Lua - handling concatenation of List

We use .. operator to concatenate elements in Lua. We can use __concat() metamethod to implement concatenation of list items with any string using .. operator. We can define how to concatenate elements of the list.

Syntax

__concat(table, value)

where

• table represents the current table whose behavior is to be modified.

• value to be used while concatenation.

Example - Concatenation of List without __concat() implementation

.. operator works with numbers and strings. In case of list, it will throw error as shown below−

main.lua

-- list with non-contigous indexes
local list = {"Welcome", "To", "Tutorialspoint"}

-- concatenate list using space string
local concatenatedString = list.." "

print(concatenatedString)

Output

When we run the above code, we will get the following output−

lua: main.lua:5: attempt to concatenate a table value (local 'list')
stack traceback:
	main.lua:5: in main chunk
	[C]: in ?

As list is a table, we cannot directly concatenate list items with string using .. operator.

Example - Solution to concatenate list with String

We can implement __concat(table, value) metamethod to concatenate table with string−

main.lua

-- list of strings
local list = {"Welcome", "To", "Tutorialspoint"}

-- create a metatable for concatenation
local metatableConcat = {
   __concat = function(list, value)
      return table.concat(list, value)
   end
}

setmetatable(list,metatableConcat )
-- concatenate list using space
local concatenatedString = list.." "
-- print the concatenated string
print(concatenatedString)

Output

When the above code is built and executed, it produces the following result −

Welcome To Tutorialspoint

Explanation

• local list = {"Welcome", "To", "Tutorialspoint"} is used to create a list of strings.

• local metatableConcat is used to create a meta table with __concat() method.

• __concat = function(list, value) is the implementation of metamethod __concat(table, value) method.

• return table.concat(list, value)concatenates entries of the list using the value as separator.

• When list.." " is called then __concat metamethod is called and a concatenated string is returned.