Maps are a data structure which associate values of the same type to values of the same type. The former are called key and the latter values. Together they make up a binding. An additional requirement is that the type of the keys must be comparable, in the Michelson sense.
Declaration
Maps are declared as
type balances = map<string, nat>;
Instantiation
To create an empty map :
let empty: balances = Map.empty;
To create an non-empty map :
let moves: balances = Map.literal(list([
["tim", 5 as nat],
["mark", 0 as nat]
]));
Access
Use the postfix [] operator to read a value of the map :
let my_balance: option<nat> = Map.find_opt("tim", moves);
ℹ️ The keyword option shows that this value is optional. More on this later.
Update
The values of a map can be updated using Map.update built-in function:
let user_balances: balances = Map.update("tim", Some(14 as nat), moves);
Insertion
To add a new value in the map, use Map.add :
let user_balances: balances = Map.add("ed", 39 as nat, moves);
Removal
A key-value can be removed from the mapping as follows :
Map.remove("tim", moves);
Big Maps
Maps load their entries into the environment, which is fine for small maps, but for maps holding millions of entries, the cost of loading such map would be too expensive. For this we use big_maps. Their syntax is the same as for regular maps.
Your mission
1- Notice we defined coordinates as a 3D tuple.
2- Define the type name_to_coordinates as a mapping from the celestial body name to its coordinates.
3- Create a new map called star_map and add values for earth at 2,7,1 , the sun at 0,0,0 and alpha-centauri at 2232,7423,12342 .