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Use table key inside same (anonymous) table
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Closed 5 years ago.
So I'm trying something which i think should be very easy, but I just can't get it to work...
Basically what I'm trying to do is:
myTable = {
a = 1,
b = a + 1
}
This is not working, I get the error that "a" is nil. Reasonable.
What i have already tried is
myTable = {
a = 1,
b = myTable.a + 1
}
and
myTable = {
a = 1,
b = self.a + 1
}
But it gives me the error me that "myTable"/"self" is nil.
I got the feeling that the solution is rather simple but i couldn't find it out by myself and google wasn't that helpful as well.
There's no way of doing this in one statement (at least not without calling any functions or using metatables). That's because in a statement like foo = bar, the foo variable isn't assigned until the bar expression has been evaluated.
In your second example, the myTable variable isn't assigned until the closing curly brace, so the myTable in myTable.a + 1 is treated as an unnassigned global variable, and gets a value of nil. The self in your third example is the same, only you don't try to assign anything to it later. (In Lua, self is only special inside functions written with the colon syntax.)
To do what you want to do, you have to do something like this:
myTable = {
a = 1
}
myTable.b = myTable.a + 1
Or this:
local a = 1
myTable = {
a = a,
b = a + 1
}
Related
How can I get variables that not declared yet?
Here are simple example:
a = b
b = 123
What I want from these 2 lines is a << 123. But obv it doesn't work.
I know the easy way to get the answer a = 123 is cut 1st line and paste it to lower than 2nd line.
But I'm in some problem. I need some function like 'WillDeclaredVar()' that I can use in like this:
a = WillDeclaredVar(b)
sheepCount = 123
b = sheepCount
print(a)
so I can get the answer '123'.
Or there are any built-in functions that will allows me to do similar thing?
===
I think the link given by timrau is not telling my case. the key point is how to get Variables 'that not declared yet'.
===
Adding actual Code:
triggerCount = 0 -- Counting number of 'Trigger' function
local Trigger = function (t)
triggerCount = triggerCount + 1
return Trigger (t)
end
-- following Triggers are same as while statement.
-- following Triggers doing: Add 1 MarineCount until get 64000 MarineCount
Trigger { -- Here the Trigger function. Now triggerCount = 1.
players = {P1}
actions = {
SetDeaths(P1, Add, 1, "Terran Marine")
},
flag = {preserved},
}
Portal(LoopStart);
-- function Portal(VariableName) returns VariableName = triggerCount. So LoopStart = 1.
Trigger { -- Now triggerCount = 2.
players = {P1}
actions = {
LinkList(LoopEnd, LoopStart);
-- function LinkList(From, To) changes 'From' Trigger's next pointer to the 'To' Trigger.
-- But now the problem happens. Because 'LoopEnd' is not declared yet.
},
flag = {preserved},
}
Trigger { -- Now triggerCount = 3.
players = {P1}
conditions = {
Deaths(P1, Exactly, 64000, "Terran Marine");
}
actions = {
_LinkList(LoopEnd);
-- Reset LoopEnd's next pointer(= LoopEscape) if MarineCount hits 64000
},
flag = {preserved},
}
Portal(LoopEnd); -- LoopEnd = 3.
Changing Order of Triggers will break the Trigger logic(while statement).
All i want is get easy to coding. To put in bluntly, I don't need to solve this problem(get undeclared var). I can imagine a few ways to avoid it. But if i using these ways then the coding work will be very complicated and the difficulty of coding will increases greatly. The difficulty made me stop coding in recent months.
How can I get variables that not declared yet?
Short of time travel, you can't.
Your example code doesn't explain the motivation for the question, because this:
a = WillDeclaredVar(b)
sheepCount = 123
b = sheepCount
print(a)
Can trivially be rearranged into this:
sheepCount = 123
b = sheepCount
a = WillDeclaredVar(b)
print(a)
It would be easier to answer your question if you showed the actual problem you're trying to solve (to avoid an XY problem).
However, as stated there are few things we can note.
First, you need to distinguish between declaring a variable and giving it a value. In Lua you can say:
local b
To declare b as a local variable, which presumably will make a slot for it in the stack frame and let you bind closures to it, before you give it a value. However, the line:
a = WillDeclaredVar(b)
Will pass WillDeclaredVar the value that b currently has, and there's no way for a to change retroactively as a result of b being assigned a new value. That's simply not going to happen, ever. Neither a nor WillDeclaredVar are even aware that b exists, they are receive the value it contains at the point of call.
You could however bind the variable b to a closure which will fetch b's current value when needed.
-- declare b before giving it a value, aka "forward reference"
local b
a = function() return b end
sheepCount = 123
b = sheepCount
print(a()) -- call a to get b's current value
Another way to do that would be to make b a global variable, which is really just a key into your environment table, so you could say:
a = WillDeclaredVar('b')
And have a be some object that can fetch the current value of __ENV['b'] when required.
However, neither of these will support this syntax:
print(a)
a needs to be a function, something that looks up the value of b when needed rather than simply holding a previously computed value. You could do it in this particular instance (i.e. a needs to be convertable to a string), by creating a proxy object that implements __tostring.
function WillDeclaredVar(variableName)
local proxy = { environment = _ENV or _G, variableName = variableName }
return setmetatable(proxy, {
__tostring = function(proxy)
return proxy.environment[proxy.variableName]
end
})
end
-- a will compute a value based on the current value of b when needed
a = WillDeclaredVar('b')
sheepCount = 123
b = sheepCount
print(a)
Output:
123
To make var1 be a reference for var2 write var1 = ReferenceF or var2 (please note a space inside "ReferenceFor"!)
do
local values, references, reference_flag = {}, {}
setmetatable(_G, {
__index = function (_, name)
if name == 'ReferenceF' then
reference_flag = true
elseif reference_flag then
reference_flag = false
return {[references] = name}
elseif references[name] then
return _G[references[name]]
else
return values[name]
end
end,
__newindex = function (_, name, val)
if type(val) == 'table' and val[references] then
references[name] = val[references]
else
values[name] = val
end
end
})
end
a = ReferenceF or b -- a is Reference For b
b = ReferenceF or c -- b is Reference For c
sheepCount = 123
c = sheepCount
print(a, b, c) --> 123 123 123
I have a variable in groovy like below:
project.Map
{
time.'1 1 * ?' = ['T1']
time.'2 1 * ?' = ['T2']
templates.'T1' = ['Z','X','Y']
templates.'T2' = ['Q']
}
Sorry but I am new to groovy ,when i try to access the individual
variable values in project.map how do i access them
i tried something like below
log.info(grailsApplication.config.project.Map.time[1])
log.info(grailsApplication.config.project.Map.get('time.'2 1 * ?'' ))
log.info(grailsApplication.config.project.Map.get('time[0]' ))
log.info(grailsApplication.config.project.Map.time.get('1 1 * ?'))
but they all print null value or object references.how do i access values for
time and templates both within a for loop and without it.
please see http://grails.org/doc/latest/guide/conf.html#config for the ways the config is allowed to nest. your outer syntax is especially mentioned to not be allowed:
However, you can't nest after using the dot notation. In other words, this won't work:
// Won't work!
foo.bar {
hello = "world"
good = "bye"
}
You have to write it as
project { Map { ... } }
The inner dotted parts (with the assignment) are ok (according to the doc)
This is my first attempt to use Lua tables and I'm getting on with it quite well. I'm struggling with one thing though, heres (a small sample of) my table as it currently stands:
objects = {
["1/1/1"] = { tl = 1, startVal = 1, stopVal = 0 },
["1/1/2"] = { tl = 11, startVal = 1, stopVal = 0 },
["1/1/3"] = { tl = 22, startVal = 1, stopVal = 0 },
["1/1/4"] = { tl = 33, startVal = 1, stopVal = 0 },
}
The typical operation of this is that I use the "1/1/1" values as a lookup to the inner tables and then use those values in various functions. This all works well. Now, I need to go the other way, say I have tl = 22 coming in, I want to return the top value ("1/1/3" in this case).
I think I need to do something with the inpairs I keep seeing on the web but I'm struggling to implement. Any help would be massively appreciated.
You can't use ipairs because your table is an associated array not a sequence, so you have to use pairs. Also, there is no search function builtin to Lua, so you have to loop over all items yourself, looking for the right field value:
function findTL(tbl)
for key, data in pairs(tbl) do
if data.tl == tlSearch then
return key
end
end
end
local key = findTL(objects, 22)
If you want something a tad more object-oriented you could do this:
objects.findTL = findTL -- ok because first arg is the table to search
local key = objects:findTL(22)
isn't it better to take the value directly?
objects.1/1/1.tl
I don't know if it will work also with slashes, but if not, you may replace it by 'x' for example. Then it will be:
objects.1x1x1.tl
How would I code a function to iterate through one "pages" worth of data? Sample code would be ideal...
So say we image the size of a page is 5 items. If we had a lua table with 18 items it would need to print out:
Page 1: 1 to 5
Page 2: 6 to 10
Page 3: 11 to 15
Page 4: 16 to 18
So assume the data is something like:
local data = {}
data["dog"] = {1,2,3}
data["cat"] = {1,2,3}
data["mouse"] = {1,2,3}
data["pig"] = {1,2,3}
.
.
.
How would one code the function that would do the equivalent of this:
function printPage (myTable, pageSize, pageNum)
-- find items in "myTable"
end
So in fact I'm not even sure if a Lua table used as a dictionary can even do this? There is no specific ordering is there in such a table, so how would you be sure the order would be the same when you come back to print page 2?
The next function allows you to go through a table in an order (albeit an unpredictable one). For example:
data = { dog = "Ralf", cat = "Tiddles", fish = "Joey", tortoise = "Fred" }
function printPage(t, size, start)
local i = 0
local nextKey, nextVal = start
while i < size and nextKey ~= nil do
nextKey, nextVal = next(t, nextKey)
print(nextKey .. " = " .. nextVal)
i = i + 1
end
return nextKey
end
local nextPage = printPage(data, 2) -- Print the first page
printPage(data, 2, nextPage) -- Print the second page
I know this isn't quite in the form you were after, but I'm sure it can be adapted quite easily.
The next function returns the key after the one provided in the table, along with its value. When the end of the table is reached, it returns nil. If you provide nil as the second parameter, it returns the first key and value in the table. It's also documented in Corona, although it appears to be identical.
I am starting to learn Lua from Programming in Lua (2nd edition)
I didn't understand the following in the book. Its very vaguely explained.
a.) w={x=0,y=0,label="console"}
b.) x={math.sin(0),math.sin(1),math.sin(2)}
c.) w[1]="another field"
d.) x.f=w
e.) print (w["x"])
f.) print (w[1])
g.) print x.f[1]
When I do print(w[1]) after a.), why doesn't it print x=0
What does c.) do?
What is the difference between e.) and print (w.x)?
What is the role of b.) and g.)?
You have to realize that this:
t = {3, 4, "eggplant"}
is the same as this:
t = {}
t[1] = 3
t[2] = 4
t[3] = "eggplant"
And that this:
t = {x = 0, y = 2}
is the same as this:
t = {}
t["x"] = 0
t["y"] = 2
Or this:
t = {}
t.x = 0
t.y = 2
In Lua, tables are not just lists, they are associative arrays.
When you print w[1], then what really matters is line c.) In fact, w[1] is not defined at all until line c.).
There is no difference between e.) and print (w.x).
b.) creates a new table named x which is separate from w.
d.) places a reference to w inside of x. (NOTE: It does not actually make a copy of w, just a reference. If you've ever worked with pointers, it's similar.)
g.) Can be broken up in two parts. First we get x.f which is just another way to refer to w because of line d.). Then we look up the first element of that table, which is "another field" because of line c.)
There's another way of creating keys in in-line table declarations.
x = {["1st key has spaces!"] = 1}
The advantage here is that you can have keys with spaces and any extended ASCII character.
In fact, a key can be literally anything, even an instanced object.
function Example()
--example function
end
x = {[Example] = "A function."}
Any variable or value or data can go into the square brackets to work as a key. The same goes with the value.
Practically, this can replace features like the in keyword in python, as you can index the table by values to check if they are there.
Getting a value at an undefined part of the table will not cause an error. It will just give you nil. The same goes for using undefined variables.
local w = {
--[1] = "another field"; -- will be set this value
--["1"] = nil; -- not save to this place, different with some other language
x = 0;
y = 0;
label = "console";
}
local x = {
math.sin(0);
math.sin(1);
math.sin(2);
}
w[1] = "another field" --
x.f = w
print (w["x"])
-- because x.f = w
-- x.f and w point one talbe address
-- so value of (x.f)[1] and w[1] and x.f[1] is equal
print (w[1])
print ((x.f)[1])
print (x.f[1])
-- print (x.f)[1] this not follows lua syntax
-- only a function's has one param and type of is a string
-- you can use print "xxxx"
-- so you print x.f[1] will occuur error
-- in table you can use any lua internal type 's value to be a key
-- just like
local t_key = {v=123}
local f_key = function () print("f123") end
local t = {}
t[t_key] = 1
t[f_key] = 2
-- then t' key actualy like use t_key/f_key 's handle
-- when you user t[{}] = 123,
-- value 123 related to this no name table {} 's handle