Mappings are fundamental data structures in Solidity that create relationships between two values using key-value pairs. Think of them as hash tables or dictionaries in other programming languages.

A mapping uses the syntax

mapping(KeyType => ValueType)

where KeyType can be any built-in value type and ValueType can be any type including another mapping or array.

Basic example of a mapping:

mapping(address => uint) public balances;

Key characteristics of mappings:

• All possible keys exist by default and are initialized to zero-value
• You cannot get the size/length of a mapping
• Keys are not stored, only their keccak256 hashes
• Cannot iterate through mappings directly
• Extremely gas-efficient for lookups

Common use cases:

• Token balances tracking
• User permissions and roles
• Game state storage
• Relationship tracking between addresses

Example of nested mappings:

mapping(address => mapping(uint => bool)) public nestedMapping;

Best practices:

• Always validate inputs before storing
• Consider using structs for complex value types
• Be cautious with public mappings as they create automatic getter functions
• Use events to track mapping changes
• Consider mapping alternatives if iteration is needed

Mappings are perfect for direct lookups but if you need enumeration or size tracking, consider combining them with arrays or implementing a separate counting mechanism.

Remember: Values in mappings cannot be deleted in the traditional sense - they can only be reset to their default values using the delete keyword.

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