Mapping

Mapping is a data structure in Solidity that allows a programmer to store, retrieve and modify data in a key-value pair format. It is similar to a hash table or dictionary in other languages.

Example:

Let's assume we want to store a list of people and their ages. We can do this with a mapping like this:

mapping (address => uint) public peopleAges;

Initialization & Assignment

• On Initialization, all possible key-value pairs are already initialized.

• Accessing unassigned key-value pairs will return the default value for that value type.

• This is because the hash that points to the location in storage has not been generated & we have not written a value thus far.

Assignment of values must be done inside a function

• cannot simply do myMapping[1] = "test" (like python)

Example

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;

contract SimpleMappingExample {

    mapping(uint => bool) public myMapping;
    mapping(address => bool) public myAddressMapping;

    function setValue(uint _index) public {
        myMapping[_index] = true;
    }

    function setMyAddressToTrue() public {
        myAddressMapping[msg.sender] = true;
    }
}

1. On initialization, myMapping[1] will return the default bool value of false.

2. Then we call setValue setting -> myMapping[1] = true

3. Subsequently, when we call myMapping[1], we are returned true as set earlier.

Nested Mapping

mapping(address => mapping(uint256 => string)) public userData;

// accessing nested mapping
userData[address][uint256] = 'John'

• This creates a mapping from an address to a mapping of uints to strings.

• This allows us to store a string value associated with a given address and a given uint.

• This could be used to store user data associated with a particular address and a particular event or transaction.