Lua - Tables as Sets

Introduction

A Set is a colletion which cannot have duplicate elements. There are many use cases of a Set like−

• Reserved keywords

• Unique numbers

• Mathematical Operators

• and so on...

In Lua, we can create a set using table construct very easily as a table uses associative key values. For example, if we want to make a check that a expression should not be using any of the given reserved keyword then we can create a table or reserved keywords as indexes and corresponding value as true.

reserved = {
   ["while"] = true, ["end"] = true, ["function"] = true, ["local"] = true,
   ["for"] = true, ["then"] = true, ["do"] = "true"
}

Now we can make a check on reserved keywords using following syntax.

for index, word in ipairs(allwords) do
   -- if word is present in reserved set
   if reserved[word] then
      print(word, "is a reserved keyword.")         
      break
   end
   -- do the regular processing
end

Complete example of Set of Reserved keywords

Now let's see the complete code in action.

main.lua

-- set of reserved keywords
reserved = {
   ["while"] = true, ["end"] = true, ["function"] = true, ["local"] = true,
   ["for"] = true, ["then"] = true, ["do"] = "true", ["if"] = "true"
}

-- collection of words
allwords = {"value", "=", "30", "if", "index", "=", "4", "and", "error", "=", "1"}

-- iterate through all words
for _, word in ipairs(allwords) do
   -- if word is present in reserved set
   if reserved[word] then
      print("Error: ", word, "is a reserved keyword.")    
      return
   end
   -- do the regular processing
end
print("all good")

Output

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

Error: 	if	is a reserved keyword.

Function to create a Set

We can create a function which can give the Set implementation as required above. See the syntax below:

-- function to return a set on given values
function Set (list)
   -- initialize the empty set
      local set = {}

   -- iterate over values	  
   for _, value in ipairs(list) do 
      set[value] = true 
   end
   
   -- return the set
   return set
end

Now we can call the above function and get the set of prime numbers as shown below:

keywords = { "2", "3", "5", "7", "11", "13", "17", "19"}

primeNumbers = Set(list)

Complete example of Search of Prime Numbers

Now let's see the complete code in action.

main.lua

-- function to return a set on given values
function Set (list)
   -- initialize the empty set
   local set = {}

   -- iterate over values	  
   for _, value in ipairs(list) do 
      set[value] = true 
   end
   
   -- return the set
   return set
end

list = { "2", "3", "5", "7", "11", "13", "17", "19" }

primeNumbers = Set(list)

-- collection of words
allwords = {"1","12","13","4","51","16","18" }

-- iterate through all words
for _, word in ipairs(allwords) do
   -- if word is present in primae numbers
   if primeNumbers[word] then
      print("A Prime Number found: ", word)    
      return
   end
   -- do the regular processing
end
print("all good")

Output

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

A Prime Number found: 	13