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 '#'
Related
After reading this topic and after experimenting a bit, I am trying to understand how the Lua length operator works when a table contains nil values.
Before I started to investigate, I thought that the length was simply the number of consecutive non-nil elements, starting at index 1:
print(#{nil}) -- 0
print(#{"o"}) -- 1
print(#{"o",nil}) -- 1
print(#{"o","o"}) -- 2
print(#{"o","o",nil}) -- 2
That looks pretty simple, right?
But my headache started when I accidentally added an element after a nil-terminated table:
print(#{"o",nil,"o"})
My guess was that it should probably print 1 because it would stop counting when the first nil is found. Or maybe it should print 2 if the length operator is greedy enough to look for non-nil elements after the first nil. But the above code prints 3.
So I’ve ran several other tests to see what happens:
-- nil before the end
print(#{nil,"o"}) -- 2
print(#{nil,"o","o"}) -- 3
print(#{"o",nil,"o"}) -- 3
-- several nil elements
print(#{"o",nil,nil}) -- 1
print(#{nil,"o",nil}) -- 0
print(#{nil,nil,"o"}) -- 3
I should mention that repl.it currently uses Lua 5.1.5 which is rather old, but if you test with the Lua demo, which currently uses Lua 5.3.5, you’ll get the same results.
By looking at those results and by looking at this answer, I assume that:
if the last element is not nil, the length operator returns the full size of the table, including nil entries if any
if the last element is nil, it counts the number of consecutive non-nil and stops counting at the first nil
Are those assumptions correct?
Can we predict a 100% well-defined behavior when a table contains one or several nil values?
The Lua documentation states that the length of a table is only defined if the table is a sequence. Does that mean that the length operator has undefined behavior for non-sequences?
Apart from the length operator, can nil values cause any trouble in a table?
We can predict some behaviour, but it is not standardised, and as such you should never rely on it. It's quite possible that the behaviour may change within this major version of Lua.
Should you ever need to fill a table with nil values, I suggest wrapping the table and replace holes with a unique placeholder value (eg. NIL={}; if v==nil then t[k]=NIL end, this is quite cheap to test against and safe.).
That said...
As there is even a difference in the result of # depending on how the table is defined, you'll have to distinguish between statically defined (constant) tables and dynamic defined (muted) tables.
Static table definitions:
#{nil,nil,nil,nil,nil, 1} -- 6
#{3, 2, nil, 1} -- 4
#{nil,nil,nil, 1, 1,nil} -- 0
#{nil,nil, 1, 1, 1,nil} -- 5
#{nil, 1, 1, 1, 1,nil} -- 5
#{nil,nil,nil,nil, 1,nil} -- 0
#{nil,nil, 1,nil, 1,nil,nil} -- 5
#{nil,nil,nil, 1,nil,nil, 1,nil} -- 4
Using this kind of definition, as long as the last value is non-nil, you will get a length equal to the position of the last value. If the last value is nil, Lua starts a (non-linear) search from the tail until it finds the first non-nil value.
Dynamic data definition
local x={}; x[5]=1;print(#x) -- 0
local x={}; x[1]=1;x[2]=1;x[3]=1;x[5]=1;print(#x) -- 3
local x={}; x[1]=1;x[2]=1;x[4]=1;x[5]=1;print(#x) -- 5
#{[5]=1} -- 0
local x={nil,nil,nil,1};x[5]=1;print(#x) -- 0
As soon as the table was changed once, the operator works the other way (that includes static definitions with []). If the first element is nil, # always returns 0, but if not it starts a search that I did not investigate further (I guess you can check the sources, though I don't think it's a standard binary search), until it finds a nil value that is preceded by a non-nil value.
As said before, relying on this behaviour is not a good idea, and invites lots of issues down the road. Though if you want to make a nasty unmaintainable program to mess with a colleague, that's a sure way to do it.
When a table is a sequence (all numeric keys start at 1 and there are no nil gaps), # is defined to be precisely the count of those elements.
For non-sequence tables, it is a bit more complicated. Lua 5.2 seems to leave the result as undefined. For 5.1 and 5.3, the result of the operation is a border.
A border in a table is any positive index that contains a non-nil value followed by nil, or 0 if the first element is nil. # is defined to return any value that satifies these conditions.
Looking at it from another perspective, since tables contain an "array" part and a "map" part, Lua has no way of knowing where the "map" indices start. For example, you can create a table with 1000 values and then set the first 999 of them to nil; that could leave you with a table of "size" 1000. However, you can also start with an empty table and set the 1000th element, having a table of "size" 0 but still structurally equivalent to the first one. The result of # is then simply the first valid value the internal algorithm finds.
The length operator produces undefined behaviour for tables that aren't sequences (i.e. tables with nil elements in the middle of the array). This means that even if the Lua implementation always behaves in a certain way, you shouldn't rely on that behaviour, as it may change in future versions of Lua, or in different implementations like LuaJIT.
You can use nils in tables - there is nothing wrong with that - just don't use the length operator on a table which might have nils before non-nil values.
The post you linked to contains more details about how the actual algorithm works. It mentions counting elements with a "binsearch", i.e. a binary search. This is not the same as just counting the elements one by one - if there are nils in the table, then depending on their exact position, the binary search algorithm may treat them as the end of the table, or may just ignore them.
To sum up, the algorithm is harder to predict than you were assuming, and even though it is technically possible to predict what will happen in any given case, you shouldn't rely on that behaviour as it is liable to change.
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.
I have been learning about metatables in Lua and I wanted to implement range operators as in Ruby, so I used this model
debug.setmetatable(1, {
__concat = function(a, b)
if a > b then
error(table.concat({
"attempt to create a range of values with a",
"minimum larger than the maximum"
}, " "))
elseif a == b then
return a
else
return unpack((function(nStart,nEnd)
local nTable = {}
for it = nStart,nEnd do
table.insert(nTable, it)
end
return nTable
end)(a, b))
end
end
})
print(6 .. 6)
But it seems that that it continues to use the default behavior. Is there any way to get this to work? I am aware that I could make a function to emulate the behavior and call it with range(n,n2) or similar but that defeats the purpose. Thanks.
Please see section 3.4.5 of the Lua 5.2 manual.
The string concatenation operator in Lua is denoted by two dots ('..'). If both operands are strings or numbers, then they are converted to strings according to the rules mentioned in §3.4.2. Otherwise, the __concat metamethod is called (see §2.4).
If you want to change this behavior, look into lvm.c, specifically the luaV_concat function.
The, well, special specification of Lua's length operator made me wonder whether Lua would be "allowed" to return a negative value in a situation like
#{[-5]=1,[-1]=3}
It says:
The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil;
n=-5 and n=-1 would meet this criterion in my example, right?
moreover, if t[1] is nil, n can be zero.
Right, it can be zero, but it's not guaranteed, right?
For a regular array, with non-nil values from 1 to a given n, its length is exactly that n, the index of its last value.
This isn't the case here, so it doesn't apply.
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).
This is the case here, so again, n=-5 and n=-1 would be valid return values, right?
Can I be entirely certain that Lua always returns 0 for the example table, or any other table containing only negative indices? If (hypothetically) I'd be writing a Lua interpreter and would return either of those values, would I be conforming with the specifications?
Edit
Obviously, the way Lua is implemented, it does not return negative values. I felt the length operator is somewhat underdocumented and I see that Lua 5.2's documentation has changed. It now says:
Unless a __len metamethod is given, the length of a table t is only defined if the table is a sequence, that is, the set of its positive numeric keys is equal to {1..n} for some integer n. In that case, n is its length. Note that a table like
{10, 20, nil, 40}
is not a sequence, because it has the key 4 but does not have the key 3.
So, it now talks about positive numeric keys, that's much clearer. I'm left wiser but not totally happy with the documentation. When it says the "length is only defined if the table is a sequence", it should also state that even if the table is not a sequence a value is returned, but the behavior is undefined. Also, this table looks pretty much like a sequence:
a = setmetatable(
{0},
{
__index = function(t,k)
return k < 10 and k or nil
end
}
)
i = 1
while a[i] do
print(a[i])
i = i+1
end
--[[ prints:
0
2
3
4
5
6
7
8
9
]]
print(#a)
-- prints: 1
However, this is becoming nitpicking as it's pretty clear that it wouldn't make sense to take into account what mess __index might make. And Stackoverflow is certainly not the place to complain about documentation that could be more precise.
As you have noted, the specification of the length operator has changed between 5.1 and 5.2.
Can I be entirely certain that Lua always returns 0 for the example table, or any other table containing only negative indices?
You can for the current reference implementation, which ensures that for ilen defined
function ilen (xs)
local i=0
while xs[i+1] do i=i+1 end
return i
end
we always have #xs >= ilen(xs) - see the definition of luaH_getn in the ltable.c source. But the specification now deliberately does not promise this behaviour: a conformant implementation can return nil or raise an exception for attempts to find the length of tables that are not sequences.
From the text in reference link. The answer is NO.
I think your confusing the fact that if a NIL is found then the length of the table is deemed to be position the NIL was found -1.
Therefore if t(1) is NIL then 1 - 1 = 0 so the table length is 0.
If the length of a table was 5 then the next position or t(6) IS or WOULD BE NIL
The length of a table t is defined to be any integer index n such that t[n] is not nil and t[n+1] is nil; moreover, if t[1] is nil, n can be zero.
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 ;)