How do I sort a simple Lua table alphabetically?

Question

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.

Answer 1

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.

Answer 2

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,....

Answer 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)