I have some lua code like this:
_table = {
stuff = {
item1 = {Name="Stack",Rarity="Over"};
item2 = {Name="Flow",Rarity="Com"}
};
};
print("placeholder") -- example thing
_stuff = _table.stuff
for i = 1, #_stuff do
print(_stuff[i].Name)
end
The output is this:
placeholder
I've tried to look at stuff but I don't think it was related to my problem.
Lua tables conceptually are "maps": They map keys to values. Your table _table maps the key stuff to the {item1 = ..., item2 = ...} table. Your stuff table maps the string "item1" to the {Name="Stack",Rarity="Over"} table and the string "item2" to the {Name="Flow",Rarity="Com"} table.
The length operator # completely ignores string keys. It returns an integer i such that t[i] ~= nil and t[i+1] == nil. In particular, it returns 0 if there are no integer keys in the table. This is the case for your stuff table: You only have string keys. Thus your loop never runs, as the limit is 0 and i = 1 > 0 already is the case before the first loop iteration.
Put simply, you're trying to iterate over a "dictionary" (a table with string keys) as if it were a list. Lua tables have both a "list part" (integer keys from 1 to n, where usually n = #t) and a "hash part" (of all other keys). The "list part" is empty in your case, so there's nothing for you to iterate over.
To iterate over all key-value pairs of any table (list or dictionary) in an undefined order, use pairs (rather than ipairs or iterating over indices i, which only works for lists):
for itemname, item in pairs(_stuff) do
print(item.Name)
end
If you want a defined order of iteration, either build a second table of keys...
local order = {"item1", "item2"}
for _, key in ipairs(order) do
print(_stuff[key].Name)
end
... or use the integer keys ("list part") of the table:
_table = {
stuff = {
{key = "item1", Name="Stack", Rarity="Over"};
{key = "item2", Name="Flow", Rarity="Com"};
};
};
_stuff = _table.stuff
for _, key in ipairs(order) do
for i = 1, #_stuff do
print(_stuff[i].Name)
end
end
then works as expected.
Related
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)
In the command
table.insert(table, data)
how can you use that but for the inserts have string keys?
PSEUDO CODE
tableOfStuff = {cat, pig, hat, lemon}
t = {}
for i=1, #tableOfStuff do
table.insert(t, key=tableOfStuff[i], data=tableOfStuff[i])
end
So I end up with a table...
t['cat'] == 'cat'
t['dog'] == 'dog'
etc.....
EDIT
I think my example confused people... I am asking how to use "insert.table" but insert tings with string keys...
table.insert(table,data,stringkey)
something like this?
Creating One Table
If all you want is to create a table with strings as keys, then check out Table Constructors, you have a couple options.
Option 1:
t = { key1 = "value1", key2 = "value2" }
--or like this:
t = { ["key1"] = "value1", ["key2"] = "value2" }
Option 2: (create an empty table first)
t = {}
t.key1 = "value1"
--or like this
t["key2"] = "value2"
It looks like you want the keys and values to be the same string and that is possible. Just write the same thing for key1 and value1. So t["cat"] = "cat".
Using Two Tables
Based on your example code, it looks like you want to take an existing table of strings and create from that a new table with strings as both the keys and the values. To do that:
table1 = { "cat", "pig", "hat", "lemon" }
table2 = {}
for i=1, #table1 do
table2[ table1[i] ] = table1[i]
end
--test
print table2["cat"]
Here is a good lesson about tables in Lua: Lua Tables Tutorial
The comment is right.You needn't and you can't use table.insert.You can see the document table.insert.It's only support the number.It' used for the array part of table.But you're using the hash part of a table.
code:
tableOfStuff = {"cat", "pig", "hat", "lemon"}
t = {}
for i=1, #tableOfStuff do
local szKey = tableOfStuff[i];
t[szKey] = tableOfStuff[i]; -- the value can be the others.
end
Is this script:
local data =
{
{ "data1", "1"},
{ "data5", "2"},
{ "3453453", "3"},
{ "zzz", "4"},
{ "222", "5"},
{ "lol", "6"},
{ "asdf", "7"},
{ "hello", "8"},
}
local function test()
local count = #data
for i = 1, count do
print(data[i][1] .. " = " .. data[i][2])
end
end
test()
Guaranteed to output:
data1 = 1
data5 = 2
3453453 = 3
zzz = 4
222 = 5
lol = 6
asdf = 7
hello = 8
If not then why, and what is best way performance wise to make it so?
I read something about pairs VS ipairs not returning a fixed order of results
ipairs is an iterator of the array elements of a table, in order from first to last. "Array elements" being defined as the members of a table with keys that are numeric values on the range [1, #tbl], where #tbl is the length operator applied to the table.
pairs is an iterator over all of the elements of a table: array and non-array elements alike. Non-array elements of a table have no intrinsic order to Lua, so pairs will return them in any order. And even though the array elements do technically have an order, pairs will not make an exception for them; it always operates in an arbitrary order.
Your code works like ipairs: iterating over each of the numeric keys of the table from 1 to its length.
In Lua how would one pop/remove the next item (any order) in a key-value-pair table?
Is this possible without having to iterate using pairs?
There is a primitive function next, you can call next(t,k), where k is a key of the table t, returns a next key in the table, in an arbitrary order, and the value associated with this key.
If k is nil, next(t,k) returns the first element if there is one. So you can iterate the table from calling next(t,nil) and end when the next key is nil.
This is an simple example to demonstrate the use of next:
local t = {a = "va", b = "vb", c = "vc"}
local k,v = next(t,nil)
print(k,v)
k,v = next(t,k)
print(k,v)
k,v = next(t,k)
print(k,v)
k,v = next(t,k)
print(k,v)
Output:
a va
c vc
b vb
nil nil
The global function next is useful here. The docs explain it pretty well in general. To use it iteratively, this is 'key':
You may ... modify existing fields. In particular, you may clear
existing fields.
A simple pop function:
-- removes and returns an arbitrary key-value pair from a table, otherwise nil
local pop = function (t)
local key, value = next(t)
if key ~= nil then
t[key] = nil
end
return key, value
end
Demo:
local test = { "one", c = "see", "two", a = "ayy", b = "bee", "three" }
assert(next(test), "Table was expected to be non-empty")
local key, value = pop(test)
while key do
print(key, value)
key, value = pop(test)
end
assert(not next(test), "Table was expected to be empty")
If you run the demo multiple times, you might see the arbitrariness of the table sequence.
In Lua, you can create a table the following way :
local t = { 1, 2, 3, 4, 5 }
However, I want to create an associative table, I have to do it the following way :
local t = {}
t['foo'] = 1
t['bar'] = 2
The following gives an error :
local t = { 'foo' = 1, 'bar' = 2 }
Is there a way to do it similarly to my first code snippet ?
The correct way to write this is either
local t = { foo = 1, bar = 2}
Or, if the keys in your table are not legal identifiers:
local t = { ["one key"] = 1, ["another key"] = 2}
i belive it works a bit better and understandable if you look at it like this
local tablename = {["key"]="value",
["key1"]="value",
...}
finding a result with : tablename.key=value
Tables as dictionaries
Tables can also be used to store information which is not indexed
numerically, or sequentially, as with arrays. These storage types are
sometimes called dictionaries, associative arrays, hashes, or mapping
types. We'll use the term dictionary where an element pair has a key
and a value. The key is used to set and retrieve a value associated
with it. Note that just like arrays we can use the table[key] = value
format to insert elements into the table. A key need not be a number,
it can be a string, or for that matter, nearly any other Lua object
(except for nil or 0/0). Let's construct a table with some key-value
pairs in it:
> t = { apple="green", orange="orange", banana="yellow" }
> for k,v in pairs(t) do print(k,v) end
apple green
orange orange
banana yellow
from : http://lua-users.org/wiki/TablesTutorial
To initialize associative array which has string keys matched by string values, you should use
local petFamilies = {["Bat"]="Cunning",["Bear"]="Tenacity"};
but not
local petFamilies = {["Bat"]=["Cunning"],["Bear"]=["Tenacity"]};