Mappings are fundamental data structures in Solidity that function as hash tables, storing key-value pairs with instant data retrieval. They're essential for building efficient smart contracts on Ethereum.

A mapping is declared using the syntax

mapping(KeyType => ValueType) VariableName

. For example:

mapping(address => uint) public balances;

Key features of Solidity mappings:

• Instant value lookup using keys
• Cannot be iterated over directly
• All possible keys exist by default (initialized to zero/empty)
• Key-value pairs are not actually stored in memory

Common use cases:

• Token balances tracking
• User permissions management
• State tracking for addresses
• Relationship mapping between different data types

Example implementation for a basic token contract:

mapping(address => uint256) public balances;

function transfer(address recipient, uint256 amount) public {
    require(balances[msg.sender] >= amount);
    balances[msg.sender] -= amount;
    balances[recipient] += amount;
}

Best practices:

• Always initialize mappings in the contract's constructor
• Use appropriate key types (address, uint, bytes32 are common)
• Include proper access controls
• Consider using nested mappings for complex data structures

Important limitations:

• Cannot get a list of all keys
• No built-in way to determine size
• Keys are not stored in the blockchain
• Values can only be accessed if you know the key

Mappings are memory-efficient since they only store non-zero values, making them perfect for sparse data sets like token balances or user permissions in decentralized applications.

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