Concatenation of strings in Lua

Question

In many languages you can concatenate strings on variable assignment   I have a scenario  using the Lua programming language  where I need to append the output of a command to an existing variable   Is there a functional equivalent in Lua to the below examples   Examples of other languages         PERL        filename    checkbook    filename      tmp                           C        string filename    checkbook   filename      tmp                     Thank you in advance for your help

User · Answer

If you are asking whether there s shorthand version of operator    - no there isn t  You cannot write a     b  You ll have to type it in full  filename   filename      tmp

User · Answer

Concatenation       The string concatenation operator in Lua is denoted by two dots         If both operands are strings or numbers  then they are converted to strings according to the rules mentioned in   2 2 1  Otherwise  the  concat  metamethod is called  see   2 8      from  http   www lua org manual 5 1 manual html 2 5 4

User · Answer

As other answers have said  the string concatenation operator in Lua is two dots    Your simple example would be written like this   filename    checkbook  filename   filename      tmp    However  there is a caveat to be aware of  Since strings in Lua are immutable  each concatenation creates a new string object and copies the data from the source strings to it  That makes successive concatenations to a single string have very poor performance    The Lua idiom for this case is something like this   function listvalues s      local t           for k v in ipairs s  do         t  t 1    tostring v      end     return table concat t   n   end   By collecting the strings to be concatenated in an array t  the standard library routine table concat can be used to concatenate them all up  along with a separator string between each pair  without unnecessary string copying   Update  I just noticed that I originally wrote the code snippet above using pairs   instead of ipairs      As originally written  the function listvalues   would indeed produce every value from the table passed in  but not in a stable or predictable order  On the other hand  it would include values who s keys were not positive integers in the span of 1 to  s  That is what pairs   does  it produces every single  key value  pair stored in the table   In most cases where you would be using something like listvaluas   you would be interested in preserving their order  So a call written as listvalues 13  42  17  4  would produce a string containing those value in that order  However  pairs   won t do that  it will itemize them in some order that depends on the underlying implementation of the table data structure  It is known that the order not only depends on the keys  but also on the order in which the keys were inserted and other keys removed   Of course ipairs   isn t a perfect answer either  It only enumerates those values of the table that form a  sequence   That is  those values who s keys form an unbroken block spanning from 1 to some upper bound  which is  usually  also the value returned by the   operator   In many cases  the function ipairs   itself is better replaced by a simpler for loop that just counts from 1 to  s  This is the recommended practice in Lua 5 2 and in LuaJIT where the simpler for loop can be more efficiently implemented than the ipairs   iterator    If pairs   really is the right approach  then it is usually the case that you want to print both the key and the value  This reduces the concerns about order by making the data self-describing  Of course  since any Lua type  except nil and the floating point NaN  can be used as a key  and NaN can also be stored as a value  finding a string representation is left as an exercise for the student  And don t forget about trees and more complex structures of tables

User · Answer

Strings can be joined together using the concatenation operator       this is the same for variables I think

[lua] Concatenation of strings in Lua

Examples related to lua

Examples related to concatenation