Concatenating Various Tables In Lua

Question

I've been searching for answer to this question for a couple of days i manages to somehow use a trick to just omit this Concatenation part and just use several seperate loops to re-insert different values into the same table... but my question is

By default, table.sort uses < to compare array elements, so it can only sort arrays of numbers or arrays of strings. Write a comparison function that allows table.sort to sort arrays of mixed types. In the sorted array, all values of a given type should be grouped together. Within each such group, numbers and strings should be sorted as usual, and other types should be sorted in some arbitrary but consistent way.

A = { {} , {} , {} , "" , "a", "b" , "c" , 1 , 2 , 3 , -100 , 1.1 , function() end , function() end , false , false , true }

as i said i solved this using different for loops but is there a way to just analyse every element of the table then assign it to a different Table??? Like : "Tables,Funcs,Nums,Strings,..." then after analysing finished just concatenate them together to have the same table just in sorted version.

My Inefficient Answer To This Was :

function Sep(val)
local NewA = {}

   for i in pairs(val) do
      if type(val[i]) == "string" then
     table.insert(NewA,val[i])
      end
   end
for i in pairs(val) do
      if type(val[i]) == "number" then
     table.insert(NewA,val[i])
      end
   end

for i in pairs(val) do
      if type(val[i]) == "function" then
     table.insert(NewA,tostring(val[i]))
      end
   end

for i in pairs(val) do
      if type(val[i]) == "table" then
     table.insert(NewA,tostring(val[i]))
      end
   end

for i in pairs(val) do
      if type(val[i]) == "boolean" then
     table.insert(NewA,tostring(val[i]))
      end
   end

for i in pairs(NewA) do
   print(NewA[i])
end
end

tozka · Accepted Answer · 2012-10-09 14:29:39Z

2

As far as I understand you want to sort them first by type and then by value: You can write your own custom predicate and pass it to sort

-- there would be need of custom < function since tables cannot be compared
-- you can overload operator < as well I suppose.
 function my_less (lhs, rhs)
    if (type (lhs) ~= "number" or type (lhs) ~= "string") then
        return tostring (lhs) < tostring (rhs) 
    else
        return lhs < rhs;
    end;
 end;

 -- the custom predicate I mentioned
 function sort_predicate (a,b)
    -- if the same type - compare variable, else compare types
    return (type (a) == type (b) and my_less (a, b)) or type (a) < type (b);
 end

 table.sort (A, sort_predicate);

edited Oct 9, 2012 at 14:29

answered Oct 9, 2012 at 13:55

tozka

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8 Comments

Mike Corcoran Over a year ago

you could boil this whole thing down to like 4 lines. all you need to do is check if the type(lhs) == "number" and if so return lhs < rhs, otherwise you return tostring(lhs) < tostring(rhs), right? there isn't really any reason to check for the various types when you're doing the same thing for everything but numbers.

T0M XeOn LuCiFeR Over a year ago

i don't get why we have to enter two values in the my_less function? isn't it just an input for of type Table in a function and then just sorting that thing out in a pre-defined manner...?

Puppy Over a year ago

It's a comparison function- you compare. Comparison inherently involves comparing at least two objects.

tozka Over a year ago

I was trying to imply that for each type you can write any kind of opreator< . They don't have to be tostring < tostring. In any case I have not even mentioned all types like threads or userdata.

T0M XeOn LuCiFeR Over a year ago

I just updated the question an added my question which as @MikeCorcoran Mentioned there's no need for extera types especially the way i did it, but could you pleae explain the code a little bit more, i'm new following "Programming in Lua, Wrox" ==>> and i didn't get the answer for a way of concatenating two or more different table values together and adding them into a Single Table holding all of them.

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Paul Kulchenko · Accepted Answer · 2012-10-10 20:37:13Z

0

A = { {}, {}, {}, "", "a", "b", "c", "2", "12", 1, 2, 3, -100, 1.1, 12, 11,
  function() end, function() end, false, false, true }

table.sort(A, function(a,b)
  if type(a) == type(b) and type(a) == 'number' then return a < b end
  return type(a)..tostring(a) < type(b)..tostring(b) end)

Will give you this:

{false, false, true, function() end, function() end,
 -100, 1, 1.1, 2, 3, 11, 12, "", "12", "2", "a", "b", "c", {}, {}, {}}

[Updated based on @tozka's comments]

edited Oct 10, 2012 at 20:37

answered Oct 9, 2012 at 17:14

Paul Kulchenko

26.9k3 gold badges43 silver badges64 bronze badges

7 Comments

tozka Over a year ago

This won't sort the numbers in the correct order A = { 1, 2, 21, 12 } will give this 1, 12, 2, 21

Paul Kulchenko Over a year ago

@tozka, updated with a slightly more complex version that should handle that.

tozka Over a year ago

Now you have almost the oposite problem, while A = { 2, 12 } will sort all right (because we are sorting numbers). A = { "2", "12" } (note that now we are sorting strings and not numbers) will give { "2", "12" } where the correct sorted order of strings is { "12", "2" }

Paul Kulchenko Over a year ago

Right; I'll argue it's more 'natural' this way ;)

Paul Kulchenko Over a year ago

@downvoters, would be nice to have a comment on what you disagree with. Style? Substance? Formatting?

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