I can't write/encode JSON files properly. Code produces different results - json

Question

I am having problems with the JSON file in my Corona game. Basically, the game gives you trophies (cards) when you reach certain points. The card information is then written into a JSON file. When you start the game, it checks if the file "playerCards.json" already exists, and if not, it creates such file with the following structure:
{"common":[],"uncommon":[],"rare":[]}
Later in the game, the player finally receives one card. The (simplified) code below runs:
local category = "common"
local rdm = math.random(1,20)
array = loadFile("playerCards.json")
array[category][rdm] = collection[category][rdm]
writeFile("playerCards.json", array)
Collection is a preloaded Lua table structured like this: {"common" = {"001", "002", "003",..., "020"}}. For the sake of the question, I've restricted the cards to a certain category (common). Let's suppose the player won card number 3, so the code should run like this:
array["common"][3] = collection["common"][3]
And the resulting table array would be:
array = {"common" = {nil, nil, "003"}}
When I use the function writeFile("playerCards.json", array) I am encoding the table above into the file playerCards.json. For now, this code works perfectly, and the resulting JSON file is as follows:
{"common":[null,null,"003"],"uncommon":[],"rare":[]}
The problem comes when the player gets a card above 9, for example, 15. When written, the JSON file becomes this:
{"common":{"3":"003","15":"015"},"uncommon":[],"rare":[]}
How can the same code produce such different results? Can you help me solve this problem? If you need it, here is the code for the load and write functions:
local loadFile = function(name)
local data = nil
local path = system.pathForFile(name, system.DocumentsDirectory)
handle = io.open(path, "r")
if handle then
data = json.decode(handle:read("*a"))
io.close(handle)
end
return data
end
local writeFile = function(name, data)
local path = system.pathForFile(name, system.DocumentsDirectory)
local handle = io.open(path, "w+")
if handle then
handle:write(json.encode(data))
io.close(handle)
end
end

Answer 1

The problem is that Lua does not distinguish between list and mapping objects (tables are used for both) while JSON does.
This creates ambiguity when serializing to JSON; the serializer has to determine whether a Lua table should serialize to an array or an object. Most serializers do this by checking if the keys of the table are roughly sequential integers, and only serializing to an array if so. If your array is too 'sparse', as in your second example, then the serializer thinks its a map with numerical keys instead.
I don't know about Corona, but some Lua JSON libraries I have seen include a method to mark tables explicitly as arrays. Alternatively, you can fill empty slots of the array with a placeholder value like false instead of nil.