Nested Loops in Lua

Now that you have learned how to use both for and while loops in Lua, it is time to take your skills a step further. In this lesson, you will explore how to use loops inside other loops — these are called nested loops. This is a natural next step after learning about single loops, and it will help you handle more complex data and tasks in your programs.

Nested loops are useful when you need to work with multiple sequences or layers of information at the same time. For example, imagine you are planning a trip and, for each country you visit, you want to list the sights you want to see. You have a table in which each country is linked to a list of sights. To print out all the sights for every country, you need a loop inside another loop.

Here is a sample code snippet to show how this works:

local country_sights = {
    France = {"Eiffel Tower", "Louvre Museum"},
    Italy = {"Colosseum", "Piazza San Marco"},
    Spain = {"Park Güell", "The Alhambra"},
    Japan = {"Mt. Fuji", "Fushimi Inari Taisha"}
}

for country, sights in pairs(country_sights) do
    print("***In " .. country .. ", I want to see:")
    for _, sight in ipairs(sights) do
        print(sight)
    end
end

Let's break down what happens in the code above:

• The variable country_sights is a table where each key is a country and each value is a list (table) of sights in that country.
• The outer for loop uses pairs to go through each country and its list of sights.
• For each country, the code prints a header line showing the country name.
• The inner for loop uses ipairs to go through the list of sights for the current country.
• Each sight is printed on its own line.

This structure allows you to process and display information that is organized in layers — in this case, countries and their sights. The outer loop handles the first layer (countries), and the inner loop handles the second layer (sights within each country). Nested loops like this are a powerful way to work with grouped or hierarchical data in Lua.

• pairs is used to iterate over all key-value pairs in a table, no matter what the keys are (they can be strings or numbers, and in any order). In our example, country_sights uses country names as keys, so we use pairs(country_sights) to go through each country and its list of sights.

• ipairs is used to iterate over tables that act like arrays, where the keys are consecutive integers starting from 1. It goes through the elements in order and stops at the first nil value. In our example, each value in country_sights is a list of sights, so we use ipairs(sights) to go through each sight in order.