Mappings

A mapping is a key-value data structure in smart contracts that acts like a hash table, providing a way to store and retrieve data based on a unique key. It is the most commonly used structure for storing user balances, ownership information, and protocol state.

Unlike arrays, mappings do not require iterating over the entire structure to find a value; instead, they provide constant-time access to any entry. This makes them highly efficient for managing large datasets.

However, mappings cannot be enumerated, meaning you cannot easily iterate over all the keys without maintaining an auxiliary list. They are fundamental to almost every decentralized finance protocol, providing the backbone for accounting and state management.

Understanding how to use mappings effectively, including managing their limitations, is a core skill for any blockchain developer.