
__index: The indexing access table[key]. ... The metamethod is looked up in table.
It says the same thing for __newindex, but not for any other metamethod.
If this were true (which it's not), it would be a major departure from previous versions of Lua. The following code outputs nil, as I would expect, but it's inconsistent with the doc.
#!/usr/bin/env lua5.3
local proto = {a = 54}
local t0 = {__index = proto}
print(t0.a)
To be clear: If the doc was correct, I would expect t0 in the above code to only require an __index field without an actual metatable for t0.a to be 54. So does anyone know what's going on with the doc?
You are incorrectly interpreting the meaning of the term. To say that it is "metamethod Y is looked up in X" does not mean that it searches the X table for an entry named Y. It means that it gets the metatable for X and looks up an entry named Y, as if by rawget(getmetatable(X) or {}, "Y"), as specified in the docs.
This terminology is repeatedly used in the metamethod descriptions. For example:
First, Lua will check the first operand (even if it is valid). If that operand does not define a metamethod for __add, then Lua will check the second operand.
It's not asking if the first (or second) operand have a method __add; it asks if they have a metamethod __add.
As you can see from the __add example, you have to specify which of the operands it tries to get metamethods from, and in which order. For table[key], the point of the text is that doesn't try to get the metamethod from key, only from table. That may seem a tad bit obvious, but completeness is better than incompleteness.
I try to make efficiently a copy of a lua table. I have written the following function copyTable() that works well (see below). But I imagined I could have something more efficient using the "passing by value" mechanism of the functions. I made a few tests to explore this mechanism :
function nop(x)
return x
end
function noop(x)
x={}
return x
end
function nooop(x)
x[#x+1]=4
return x
end
function copyTable(datatable)
local tblRes={}
if type(datatable)=="table" then
for k,v in pairs(datatable) do tblRes[k]=copyTable(v) end
else
tblRes=datatable
end
return tblRes
end
tab={1,2,3}
print(tab) -->table: 0x1d387e0 tab={1,2,3}
print(nop(tab)) -->table: 0x1d387e0 tab={1,2,3}
print(noop(tab)) -->table: 0x1e76f90 tab={1,2,3}
print(nooop(tab)) -->table: 0x1d387e0 tab={1,2,3,4}
print(tab) -->table: 0x1d387e0 tab={1,2,3,4}
print(copyTable(tab)) -->table: 0x1d388d0
We can see that the reference to the table is transferred unchanged through the functions (when I just read it or add things) except within noop() where I try a radical modification of the existing.
I read Bas Bossink and the answer made by Michael Anderson in this Q/A. Regarding the passing or tables as arguments, they emphasized the difference between "arguments passed by ref" and "arguments passed by values and tables are references" with examples where this difference appears.
But what does that mean precisely ? Do we have a copy of the reference, but what difference does that make with a passing through ref since the data pointed and therefore manipulated is still the same, not copied ? Is the mechanism in noop() specific when we try to affect nil to the table, specific to avoid the deletion of the table or in which cases does it trigger (we can see with nooop() that it is not always the case when the table is modified) ?
My question : how the mechanism of passing tables really works ? Is there a way to make a more efficient way to copy the data of a table without the burden of my copyTable ?
The rules of argument passing in Lua is similarly to C: everything is pass by value, but tables and userdata are passed around as pointers. Passing a copy of a reference does not appear so different in usage, but it is completely different than passing by reference.
For example, you brought this part up specifically.
function noop(x)
x={}
return x
end
print(noop(tab)) -->table: 0x1e76f90 tab={1, 2, 3}
You are assigning the value for the new table[1] into variable x (x now holds a new pointer value). You didn't mutate the original table, the tab variable still holds the pointer value to the original table. When you return from noop you are passing back the value of the new table, which is empty. Variables hold values, and a pointer is a value, not a reference.
Edit:
Missed your other question. No, if you want to deep-copy a table, a function similar to what you wrote is the only way. Deep copies are very slow when tables get large. To avoid performance issues, you might use a mechanism like "rewind tables", which keep track of changes made to them so they can be undone at later points in time (very useful in recursive with backtrack contexts). Or if you just need to keep users from screwing with table internals, write a "freezable" trait.
[1] Imagine the {} syntax is a function that constructs a new table and returns a pointer to the new table.
If you are sure that those 3 assumptions (A) are valid for "tab" (the table being copied):
There are no table keys
t1 = {}
tab = {}
tab[t1] = value
There are no repeated table values
t1 = {}
tab = {}
tab.a = t1
tab.b = t1
-- or
-- tab.a.b...x = t1
There are no recursive tables:
tab = {}
tab.a = tab
-- or
-- tab.a.b...x = tab
Then the code you provided is the smallest and almost as efficient as possible.
If A1 doesn't hold (i.e. you have table keys), then you must change your code to:
function copyTable(datatable)
local tblRes={}
if type(datatable)=="table" then
for k,v in pairs(datatable) do
tblRes[copyTable(k)] = copyTable(v)
end
else
tblRes=datatable
end
return tblRes
end
If A2 doesn't hold (i.e. you have repeated table values), then you could change your code to:
function copyTable(datatable, cache)
cache = cache or {}
local tblRes={}
if type(datatable)=="table" then
if cache[datatable] then return cache[datatable]
for k,v in pairs(datatable) do
tblRes[copyTable(k, cache)] = copyTable(v, cache)
end
cache[datatable] = tblRes
else
tblRes=datatable
end
return tblRes
end
This approach only pays off, though, if you have lots of repeated large tables. So, it is a matter of evaluating which version is faster for your actual production scenario.
If A3 doesn't hold (i.e. you have recursive tables), then your code (and both adjustments above) will enter an infinite recursive loop and eventually throw a stack overflow.
The simplest way to handle that is keeping a backtrack and throwing an error if table recursion happens:
function copyTable(datatable, cache, parents)
cache = cache or {}
parents = parents or {}
local tblRes={}
if type(datatable)=="table" then
if cache[datatable] then return cache[datatable]
assert(not parents[datatable])
parents[datatable] = true
for k,v in pairs(datatable) do
tblRes[copyTable(k, cache, parents)]
= copyTable(v, cache, parents)
end
parents[datatable] = false
cache[datatable] = tblRes
else
tblRes=datatable
end
return tblRes
end
My solution for a deepcopy function which handles recursive tables, preserving the original structure may be found here: https://gist.github.com/cpeosphoros/0aa286c6b39c1e452d9aa15d7537ac95
I am trying to modify a collection of arrays inside of a variadic function. I end up working on a copy when trying to add to the arrays and they get lost after the call. Is there any way to pass values by ref in Lua?
function myfunc(...)
local args = {...}
--do work on args--
end
"do work" doesn't actually end up doing anything but it works outside the function just fine.
Obviously I could pass an array of arrays and not use ... but that kinda defeats the purpose of using ...
In Lua, you can't just choose to pass variables by reference or not. Basic types are never passed by reference (like numbers and booleans), others are always passed by reference (like tables, userdata and strings). In the case of strings this does not matter much, because they are immutable anyhow.
So either you pass your arguments you want to work on globally as strings like this:
a=2
b=3
function myfunc(...)
local args={...}
for k,v in pairs(args) do
_G[v]=_G[v]+k
end
end
myfunc('a')
print(a) -- 3
myfunc('a','b')
print(a,b) -- 4 5
Note that this only works on globals, since locals are not kept in a table.
Working with tables makes this kind of things less painful:
function myfunc(t)
for k,v in pairs(t) do
t[k]=v+k
end
end
tab1={a=2}
myfunc(tab1)
print(tab1.a) -- 3
tab2={a=2,b=3}
myfunc(tab2)
print(tab2.a,tab2.b) -- 3 5
The purpose of using ... is grouping the whole parameter list in one varible. That has little to do with the by-reference or by-value nature of the parameters.
All natural types in Lua are passed by value, with tables being the only exception.
The simplest way to do what you want is to pass an array of arrays. If the two extra characters seem like too much typing, know that you can remove the parenthesis instead:
foo({a,b,c})
foo{a,b,c} -- equivalent
In Lua, there seem to be two ways of appending an element to an array:
table.insert(t, i)
and
t[#t+1] = i
Which should I use, and why?
Which to use is a matter of preference and circumstance: as the # length operator was introduced in version 5.1, t[#t+1] = i will not work in Lua 5.0, whereas table.insert has been present since 5.0 and will work in both. On the other hand, t[#t+1] = i uses exclusively language-level operators, wheras table.insert involves a function (which has a slight amount of overhead to look up and call and depends on the table module in the environment).
In the second edition of Programming in Lua (an update of the Lua 5.0-oriented first edition), Roberto Ierusalimschy (the designer of Lua) states that he prefers t[#t+1] = i, as it's more visible.
Also, depending on your use case, the answer may be "neither". See the manual entry on the behavior of the length operator:
If the array has "holes" (that is, nil values between other non-nil values), then #t can be any of the indices that directly precedes a nil value (that is, it may consider any such nil value as the end of the array).
As such, if you're dealing with an array with holes, using either one (table.insert uses the length operator) may "append" your value to a lower index in the array than you want. How you define the size of your array in this scenario is up to you, and, again, depends on preference and circumstance: you can use table.maxn (disappearing in 5.2 but trivial to write), you can keep an n field in the table and update it when necessary, you can wrap the table in a metatable, or you could use another solution that better fits your situation (in a loop, a local tsize in the scope immediately outside the loop will often suffice).
The following is slightly on the amusing side but possibly with a grain of aesthetics. Even though there are obvious reasons that mytable:operation() is not supplied like mystring:operation(), one can easily roll one's own variant, and get a third notation if desired.
Table = {}
Table.__index = table
function Table.new()
local t = {}
setmetatable(t, Table)
return t
end
mytable = Table.new()
mytable:insert('Hello')
mytable:insert('World')
for _, s in ipairs(mytable) do
print(s)
end
insert can insert arbitrarily (as its name states), it only defaults to #t + 1, where as t[#t + 1] = i will always append to the (end of the) table. see section 5.5 in the lua manual.
'#' operator only use indexed key table.
t = {1, 2 ,3 ,4, 5, x=1, y=2}
at above code
print(#t) --> print 5 not 7
'#' operator whenever not using.
If you want to '#' operator, then check it to table elements type.
Insert function can using any type use.But element count to work slow than '#'