Default Values

You know from the exercise on parameters that a missing argument is nil. Often there's a value that fits most calls, and you only want to pass a different one now and then. Emma usually buys one bag of seeds, for example. For such cases, the parameter gets a default value. Lua has no special way of writing it, but you know everything you need from the lesson on conditions: x or default.

local function buySeeds(crop, bags)
    bags = bags or 1
    print("Seeds: " .. crop .. ", bags: " .. bags)
end

buySeeds("carrot", 3)
buySeeds("pumpkin")

This prints:

Seeds: carrot, bags: 3
Seeds: pumpkin, bags: 1

The first line of the body fills in the default value. When the call passes bags, bags or 1 is that number. When it doesn't, bags is nil, and nil or 1 is 1. A call can leave out arguments only at the end, so parameters with a default value go last.

Many other languages write the default value right in the parentheses: function buySeeds(crop, bags = 1). Lua doesn't have that, and such a line is a syntax error: ')' expected near '='.

There's one trap. or replaces not only nil, but also false. So when the default value is true, the caller can't pass false:

local function feed(animal, hungry)
    hungry = hungry or true
    if hungry then
        print("Feeding " .. animal)
    else
        print(animal .. " isn't hungry")
    end
end

feed("Bella")
feed("Rex", false)

This prints:

Feeding Bella
Feeding Rex

Rex isn't hungry, but false or true is true. It's the same trap as with a and b or c in the lesson on conditions. For a parameter that can be false, check for nil instead:

local function feed(animal, hungry)
    if hungry == nil then
        hungry = true
    end
    if hungry then
        print("Feeding " .. animal)
    else
        print(animal .. " isn't hungry")
    end
end

feed("Bella")
feed("Rex", false)

This prints:

Feeding Bella
Rex isn't hungry

Now only a missing argument (nil) gets the default value, and false stays false. For numbers and texts, x or default is all you need, because they're never false. Even 0 and "" stay as they are, since in Lua they count as true.