Get to know Lua for loops in 4 minutes

Get to know Lua for loops in 4 minutes Seth Kenlon Wed, 11/30/2022 - 03:00 In programming, iteration is an important concept because code often must scan over a set of data several times so that it can process each item individually. Control structures enable you to direct the flow of the program based on conditions that are often established dynamically as the program is running. Different languages provide different controls, and in Lua, there's the while loop, for loop, and repeat until loop. This article covers for loops. I will cover while and repeat until loops in a separate article. For loop A for loop takes a known quantity of items and ensures that each item is processed. An "item" can be a number. It can also be a table containing several entries or any Lua data type. The syntax and logic are a little flexible, but the syntax allows for these parameters, each of which essentially describes a counter: Starting value of the counter Stop value The increment you want the counter to advance For instance, suppose you have three items and want Lua to process each. Your counter could start at 3 and last until 1, at an increment of -1. That renders the count of 3, 2, 1. mytable = { "zombie", "Halloween", "apocalypse" } for count = 3, 1, -1 do   print(count .. ": " .. mytable[count]) endRun the code to ensure all three items are getting processed: $ lua ./for.lua3: apocalypse2: Halloween1: zombieThis code effectively processed the table in "reverse" because it was a countdown. You can count up, instead: for count = 1, 3, 1 do   print(mytable[count]) endThis example processes the table from lowest index to highest: $ lua ./for.lua1: zombie2: Halloween3: apocalypseIncrements You can change the increment, too. For instance, maybe you want a zombie apocalypse without all the pomp and circumstance of Halloween: mytable = { "zombie", "Halloween", "apocalypse" } for count = 1, 3, 2 do   print(mytable[count]) endRun the code: $ lua ./for.lua zombie apocalypseThe example printed 1 and 3 because the first count was 1, which was then incremented by 2 (for a total of 3). Counter Sometimes you don't know the number of times you need Lua to iterate over data. In this case, you can set your counter to a variable populated by some other process. Also, the word count isn't a keyword. It's just what I'm using in my sample code for clarity. It's common for programmers to use something shorter, such as i or c. var = os.time() if var%2 == 0 then   mytable = { var } else   mytable = { "foo", "bar", "baz" } end for c = 1, #mytable, 1 do   print(mytable[c]) end Programming and development Red Hat Developers Blog Programming cheat sheets Try for free: Red Hat Learning Subscription eBook: An introduction to programming with Bash Bash shell scripting cheat sheet eBook: Modernizing Enterprise Java An open source developer's guide to building applications This code creates a variable containing the timestamp of when it was launched. If the timestamp is even (it has a modulo of 0 when divided by 2), then just the timestamp is placed into a table. If the timestamp is odd, it puts three strings into a table. Now you can't be sure how many times your for loop needs to run. It's either once or thrice, but there's no way to be sure. The solution is to set the starting count to 1 and the final count to the length of the table (#mytable is the built-in shortcut to determine the length of a table). It might take a few times of running the script to see both results, but eventually, you end up with something like this: $ lua ./dynamic.lua1665447960 $ lua ./dynamic.lua foo bar bazFor loops with pairs and ipairs If you've already read my article on table iteration, then you're already familiar with one of the most common for loops in Lua. This one uses the pairs or ipairs function to iterate over a table: mytable = { "zombie", "Halloween", "apocalypse" } for i,v in ipairs(mytable) do   print(i .. ": " v) endThe pairs and ipairs functions "unpack" the table and dump the values into the variables you provide. In this example, I use i for index and v for value, but the variables' names don't matter. $ lua ./for.lua1: zombie2: Halloween3: apocalypseFor loop The for loop structure is common in programming and very common in Lua due to its frequent use of tables and the pairs function. Understanding the for loop structure and the options you have when controlling it means you can make clever decisions about how to process data in Lua. Understanding the for loop structure and the options you have when controlling it means you can make clever decisions about how to process data in Lua. Image by: Opensource.com Programming What to read next How to use loops in awk This work is licensed under a Creative Commons Attribution-Share Alike 4.0 International License. Register or Login to post a comment.