How to check if a table contains an element in Lua

Question

Is there a method for checking if a table contains a value   I have my own  naive  function  but I was wondering if something  official  exists for that   Or something more efficient     function table contains table  element    for    value in pairs table  do     if value    element then       return true     end   end   return false end   By the way  the main reason I m using this functions is to use tables as sets  ie with no duplicate elements  Is there something else I could use

User · Answer

I can t think of another way to compare values  but if you use the element of the set as the key  you can set the value to anything other than nil   Then you get fast lookups without having to search the entire table

User · Answer

You can put the values as the table s keys  For example   function addToSet set  key      set key    true end  function removeFromSet set  key      set key    nil end  function setContains set  key      return set key     nil end   There s a more fully-featured example here

User · Answer

Given your representation  your function is as efficient as can be done   Of course  as noted by others  and as practiced in languages older than Lua   the solution to your real problem is to change representation   When you have tables and you want sets  you turn tables into sets by using the set element as the key and true as the value    1 to interjay

User · Answer

I know this is an old post  but I wanted to add something for posterity  The simple way of handling the issue that you have is to make another table  of value to key   ie  you have 2 tables that have the same value  one pointing one direction  one pointing the other   function addValue key  value      if  value    nil  then         removeKey key          return     end      primaryTable key    value      secodaryTable value    key end  function removeKey key      local value    primaryTable key      if  value    nil  then         return     end      primaryTable key    nil      secondaryTable value    nil end  function getValue key      return  primaryTable key  end  function containsValue value      return  secondaryTable value     nil end   You can then query the new table to see if it has the key  element    This prevents the need to iterate through every value of the other table   If it turns out that you can t actually use the  element  as a key  because it s not a string for example  then add a checksum or tostring on it for example  and then use that as the key   Why do you want to do this   If your tables are very large  the amount of time to iterate through every element will be significant  preventing you from doing it very often   The additional memory overhead will be relatively small  as it will be storing 2 pointers to the same object  rather than 2 copies of the same object  If your tables are very small  then it will matter much less  infact it may even be faster to iterate than to have another map lookup   The wording of the question however strongly suggests that you have a large number of items to deal with

[lua] How to check if a table contains an element in Lua?

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