Mapping in solidity

Mapping in Solidity acts like a hash table or dictionary in any other language.

mapping(key => value) <access specifier> <name>;

keys are mapped to a value. Data isn't stored in a mapping, only its hash value.

key: any built-in types like uint , bytes, string except for a mapping, a dynamically sized array, a contract, an enum and a struct.
value: any type including mapping, struct, arrays

They don't have a length and you can't iterate through the mapping.
e.g.

// mapping declaration
mapping(uint => string )public people

// update mapping
 people[10] = 'Mark'; // assigns a value
people[12] = 'Andrew';

 // reading  values

people[1] // reads a value

 people[unknown_key]    //will return the default value of the type, i.e '' for string or 0 for unint

Assign an array as a map value
We can assign an array as a value type to a map and can access them as we would an array like below:

contract Mapping {

    mapping(address => uint[]) scores;    

    function manipulateArrayMap() external {
        scores[msg.sender].push(1);             //assign a value; 
        scores[msg.sender].push(2);             //assign another element

        scores[msg.sender][0];                  //access the element in the map array

        scores[msg.sender][1] = 5;              //update an element in the map array in index 1

        delete scores[msg.sender][0];           //delete the element in the index 0 of the map array
    }

}

Assign another map as a map value
We can also assign another map as a map value and can access them as we would access a map like below:

contract Mapping {

    mapping(address => uint) balances;
    mapping(address => mapping(address => bool)) approved;

    function manipulateMapOfMap(spender) external {
        approved[msg.sender][spender] = true                     //assign a value to the approved map
        approved[msg.sender][spender];                           //get the value of the map

        delete approved[msg.sender][spender]                     //delete the reference
    }

}

Common Use case
associate unique Ethereum addresses with associated unique values

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