Maps

The composite type map<K, V> is used as a way to associate keys of type K with corresponding values of type V.

For example, map<Int, Int> uses Int type for its keys and values:

struct IntToInt {
    counters: map<Int, Int>;
}

Allowed types

Allowed key types:

• Int
• Address

Allowed value types:

• Int
• Bool
• Cell
• Address
• Struct
• Message

Operations

Declare, emptyMap()

As a local variable, using emptyMap() function of standard library:

let fizz: map<Int, Int> = emptyMap();
let fizz: map<Int, Int> = null; // identical to the previous line, but less descriptive

As a persistent state variable:

contract Example {
    fizz: map<Int, Int>; // Int keys to Int values
    init() {
        self.fizz = emptyMap(); // redundant and can be removed!
    }
}

Note, that persistent state variables of type map<K, V> are initialized empty by default and don’t need default values or an initialization in the init() function.

Set values, .set()

To set or replace the value under a key call the .set() method, which is accessible for all maps.

// Empty map
let fizz: map<Int, Int> = emptyMap();

// Setting a couple of values under different keys
fizz.set(7, 7);
fizz.set(42, 42);

// Overriding one of the existing key-value pairs
fizz.set(7, 68); // key 7 now points to value 68

Get values, .get()

To check if a key is found in the map by calling the .get() method, which is accessible for all maps. This will return null if the key is missing, or the value if the key is found.

// Empty map
let fizz: map<Int, Int> = emptyMap();

// Setting a value
fizz.set(68, 0);

// Getting the value by its key
let gotButUnsure: Int? = fizz.get(68);          // returns Int or null, therefore the type is Int?
let mustHaveGotOrErrored: Int = fizz.get(68)!!; // explicitly asserting that the value must not be null,
                                                // which may crush at runtime if the value is, in fact, null

// Alternatively, we can check for the key in the if statement
if (gotButUnsure != null) {
    // Hooray, let's use !! without fear now and cast Int? to Int
    let definitelyGotIt: Int = fizz.get(68)!!;
} else {
    // Do something else...
}

Delete entries, .del()

To delete a single key-value pair (single entry), use the .del() method. It returns true in the case of successful deletion and false otherwise.

// Empty map
let fizz: map<Int, Int> = emptyMap();

// Setting a couple of values under different keys
fizz.set(7, 123);
fizz.set(42, 321);

// Deleting one of the keys
let deletionSuccess: Bool = fizz.del(7); // true, because map contained the entry under key 7
fizz.del(7);                             // false, because map no longer has an entry under key 7

// Note, that assigning the `null` value to the key when using the `.set()` method
//   is equivalent to calling `.del()`, although such approach is much less descriptive
//   and is generally discouraged:
fizz.set(42, null); // the entry under key 42 is now deleted

To delete all the entries from the map, re-assign the map using the emptyMap() function:

// Empty map
let fizz: map<Int, Int> = emptyMap();

// Setting a couple of values under different keys
fizz.set(7, 123);
fizz.set(42, 321);

// Deleting all of the entries at once
fizz = emptyMap();
fizz = null; // identical to the previous line, but less descriptive

With this approach all previous entries of the map are completely discarded from the contract even if the map was declared as its persistent state variable. As a result, assigning maps to emptyMap() does not inflict any hidden or sudden storage fees.

Check if empty, .isEmpty()

The .isEmpty() method on maps returns true if the map is empty and false otherwise:

let fizz: map<Int, Int> = emptyMap();

if (fizz.isEmpty()) {
    dump("Empty maps are empty, duh!");
}

// Note, that comparing the map to `null` behaves the same as `.isEmpty()` method,
// although such direct comparison is much less descriptive and is generally discouraged:
if (fizz == null) {
    dump("Empty maps are null, which isn't obvious");
}

Convert to a Cell, .asCell()

Use .asCell() method on maps to convert all their values to a Cell type. Be mindful, that Cell type is able to store up to 1023 bits, so converting larger maps to the Cell will result in error.

As an example, this method is useful for sending small maps directly in the body of the reply:

contract Example {
    // Persistent state variables
    fizz: map<Int, Int>; // our map

    // Constructor (initialization) function of the contract
    init() {
        // Setting a bunch of values
        self.fizz.set(0, 3);
        self.fizz.set(1, 14);
        self.fizz.set(2, 15);
        self.fizz.set(3, 926);
        self.fizz.set(4, 5_358_979_323_846);
    }

    // Internal message receiver, which responds to empty messages
    receive() {
        // Here we're converting the map to a Cell and making a reply with it
        self.reply(self.fizz.asCell());
    }
}

Traverse over entries

To iterate over map entries there is a foreach loop statement:

// Empty map
let fizz: map<Int, Int> = emptyMap();

// Setting a couple of values under different keys
fizz.set(42, 321);
fizz.set(7, 123);

// Iterating over in a sequential order: from the smallest keys to the biggest ones
foreach (key, value in fizz) {
    dump(key); // will dump 7 on the first iteration, then 42 on the second
}

Read more about it: foreach loop in Book→Statements.

Note, that it’s also possible to use maps as simple arrays if you define a map<Int, V> with an Int type for the keys, any allowed V type for values and keep track of the number of items in the separate variable:

contract Iteration {
    // Persistent state variables
    counter: Int as uint32;    // counter of map entries, serialized as a 32-bit unsigned
    record: map<Int, Address>; // Int to Address map

    // Constructor (initialization) function of the contract
    init() {
        self.counter = 0; // Setting the self.counter to 0
    }

    // Internal message receiver, which responds to a String message "Add"
    receive("Add") {
        // Get the Context Struct
        let ctx: Context = context();
        // Set the entry: counter Int as a key, ctx.sender Address as a value
        self.record.set(self.counter, ctx.sender);
        // Increase the counter
        self.counter += 1;
    }

    // Internal message receiver, which responds to a String message "Send"
    receive("Send") {
        // Loop until the value of self.counter (over all the self.record entries)
        let i: Int = 0; // declare usual i for loop iterations
        while (i < self.counter) {
           send(SendParameters{
                bounce: false,              // do not bounce back this message
                to: self.record.get(i)!!,   // set the sender address, knowing that key i exists in the map
                value: ton("0.0000001"),    // 100 nanoToncoins (nano-tons)
                mode: SendIgnoreErrors,     // send ignoring errors in transaction, if any
                body: "SENDING".asComment() // String "SENDING" converted to a Cell as a message body
            });
            i += 1; // don't forget to increase the i
        }
    }

    // Getter function for obtaining the value of self.record
    get fun map(): map<Int, Address> {
        return self.record;
    }

    // Getter function for obtaining the value of self.counter
    get fun counter(): Int {
        return self.counter;
    }
}

It’s often useful to set an upper-bound restriction on such maps, so that you don’t hit the limits.

Caution

Note, that manually keeping track of number of items or checking the length of such map is very error-prone and generally discouraged. Instead, try to wrap your map into the Struct and define extension functions on it. See example in the Cookbook: How to emulate an array using a map wrapped in a Struct.

Note

This example was adapted from howardpen9/while-example-tact.

See other examples of map usage in the Cookbook:
How to emulate a stack using a map wrapped in a Struct
How to emulate a circular buffer using a map wrapped in a Struct

Serialization

It’s possible to do integer serialization of map keys, values or both to preserve space and reduce storage costs:

struct SerializedMapInside {
    // Both keys and values here would be serialized as 8-bit unsigned integers,
    // thus preserving the space and reducing storage costs:
    countersButCompact: map<Int as uint8, Int as uint8>;
}

Note

Read about other serialization options: Compatibility with FunC.

Limits and drawbacks

While maps can be convenient to work with on a small scale, they cause a number of issues if the number of items is unbounded and map can significantly grow in size:

• As the upper bound of the smart contract state size is around $65\,000$ items of type Cell, it constrains the storage limit of maps to be about $30\,000$ key-value pairs for the whole contract.

• The more entries you have in a map, the bigger compute fees you’ll get. Thus, working with large maps makes compute fees tough to predict and manage.

• Using a large map in a single contract doesn’t allow to distribute its workload. Hence, it can make the overall performance much worse compared to using a smaller map and a bunch of interacting smart contracts.

To resolve such issues you can set an upper-bound restriction on a map as a constant and check against it every time you’re setting a new value to the map:

contract Example {
    // Declare a compile-time constant upper-bound for our map
    const MaxMapSize: Int = 42;

    // Persistent state variables
    arr: map<Int, Int>; // "array" of Int values as a map
    arrLength: Int = 0; // length of the "array", defaults to 0

    // Internal function for pushing an item to the end of the "array"
    fun arrPush(item: Int) {
        if (self.arrLength >= self.MaxMapSize) {
            // Do something, stop the operation, for example
        } else {
            // Proceed with adding new item
            self.arr.set(self.arrLength, item);
            self.arrLength += 1;
        }
    }
}

If you still need a large map or an unbound (infinitely large) map, it’s better to architect your smart contracts according to the asynchronous and actor-based model of TON blockchain. That is, to use contract sharding and essentially make the whole blockchain a part of your map(s).