Concatenation of tables in Lua

Question

ORIGINAL POST

Given that there is no built in function in Lua, I am in search of a function that allows me to append tables together. I have googled quite a bit and have tried every solutions I stumbled across but none seem to work properly.

The scenario goes like this: I am using Lua embeded in an application. An internal command of the application returns a list of values in the form of a table.

What I am trying to do is call that command recursively in a loop and append the returned values, again in the form of a table, to the table from previous iterations.

---

EDIT

For those who come across this post in the future, please note what @gimf posted. Since Tables in Lua are as much like arrays than anything else (even in a list context), there is no real correct way to append one table to another. The closest concept is merging of tables. Please see the post, "Lua - merge tables?" for help in that regard.

Answer 1

Overcomplicated answers much?

Here is my implementation:

function TableConcat(t1,t2)
    for i=1,#t2 do
        t1[#t1+1] = t2[i]
    end
    return t1
end

Answer 2

If you want to concatenate an existing table to a new one, this is the most concise way to do it:

local t = {3, 4, 5}
local concatenation = {1, 2, table.unpack(t)}

Although I'm not sure how good this is performance-wise.

Answer 3

And one more way:

for _,v in ipairs(t2) do 
    table.insert(t1, v)
end

It seems to me the most readable one - it iterates over the 2nd table and appends its values to the 1st one, end of story. Curious how it fares in speed to the explicit indexing [] above

Answer 4

A simple way to do what you want:

local t1 = {1, 2, 3, 4, 5}
local t2 = {6, 7, 8, 9, 10}

local t3 = {unpack(t1)}
for I = 1,#t2 do
    t3[#t1+I] = t2[I]
end

Answer 5

To add two tables together do this

    ii=0
for i=#firsttable, #secondtable+#firsttable do
    ii=ii+1
    firsttable[i]=secondtable[ii]
end

use the first table as the variable you wanted to add as code adds the second one on to the end of the first table in order.

• i is the start number of the table or list.
• #secondtable+#firsttable is what to end at.

It starts at the end of the first table you want to add to, and ends at the end of the second table in a for loop so it works with any size table or list.

Answer 6

In general the notion of concatenating arbitrary tables does not make sense in Lua because a single key can only have one value.

There are special cases in which concatenation does make sense. One such is for tables containing simple arrays, which might be the natural result of a function intended to return a list of results.

In that case, you can write:

-- return a new array containing the concatenation of all of its 
-- parameters. Scaler parameters are included in place, and array 
-- parameters have their values shallow-copied to the final array.
-- Note that userdata and function values are treated as scalar.
function array_concat(...) 
    local t = {}
    for n = 1,select("#",...) do
        local arg = select(n,...)
        if type(arg)=="table" then
            for _,v in ipairs(arg) do
                t[#t+1] = v
            end
        else
            t[#t+1] = arg
        end
    end
    return t
end

This is a shallow copy, and makes no attempt to find out if a userdata or function value is a container or object of some kind that might need different treatment.

An alternative implementation might modify the first argument rather than creating a new table. This would save the cost of copying, and make array_concat different from the .. operator on strings.

Edit: As observed in a comment by Joseph Kingry, I failed to properly extract the actual value of each argument from .... I also failed to return the merged table from the function at all. That's what I get for coding in the answer box and not testing the code at all.

Answer 7

If you want to merge two tables, but need a deep copy of the result table, for whatever reason, use the merge from another SO question on merging tables plus some deep copy code from lua-users.

(edit Well, maybe you can edit your question to provide a minimal example... If you mean that a table

 { a = 1, b = 2 }

concatenated with another table

{ a = 5, b = 10 }

should result in

{ a = 1, b = 2, a = 5, b = 10 }

then you're out of luck. Keys are unique.

It seems you want to have a list of pairs, like { { a, 1 }, { b, 2 }, { a, 5 }, { b, 10 } }. You could also use a final structure like { a = { 1, 5 }, b = { 2, 10 } }, depending on your application.

But the simple of notion of "concatenating" tables does not make sense with Lua tables. )

Answer 8

Here is an implementation I've done similar to RBerteig's above, but using the hidden parameter arg which is available when a function receives a variable number of arguments. Personally, I think this is more readable vs the select syntax.

function array_concat(...)
    local t = {}

    for i = 1, arg.n do
        local array = arg[i]
        if (type(array) == "table") then
            for j = 1, #array do
                t[#t+1] = array[j]
            end
        else
            t[#t+1] = array
        end
    end

    return t
end

Answer 9

Here is my implementation to concatenate a set of pure-integer-indexing tables, FYI.

1. define a function to concatenate two tables, concat_2tables

2. another recursive function concatenateTables: split the table list by unpack, and call concat_2tables to concatenate table1 and restTableList

   t1 = {1, 2, 3}
   t2 = {4, 5}
   t3 = {6}
   
   concat_2tables = function(table1, table2)
       len = table.getn(table1)
       for key, val in pairs(table2)do
           table1[key+len] = val
       end
       return table1
   end
   
   concatenateTables = function( tableList )
       if tableList==nil then
           return  nil
       elseif table.getn(tableList) == 1 then
           return  tableList[1]
       else
           table1 = tableList[1]
           restTableList = {unpack(tableList, 2)}
           return concat_2tables(table1, concatenateTables(restTableList))
       end
   end
   
   tt = {t1, t2, t3}
   t = concatenateTables(tt)  
   

Answer 10

EDIT

---

Here's a better solution, the other one tended to overwrite numeric keys, the usage is still the same:

function merge(...)
  local temp = {}
  local index = 1
  local result = {}
  
  math.randomseed(os.time())

  for i, tbl in ipairs({ ... }) do
    for k, v in pairs(tbl) do
      if type(k) == 'number' then
        -- randomize numeric keys
        k = math.random() * i * k
      end
      
      temp[k] = v
    end
  end
  
  for k, v in pairs(temp) do
    if type(k) == "number" then
      -- Sort numeric keys into order
      if result[index] then
        index = index + 1
      end
      
      k = index
    end
    
    result[k] = v
  end

  return result
end

ORIGINAL

---

A wee bit late to the game, but this seems to work for me:

function concat(...)
  local result = {}
  
  for i, tbl in ipairs({...}) do
    for k, v in pairs(tbl) do
      if type(k) ~= "number" then
        result[k] = v
      else
        result[i] = v
      end
    end
  end
  
  return result
end

It might be a bit overcomplicated, but it takes an infinite amount of arguments, and works for both key-value pairs and regular "arrays" (numbers as keys). Here's an example

Answer 11

-- Lua 5.1+
function TableAppend(t1, t2)
    -- A numeric for loop is faster than pairs, but it only gets the sequential part of t2
    for i = 1, #t2 do
        t1[#t1 + 1] = t2[i] -- this is slightly faster than table.insert
    end

    -- This loop gets the non-sequential part (e.g. ['a'] = 1), if it exists
    local k, v = next(t2, #t2 ~= 0 and #t2 or nil)
    while k do
        t1[k] = v -- if index k already exists in t1 then it will be overwritten
        k, v = next(t2, k)
    end
end

Answer 12

I like the simplicity in @Weeve Ferrelaine answer, but mutations may cause many issues and in general, are not desirable.

Version with NO MUTATION.

---@param t1 {}
---@param t2 {}
function TableConcat(t1,t2)
    local tOut = {}

    for i = 1, #t1 do
        tOut[i] = t1[i]
    end

    for i = #t1, #t1 + #t2 do
        tOut[i] = t2[i]
    end

    return tOut
end

Original implementation, that's mutating t1.

function TableConcat(t1,t2)
    for i=1,#t2 do
        t1[#t1+1] = t2[i]
    end
    return t1
end

Answer 13

The other solutions here suffered from 3 issues:

1. Did not work with tables that contain key/values
2. Didn't maintain the order of the elements in the tables while merging
3. Did not work with tables with mixed numeric indices and key based indices

This solution is a variant of the original solution proposed by @kaptcha which addresses the shortcomings noted above:

--- Function to merge/join tables
--- @param ... table List of tables to be merged
--- @return table Merged table
function MergeTables(...)
    local result = {}

    for i, tbl in ipairs({...}) do
      for k, v in pairs(tbl) do
        if type(k) ~= "number" then
          result[k] = v
        else
          table.insert(result, v)
        end
      end
    end

    return result
end

Usage:

local t1 = { a={1}, b={b=2} }
local t2 = { c={3}, d={d=4} }
local tMerged = MergeTables(t1, t2)

Answer 14

use table.insert() function

table1 = {
  "param1=value1",
  "param2=value2",
  "param3=value3"
}
table2 = {
  "param21=value1",
  "param23=value2",
  "param23=value3"
}
table.insert(table1, table.concat(table2, ","))
print(table.unpack(table1));

Answer 15

Use table.concat:

http://lua-users.org/wiki/TableLibraryTutorial

> = table.concat({ 1, 2, "three", 4, "five" })
12three4five
> = table.concat({ 1, 2, "three", 4, "five" }, ", ")
1, 2, three, 4, five
> = table.concat({ 1, 2, "three", 4, "five" }, ", ", 2)
2, three, 4, five
> = table.concat({ 1, 2, "three", 4, "five" }, ", ", 2, 4)
2, three, 4