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)
I'm using json-compare gem to compare two different json files.
Example file 1:
{"suggestions": [
{
"id1": 1,
"title1": "Test",
"body1": "Test"
}
]
}
Example file 2:
{"suggestions": [
{
"id2": 1,
"title2": "Test",
"body2": "Test"
}
]
}
The gem works well and spits out a hash that looks like this:
{:update=>
{"suggestions" =>
{:update=>
{0=>
{:append=>
{"id2"=>1, "title2"=>"Test", "body2"=>"Test"},
:remove=>
{"id1"=>1, "title1"=>"Test", "body1"=>"Test"},
}
}
}
}
}
How can I parse this and return all the places where json Keys were changed? For the sake of simplicity, how would I put to the console:
id1 changed to id2
title1 changed to title2
body1 changed to body2
For the purpose of what I'm building I don't need to know changes to the values. I just need to know that id1 became id2, etc.
Except if you are relaying on key ordering there is no way to tell that id1 got replaced by id2 and title2 by title1, or that id1 became title1 and id2 became title2. Sounds like you would need specific logic related to the actual key names (in this example searching for different integer suffixes).
Maybe this can be enough for the purpose:
def find_what_changed_in(mhash, result = [])
result << mhash
return if mhash.keys == [:append, :remove]
mhash.keys.each { |k| find_what_changed_in(mhash[k], result) }
result.last
end
find_what_changed_in(changes)
#=> {:append=>{"id2"=>1, "title2"=>"Test", "body2"=>"Test"}, :remove=>{"id1"=>1, "title1"=>"Test", "body1"=>"Test"}}
Where:
changes = {:update=>
{"suggestions" =>
{:update=>
{0=>
{:append=>
{"id2"=>1, "title2"=>"Test", "body2"=>"Test"},
:remove=>
{"id1"=>1, "title1"=>"Test", "body1"=>"Test"},
...
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.
I realize this is usually not a great practice, but how would I use a string containing an integer (e.g. "7") as a table key? For example:
local myTable = {
"1" = "Foo",
"2" = "Bar"
}
If memory serves from reading the Lua manual back in the day, that should be possible with some special syntax, but what I've written above is a syntax error.
Like this:
local myTable = {
["1"] = "Foo",
["2"] = "Bar"
}
Because the keys are not valid identifiers, you can't use the syntax sugar form.