Lua - Iterator Closures

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Iterator

Iterator is a special function in lua which is used to traverse a collection item by item. Generally, a iterator returns the next element when invoked. In order to know the position of next element, either the invoker must pass a control variable to the iterator or iterator should keep a state while returning a value. In Lua, generic for loop provides a control variable to the iterator invoked and makes it very easy to use an iterator that we'll see in examples below.

Closures

Closure is a function which can access one or more local variables of the enclosing function. In Lua, we use closures to keep state of an iterator. Here we define two types of functions.

• factory function− a function with one or more local variable as state of the iterator.

• closure function− an internal function which can access the local variable of the factory function as a state and using this state, this closure function returns the next elemnt of the collection.

Example - Iterator with Closure

Consider a simple case, where we've a list of numbers and an iterator function is to be defined to traverse the array. Let's first define the iterator function−

Iterator Function

-- factory function to get the iterator
function listIterator (collection)
   local index = 0
   local size = #collection
   -- closure function
   return function ()
      index = index + 1
      if index <= size 
	  then 
	     return collection[index] 
	  end
   end
end

Here using listIterator() we're creating the iterator and every time this iterator is invoked, a new closure is created which keep increasing the index and return the next value of collection accordingly.

Using while Loop

Now using while loop, we'll showcase how to use the above iterator.

-- collection of numbers
list = {10, 20, 30}

-- create the iterator of the list
customIterator = listIterator(list)    

-- run the while loop	
while true 
do
   -- calls the iterator
   local element = customIterator()   
   
   -- if last element is reached earlier
   -- iterator will return nil as next element
   if element == nil 
   then 
      -- break the infinite loop
      break 
   end
   -- print the element
   print(element)
end

Using for loop

for loop is much easier to use as compared to while loop.

-- collection of numbers
list = {10, 20, 30}

-- run the for loop	
for element in listIterator(list)
do
   -- print the element
   print(element)
end

Complete Example of While Loop using Iterator with Closure

Following is the complete code of running a while loop on a iterator with closure.

main.lua

-- factory function to get the iterator
function listIterator (collection)
   local index = 0
   local size = #collection
   -- closure function
   return function ()
      index = index + 1
      if index <= size 
	  then 
	     return collection[index] 
	  end
   end
end

-- collection of numbers
list = {10, 20, 30}

-- create the iterator of the list
customIterator = listIterator(list)    

-- run the while loop	
while true 
do
   -- calls the iterator
   local element = customIterator()   
   
   -- if last element is reached earlier
   -- iterator will return nil as next element
   if element == nil 
   then 
      -- break the infinite loop
      break 
   end
   -- print the element
   print(element)
end

Output

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

10
20
30

Complete Example of For Loop using Iterator with Closure

Following is the complete code of running a for loop on a iterator with closure.

main.lua

-- factory function to get the iterator
function listIterator (collection)
   local index = 0
   local size = #collection
   -- closure function
   return function ()
      index = index + 1
      if index <= size 
	  then 
	     return collection[index] 
	  end
   end
end

-- collection of numbers
list = {10, 20, 30}

-- run the for loop	
for element in listIterator(list)
do
   -- print the element
   print(element)
end

Output

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

10
20
30