Lua - File Iterator

We can process content of a file without loading it completely using iterators either line by line or even word by word. Lua provides io.lines method to iterate over lines of a file.

Using iterator to read file Line by Line

In this example, we're calling io.lines within a for loop. Lua automatically opens the file, creates an iterator and close the file once loop is finished.

we will use a sample file example.txt as shown below−

example.txt

Welcome to tutorialspoint.com
Simply Easy Learning

main.lua

for line in io.lines("example.txt") do
   print(line)
end

Output

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

Welcome to tutorialspoint.com
Simply Easy Learning

Explanation

• io.lines("example.txt") returns an iterator to iterate over lines of the file.

• for loop read the next line from file everytime it calls the iterator and returns it.

• for loop continues till end of the file where iterator returns nil and loop finishes.

• Using line variable within for loop and each line is printed using print(line) statement.

Using iterator to read file Word by Word

In this example, we're combining io.lines call within a for loop with a nested for loop with string.gmatch to match a word for pattern. This way we can read files as words instead of lines.

we will use a sample file example.txt as shown below−

example.txt

Welcome to tutorialspoint.com
Simply Easy Learning

main.lua

for line in io.lines("example.txt") do
   for word in string.gmatch(line, "%w+") do
      print(word)
   end
end

Output

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

Welcome
to
tutorialspoint
com
Simply
Easy
Learning

Explanation

• io.lines("example.txt") returns an iterator to iterate over lines of the file.

• for loop read the next line from file everytime it calls the iterator and returns it.

• Nested for loop gets the itertor of words splitted from the line.

• Using word variable within nested for loop and each word is printed using print(word) statement.

Advantages of File Iterators

• Memory Efficiency − As we're processing a file line by line, entire file is not loaded into the memory and we can read every very large files easily using such iterators.

• Improved Code Readability − for...in loop is easy to understand and a very concise representation of iteration code.

• Flexibility − We can modulate code easily to process a file line by line or word by word as per our convenience.