Returning Values from Functions

In the last lesson, you passed information into functions using parameters. In this lesson, you will learn how functions can send information back using return values. Instead of only printing inside a function, you will compute a result and return it so the caller can store it, print it, or use it later.

local country_costs = {France = 1000, Italy = 800, Spain = 900}
local chosen_countries = {"France", "Italy"}

-- Define a function to calculate the trip cost.
local function calculate_trip_cost(countries, costs)
    local total_cost = 0
    for _, country in ipairs(countries) do
        total_cost = total_cost + costs[country]
    end
    return total_cost
end

Explanation:

• The first two lines create data we will use: a table of country_costs and a list of selected countries.
• The function calculate_trip_cost(countries, costs) takes two parameters: a list of countries and a lookup table of costs.
• It initializes total_cost to 0, loops through the chosen countries with ipairs, and adds each country’s cost using costs[country].
• return total_cost sends the computed total back to whoever called the function. The function itself does not print.

-- Call the function
local trip_cost = calculate_trip_cost(chosen_countries, country_costs)
print("The total cost of the trip is: $" .. trip_cost)

Explanation:

• The function call produces a value. We store that value in trip_cost.
• We then print a readable message by concatenating the string with trip_cost using ...
• Returning a value lets you reuse it anywhere in your program, not just print it once.

In Lua, a function can return more than one value by separating them with commas in the return statement.

local function min_and_max(a, b)
    if a < b then
        return a, b
    else
        return b, a
    end
end

local smallest, largest = min_and_max(5, 12)
print("Smallest:", smallest)   -- Output: Smallest: 5
print("Largest:", largest)     -- Output: Largest: 12

Explanation:

• The function min_and_max(a, b) compares two numbers and returns both the smaller and the larger value.
• When calling the function, you can assign each returned value to its own variable.
• This is useful when you want a function to provide more than one result at a time.