Lua - Table Iterator

Quiz

Table is a versatile data structure in Lua and is used to create various types like array/dictionary or to create various data structures like stack, queue, linked list etc. A table can be in numerically indexed form or in a key-value pairs based associated form.

Based on type of table, We can use following types of inbuilt iterators to navigate through table entries.

next− this is an inbuilt function to give the next value from the table when invoked. This iterator is mainly used in generic for loop and for associated table.
ipairs()− this iterator uses default keys starting from 1 and is useful where table is not having any explicit keys. It is useful for numerically indexed tables.
pairs()− this iterator is useful when table is having explicit keys.

Let's see each one of the above functions to iterate values of a table.

Example - Using next function

next function accepts a table and index to return the next value of the table.

next(table, [, index])

Here next function returns the associated value from the table as per index. If index is nil, then first value is returned. Following example showcases the same−

main.lua

-- initialize a table of fruits with associative keys 
fruits = {[2] = "banana",[4] = "orange",[6]="apple",[8] = "grapes"}

-- print the entries of table along with keys
for key,value in next,fruits do
  print(key,value)
end

Output

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

8	grapes
2	banana
4	orange
6	apple

Example - Using pairs iterator

pairs iterator function returns key-value pairs and is mainly used in associative tables. Keys are preserved but order is not preserved. See the example below−

main.lua

-- initialize table of fruits
fruits = {[2] = "banana",[4] = "orange",[6]="apple",[8] = "grapes"}

-- print the entries of table along with keys
for key,value in pairs(fruits) do
  print(key,value)
end

Output

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

8	grapes
2	banana
4	orange
6	apple

Example - Using ipairs iterator

pairs iterator function returns index-value pairs and is mainly used in tables without associative keys. See the example below−

main.lua

-- initialize table of fruits
fruits = {"banana","orange","apple","grapes"}

-- print the entries of table along with default keys
for key,value in ipairs(fruits) do
  print(key,value)
end

Output

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

1	banana
2	orange
3	apple
4	grapes

Example - a Custom Table Iterator

Following is an example of a custom stateful iterator which accepts an table and return next value.

main.lua

-- Initialize an array
array = {11, 22, 33, 44, 55, 66}

-- return a stateful iterator to get values of collection passed
function elementIterator (collection)

   local index = 0
   local count = #collection
	
   -- The closure function is returned
	
   return function ()
      index = index + 1
		
      if index <= count
      then
	     -- return the current element of the iterator
	     local value = collection[index];
         return index, value
      end		
   end	
end

-- loop through the iterator
for index, value in elementIterator(array)  
do
   -- print the element
   print(index, value)
end

Output

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

Print Page