Use Maps for Storing Values

Use mapping to store data as key-value pairs.

In Develop a smart contract, you developed a smart contract for storing and retrieving a single numeric value.

This tutorial illustrates how you can extend the functionality of your smart contract to manage one number per user. To add this functionality, you'll use the Mapping type.

The ink! language provides the Mapping type to enable you to store data as key-value pairs. For example, the following code illustrates mapping a user to a number:

// Import the `Mapping` type
use ink::storage::Mapping;

#[ink(storage)]
pub struct MyContract {
  // Store a mapping from AccountIds to a u32
  my_map: Mapping<AccountId, u32>,
}

With the Mapping data type, you can store a unique instance of the storage value for each key.

For this tutorial, each AccountId represents a key that maps to one and only one stored numeric my_map.

Each user can only store, increment, and retrieve the value associated with their own AccountId.

Initialize a Mapping

The first step is to initialize the mapping between an AccountId and a stored value.

• Specify the mapping key and the value mapped to it.

The following example illustrates how to initialize a Mapping and retrieve a value:

#![cfg_attr(not(feature = "std"), no_std)]

#[ink::contract]
mod mycontract {
    use ink::storage::Mapping;

    #[ink(storage)]
    pub struct MyContract {
        // Store a mapping from AccountIds to a u32
        my_map: Mapping<AccountId, u32>,
    }

    impl MyContract {
        #[ink(constructor)]
        pub fn new(count: u32) -> Self {
            let mut my_map = Mapping::default();
            let caller = Self::env().caller();
            my_map.insert(&caller, &count);

            Self { my_map }
        }

        // Get the number associated with the caller's AccountId, if it exists
        #[ink(message)]
        pub fn get(&self) -> u32 {
            let caller = Self::env().caller();
            self.my_map.get(&caller).unwrap_or_default()
        }
    }
}

Identifying the contract caller

In the preceding example, you might have noticed the Self::env().caller() function call.

This function is available throughout the contract logic and always returns the contract caller.

It is important to note that the contract caller is not the same as the origin caller.

If a user accesses a contract that then calls a subsequent contract, the Self::env().caller() in the second contract is the address of the first contract, not the original user.

Using the contract caller

There are many scenarios where having the contract caller available is useful.

For example, you can use Self::env().caller() to create an access control layer that only allows users to access their own values.

You can also use Self::env().caller() to save the contract owner during contract deployment.

For example:

#![cfg_attr(not(feature = "std"), no_std)]

#[ink::contract]
mod my_contract {

    #[ink(storage)]
    pub struct MyContract {
        // Store a contract owner
        owner: AccountId,
    }

    impl MyContract {
        #[ink(constructor)]
        pub fn new() -> Self {
            Self {
                owner: Self::env().caller(),
            }
        }
        /* --snip-- */
    }
}

Because you have saved the contract caller using the owner identifier, you can later write functions that check whether the current contract caller is the owner of the contract.

Add mapping to the smart contract

You are now ready to introduce a storage map to the incrementer contract.

To add a storage map to the incrementer contract:

1. Open a terminal shell on your computer, if needed.

2. Verify that you are in the incrementer project folder.

3. Open the lib.rs file in a text editor.

4. Import the Mapping type.

   #[ink::contract
   mod incrementer {
       use ink::storage::Mapping;

5. Add the mapping key from AccountId to the i32 data type stored as my_map.

   pub struct Incrementer {
       value: i32,
       my_map: Mapping<AccountId, i32>,
   }

6. In the new constructor create a new Mapping and use that to initialize your contract.

   #[ink(constructor)]
   pub fn new(init_value: i32) -> Self {
       let mut my_map = Mapping::default();
       let caller = Self::env().caller();
       my_map.insert(&caller, &0);
   
       Self {
           value: init_value,
           my_map,
       }
   }

7. In the default constructor add a new default Mapping along with the already defined default value.

   #[ink(constructor)]
   pub fn default() -> Self {
   Self {
           value: 0,
           my_map: Mapping::default(),
       }
   }

8. Add a get_mine() function to read my_map using the Mapping API's get() method and return my_map for the contract caller.

   #[ink(message)]
   pub fn get_mine(&self) -> i32 {
        let caller = self.env().caller();
        self.my_map.get(&caller).unwrap_or_default()
   }

9. Add a new test to the initialize accounts.

    #[ink::test]
   fn my_map_works() {
       let contract = Incrementer::new(11);
       assert_eq!(contract.get(), 11);
       assert_eq!(contract.get_mine(), 0);
   }

10. Save your changes and close the file.

11. Use the test subcommand and nightly toolchain to test your work by running the following command:

    cargo test

    The command should display output similar to the following to indicate successful test completion:

    running 3 tests
    test incrementer::tests::default_works ... ok
    test incrementer::tests::it_works ... ok
    test incrementer::tests::my_map_works ... ok
    
    test result: ok. 3 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Insert, update, or remove values

The final step in the Incrementer contract is to allow users to update their own values.

You can use calls to the Mapping API to provide this functionality in the smart contract.

The Mapping provides direct access to storage items.

For example, you can replace a previous value held for a storage item by calling Mapping::insert() with an existing key.

You can also update values by first reading them from storage using Mapping::get(), then update the value with Mapping::insert().

If there is no existing value at a given key, Mapping::get() returns None.

Because the Mapping API provides direct access to storage, you can use the Mapping::remove() method to remove the value at a given key from storage.

To add insert and remove functions to the contract:

1. Open a terminal shell on your computer, if needed.

2. Verify that you are in the incrementer project folder.

3. Open the lib.rs file in a text editor.

4. Add an inc_mine() function that allows the contract caller to get the my_map storage item and insert an incremented value into the mapping.

   #[ink(message)]
   pub fn inc_mine(&mut self, by: i32) {
       let caller = self.env().caller();
       let my_value = self.get_mine();
       self.my_map.insert(caller, &(my_value + by));
   }

5. Add a remove_mine() function that allows the contract caller to clear the my_map storage item from storage.

   #[ink(message)]
   pub fn remove_mine(&self) {
       let caller = self.env().caller();
       self.my_map.remove(&caller)
   }

6. Add a new test to verify that the inc_mine() functions works as expected.

   #[ink::test]
   fn inc_mine_works() {
       let mut contract = Incrementer::new(11);
       assert_eq!(contract.get_mine(), 0);
       contract.inc_mine(5);
       assert_eq!(contract.get_mine(), 5);
       contract.inc_mine(5);
       assert_eq!(contract.get_mine(), 10);
   }

7. Add a new test to verify that the remove_mine() functions works as expected.

   #[ink::test]
   fn remove_mine_works() {
       let mut contract = Incrementer::new(11);
       assert_eq!(contract.get_mine(), 0);
       contract.inc_mine(5);
       assert_eq!(contract.get_mine(), 5);
       contract.remove_mine();
       assert_eq!(contract.get_mine(), 0);
   }

8. Check your work using the test subcommand:

   cargo test

   The command should display output similar to the following to indicate successful test completion:

   running 5 tests
   test incrementer::tests::default_works ... ok
   test incrementer::tests::it_works ... ok
   test incrementer::tests::remove_mine_works ... ok
   test incrementer::tests::inc_mine_works ... ok
   test incrementer::tests::my_map_works ... ok
   
   test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

Next steps

In this tutorial, you learned how to use the ink::storage::Mapping type and Mapping API in a smart contract. For example, this tutorial illustrated:

• How to initialize a mapping for storing key-value pairs.

• How to identify and use the contract caller in a smart contract.

• How to add functions that enable users to insert and remove the values stored for them in a map using a smart contract.

You can find an example of the final code for this tutorial in the assets for the smart-contracts.

You can learn more about smart contract development in the following topics:

• Build an ERC20 token contract
• Troubleshoot smart contracts