Suppose I have the following:
table = {a = {1, 2}, b = {3, 4}}
input = "a" -- abstracted away; it's a RV from another function.
You can use table.a[1] to get 1; however, I want to get it from the input variable - which is the return value of another function that I have, which returns the string "a" and not just a.
Now, this is where the error from here comes into play:
When I did table[input], it returned a table object, so then when I tried table[input][1], it had the calling a table error.
Is it possible to get 1 using indexing with the input "a"? If so, could someone let me know how this works? Thanks!
for any table if you use . to index it, it will use a string index.
When you use [] to index it, this means you can use any sort of datatype and it allows you to also use variables.
local someTable = {
a = "hello",
b = "world"
c = "!"
}
-- I can do
print(someTable.a) -- prints "hello"
-- and
print(someTable["b"]) -- prints "world"
-- however
print(someTable[c]) -- will error since there is no c variable
------
-- note that this would be valid
local someVariable = "c"
print(someTable[someVariable]) -- prints "!"
Going back to your case. tbl1 is using [] and using the variable input. However tbl2 is using the string "input" as the index in your table. Your table does not contain "input" as a key, so it returns nil
I hope I understand your question correctly, I apologize if not. If you want to, for example, send a specific part of a table to a user after they specified which key of the table they want then you're already doing everything correctly.
The tbl1 table contains the key "a" part of the main table. You can't print a table, but you can either print specific values by using tbl1[number] or do this:
table = {a = {1, 2}, b = {3, 4}}
input = "b"
tbl1 = table[input]
for _, v in pairs(table) do
print(v)
end
-- Expected output:
-- 3
-- 4
Related
I have been trying to figure out how to access a variable from within a table based on a sting that the user inputs but have been unable to make it work. How do I collect the specified value using a string? The intent is to enable the user to grab the specific value off the table based on their input, the table itself will not always have the same values to work with (eg. the table may change from t={a=1,b=2} to t={z=1,v=3} or any other iteration based off initial table selection).
The code I have been trying to use is as follows:
local user_input = "a"
local table = {a = 1, b = 2}
for k, v in pairs(table) do
print(v.user_input)
end
The desired value = 1 but it returns NIL instead. I understand that print(v.a) would return the desired value, but I am looking for a way to grab different information based on the user input. I have tried the following but have been unsuccessful as they do not return the desired value:
local user_input = "a"
local table = {a = 1, b = 2}
for k, v in pairs(table) do
local item = tostring("v." ..user_input)
print(item)
end
---------------------------
local user_input = "a"
local table = {a = 1, b = 2}
for k, v in pairs(table) do
local item = tostring("v." ..user_input)
print(_G[item])
end
I have been trying to figure out how to access a variable from within a table based on a sting that the user inputs
I am not sure to understand the issue, but the given requirement is pretty trivial.
user_input = "a"
table = {a = 1, b = 2}
print(table[user_input])
This will print "1".
If there is a more complicated table, it might change a little bit:
user_input = "a"
table = {
a = { "a", "b" },
b = { "c", "d" }
}
for k,v in pairs(table[user_input]) do
print(k, v)
end
It will get the sub-table associated to the key "a" and will print:
1 a
2 b
I have already seen many threads with examples of how to do this, the problem is, I still can't do it.
All the examples have tables with extra data. For example somethings like this
lines = {
luaH_set = 10,
luaH_get = 24,
luaH_present = 48,
}
or this,
obj = {
{ N = 'Green1' },
{ N = 'Green' },
{ N = 'Sky blue99' }
}
I can code in a few languages but I'm very new to Lua, and tables are really confusing to me. I can't seem to work out how to adapt the code in the examples to be able to sort a simple table.
This is my table:
local players = {"barry", "susan", "john", "wendy", "kevin"}
I want to sort these names alphabetically. I understand that Lua tables don't preserve order, and that's what's confusing me. All I essentially care about doing is just printing these names in alphabetical order, but I feel I need to learn this properly and know how to index them in the right order to a new table.
The examples I see are like this:
local function cmp(a, b)
a = tostring(a.N)
b = tostring(b.N)
local patt = '^(.-)%s*(%d+)$'
local _,_, col1, num1 = a:find(patt)
local _,_, col2, num2 = b:find(patt)
if (col1 and col2) and col1 == col2 then
return tonumber(num1) < tonumber(num2)
end
return a < b
end
table.sort(obj, cmp)
for i,v in ipairs(obj) do
print(i, v.N)
end
or this:
function pairsByKeys (t, f)
local a = {}
for n in pairs(t) do table.insert(a, n) end
table.sort(a, f)
local i = 0 -- iterator variable
local iter = function () -- iterator function
i = i + 1
if a[i] == nil then return nil
else return a[i], t[a[i]]
end
end
return iter
end
for name, line in pairsByKeys(lines) do
print(name, line)
end
and I'm just absolutely thrown by this as to how to do the same thing for a simple 1D table.
Can anyone please help me to understand this? I know if I can understand the most basic example, I'll be able to teach myself these harder examples.
local players = {"barry", "susan", "john", "wendy", "kevin"}
-- sort ascending, which is the default
table.sort(players)
print(table.concat(players, ", "))
-- sort descending
table.sort(players, function(a,b) return a > b end)
print(table.concat(players, ", "))
Here's why:
Your table players is a sequence.
local players = {"barry", "susan", "john", "wendy", "kevin"}
Is equivalent to
local players = {
[1] = "barry",
[2] = "susan",
[3] = "john",
[4] = "wendy",
[5] = "kevin",
}
If you do not provide keys in the table constructor, Lua will use integer keys automatically.
A table like that can be sorted by its values. Lua will simply rearrange the index value pairs in respect to the return value of the compare function. By default this is
function (a,b) return a < b end
If you want any other order you need to provide a function that returs true if element a comes befor b
Read this https://www.lua.org/manual/5.4/manual.html#pdf-table.sort
table.sort
Sorts the list elements in a given order, in-place, from list[1] to
list[#list]
This example is not a "list" or sequence:
lines = {
luaH_set = 10,
luaH_get = 24,
luaH_present = 48,
}
Which is equivalent to
lines = {
["luaH_set"] = 10,
["luaH_get"] = 24,
["luaH_present"] = 48,
}
it only has strings as keys. It has no order. You need a helper sequence to map some order to that table's element.
The second example
obj = {
{ N = 'Green1' },
{ N = 'Green' },
{ N = 'Sky blue99' }
}
which is equivalent to
obj = {
[1] = { N = 'Green1' },
[2] = { N = 'Green' },
[3] = { N = 'Sky blue99' },
}
Is a list. So you could sort it. But sorting it by table values wouldn't make too much sense. So you need to provide a function that gives you a reasonable way to order it.
Read this so you understand what a "sequence" or "list" is in this regard. Those names are used for other things as well. Don't let it confuse you.
https://www.lua.org/manual/5.4/manual.html#3.4.7
It is basically a table that has consecutive integer keys starting at 1.
Understanding this difference is one of the most important concepts while learning Lua. The length operator, ipairs and many functions of the table library only work with sequences.
This is my table:
local players = {"barry", "susan", "john", "wendy", "kevin"}
I want to sort these names alphabetically.
All you need is table.sort(players)
I understand that LUA tables don't preserve order.
Order of fields in a Lua table (a dictionary with arbitrary keys) is not preserved.
But your Lua table is an array, it is self-ordered by its integer keys 1, 2, 3,....
To clear up the confusing in regards to "not preserving order": What's not preserving order are the keys of the values in the table, in particular for string keys, i.e. when you use the table as dictionary and not as array. If you write myTable = {orange="hello", apple="world"} then the fact that you defined key orange to the left of key apple isn't stored. If you enumerate keys/values using for k, v in pairs(myTable) do print(k, v) end then you'd actually get apple world before orange hello because "apple" < "orange".
You don't have this problem with numeric keys though (which is what the keys by default will be if you don't specify them - myTable = {"hello", "world", foo="bar"} is the same as myTable = {[1]="hello", [2]="world", foo="bar"}, i.e. it will assign myTable[1] = "hello", myTable[2] = "world" and myTable.foo = "bar" (same as myTable["foo"]). (Here, even if you would get the numeric keys in a random order - which you don't, it wouldn't matter since you could still loop through them by incrementing.)
You can use table.sort which, if no order function is given, will sort the values using < so in case of numbers the result is ascending numbers and in case of strings it will sort by ASCII code:
local players = {"barry", "susan", "john", "wendy", "kevin"}
table.sort(players)
-- players is now {"barry", "john", "kevin", "susan", "wendy"}
This will however fall apart if you have mixed lowercase and uppercase entries because uppercase will go before lowercase due to having lower ASCII codes, and of course it also won't work properly with non-ASCII characters like umlauts (they will go last) - it's not a lexicographic sort.
You can however supply your own ordering function which receives arguments (a, b) and needs to return true if a should come before b. Here an example that fixes the lower-/uppercase issues for example, by converting to uppercase before comparing:
table.sort(players, function (a, b)
return string.upper(a) < string.upper(b)
end)
Lets say that I have a dictionary in Lua (i.e. a Lua table that contains indexes of type string) like so:
local my_dictionary = {a = 123; b = 321; c = 456; d = 654};
What I am trying to do is create an iterator function that can iterate over a table even if its indexes are of type string; kind of like pairs, however whenever I try to call next() to get the next index,value it will only return the index,value if the index is of type int. An idea I had was maybe to call (index):byte(1, -1) and add up the tuple of ints, and use that as a sort of pretend index, just to keep track of the indexes, but I do not think that would work with next. Here is basically what I have so far:
local function each(list)
if #list > 0 then
local function my_itr(lst, ind)
return next(lst, ind);
end
return my_itr, List, 0;
end
return function() end, nil, nil;
end
this only works for a table with int indexes (an array table), so I was wondering if anyone could help me out.
Thanks.
Edit: To make this less vague here is an example piece of code of what I am trying to accomplish:
local mytable = {a = 123; b = 321; 3, 2, 1, c = "bca"};
for i,v in each(mytable) do
print(i,v);
end
what it should output:
>a 123
>b 321
>1 3
>2 2
>3 1
>c bca
The output would not have to be in exact order.
It should work exactly as you want it to with a couple of tweaks: fix typo in List and pass nil instead of 0:
local function each(list)
local function my_itr(lst, ind)
return next(lst, ind)
end
return my_itr, list, nil
end
local mytable = {a = 123; b = 321; 3, 2, 1, c = "bca"}
for i,v in each(mytable) do
print(i,v)
end
This prints the following for me, which is what you'd need:
1 3
2 2
3 1
a 123
b 321
c bca
You can achieve this very behavior by using pairs. Don't confuse it with ipairs though - these are two different table traversal functions!
While ipairs only traverses integer keys of the table (usually it also stops at the first non-existent integer key), pairs traverses all key-value pairs in the table.
So, by writing
local mytable = {a = 123; b = 321; 3, 2, 1, c = "bca"};
for i, v in pairs(mytable) do
print(i, v);
end
You'll get all key-value pairs printed, in some random order. Here's the demo.
As a sidenote, there's no such thing as 'dictionary' in Lua - all associative arrays are referred to as 'tables'.
Let's suppose I want to store a list of element. Including some nil values. The position of the values is significant, and I need to represent the absence of a value in the list at a given position.
Here is a problem:
a = {1,2,3,nil,4}
for k,v in ipairs(a) do
print(k,v)
end
print(a[4])
print(a[5])
The for loop will only print elements 1,2 and 3. It stops at nil. The first print statement prints nil, but I'm not sure if it is actually stored in the table or not. (Who knows?) The second print statement prints 4 - as expected.
So here is the question: how to represent a list of elements in a table, and iterate through them efficiently? Given the conditions above, e.g. the position is significant, and some of the positions are "empty". In other words: have no value, but the absence of that value at that position has a meaning.
This is module "null.lua"
local function null(...)
local t, n = {...}, select('#', ...)
for k = 1, n do
local v = t[k]
if v == null then t[k] = nil
elseif v == nil then t[k] = null
end
end
return (table.unpack or unpack)(t, 1, n)
end
_G.null = null
Use null() as encoder and decoder
require("null")
a = {null(1,2,3,nil,4)}
-- the same could be done element-by-element
-- a = {null(1),null(2),null(3),null(nil),null(4)}
for k,v in ipairs(a) do
v = null(v)
print(k,v)
end
print(null(a[4]))
print(null(a[5]))
Lua tables can be used to create any Abstract Data Structure, in your case you indicated that you want a "list". A Lua table is a data structure that combines numeric index based access with key:value access.
Based on your example, you are using the numeric index feature of tables that let you iterate (with ipairs()) through those values. You will not be able to put nil into the table since the numeric index stops at the first nil entry. The remaining values in the table are stored as key:value pairs.
There are several work-arounds, but it depends on why you want a nil in the list. The simplest approach is to use the string "nil" rather than the native data type nil.
a = {1, 2, 3, "nil", 4}
for k,v in ipairs(a) do
print(k,v)
end
The result of this code is:
1 1
2 2
3 3
4 nil
5 4
Because of the way Lua implements strings, there is not a performance penalty for comparing to the string "nil" versus comparing to the native type nil.
The issue of "holes" (caused by nil) in an array are discussed in Programming in Lua, Chapter 5 Tables. Roberto Ierusalimschy recommendation is to track the size of the array to avoid problems with holes.
The following code shows an Object Oriented approach to tracking the size of the list. There are many possible variations on this theme.
function makeList(...)
local list = table.pack(...)
list.length =
function(self) return self.n
end
list.append =
function(self, value)
self.n = self.n + 1
self[self.n] = value
end
list.print =
function(self)
for i = 1, self.n do print(i, self[i]) end
end
return list
end
a = makeList(1, 2, 3, nil, 4)
a:append(5)
a:print()
print(a:length())
The result is:
1 1
2 2
3 3
4 nil
5 4
6 5
6
Note that the function table.pack creates a field 'n' which contains the correct number of items even when 'nil' is present. See PIL chapter 6.2, Variadic Functions for a complete explanation.
Don't just hack something together, write your own datastructure for this. If you "overload" ipairs (by writing an appropriate iterator) you can use it as a table:
function create(...)
local t = table.pack(...)
local self = {
num = t.n,
elements = { ... }
}
return self
end
function elements(t)
local f = function(s, i)
i = i + 1
if i <= s.num then
return i, s.elements[i]
end
end
return f, t, 0
end
local seq = create(1, 2, nil, 3)
print(seq.num)
for i, e in elements(seq) do
print(i, e)
end
-- results:
-- 4
-- 1 1
-- 2 2
-- 3 nil
-- 4 3
You could know define a metatable for this structure and have it use its own ipairs, so you don't even have to change the name.
Well, you can't store nil in the table without issues.
The most simple solution here would be to introduce your own unique value.
local mynil = {} -- every new table is unique!
a = {1,2,3,mynil,4}
for k,v in ipairs(a) do
if (v == mynil) then
v = nil
end
print(k,v)
end
No more issues with "nil" string that might be stored in the table as well, the minor issue is one more comparison. ipairs or any other iterator will show that the key with mynil value exists. That means you can separate mynil key existence with missing key =nil.
P.S. If you want to shift your list, you may consider table.remove(list, key) function.
The answer to this is rather simple, and these "workaround a" suggested is definitely overkill. Just keep track of the number of items in your table whenever it's changed (note: do not use #, you have too keep track manually to deal with nil values) and use a numeric for loop to iterate over it.
Currently I'm stuck on this:
t = {['79402d'] = {'-5.4','5','1.6'}, ['5813g1'] = {'3','0.15','18'}}
Now i need to loop through this table to check if name == t[1], but how can i do so?
I tried doing something like: for i=1,#t,1 do print(t[i]) but it doesn't seem to work.
I hope you can help me guys ;)
Not sure why it didn't worked first time but i solved my problem with:
for a,b in pairs(t) do
print(a, b[1], b[2], b[3])
end
Please note that the length operator # will give you the correct number of elements in a table only in a special case. in your case #t will return 0, hence your for loop does nothing.
Please refer to https://www.lua.org/manual/5.3/manual.html Section 3.4.7 – The Length Operator for details on how to use the lenght operator.
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. so #t will be zero if you have no t[1] or n-1 if t[n] is the first nil value in your table starting from t[1].
As you have no t[1] but only a t['79402d'] and a t['5813g1'] your for loop indexing t[i] would not work anyway.
Please read 3.4.9 – Table Constructors on how table construction works.
t = {"hello", "world"}
is the same as
t = {}
t[1] = "hello"
t[2] = "world"
(here t[1] is "hello" and #t is 2
whereas t = {['key1'] = "hello", ['key2'] = "world"}
equals
t = {}
t['key1'] = "hello"
t['key2'] = "world"
so t[1] here is nil and #t is 0
# operator returns lenght of array part of the table. Your table is not an array (i.e. a table with non-nil values from index 1 to a given n). Because of that your loop is not iterating any elements.
Use pairs to iterate over all keys in the table regardless of what they are.