What Is the Mapping Data Type in Solidity?


Larry Thompson

When programming in Solidity, a smart contract language for Ethereum blockchain, you may come across the mapping data type. A mapping is a key-value store that allows you to associate values with unique keys. In this article, we will explore what the mapping data type is and how to use it effectively in your Solidity contracts.

What Is a Mapping?

A mapping is similar to an associative array or a dictionary in other programming languages. It consists of key-value pairs, where each key is unique and associated with a value. In Solidity, mappings are declared using the following syntax:

mapping(keyType => valueType) mappingName;

The <keyType> and <valueType> can be any valid Solidity data type, such as uint, address, or even custom structs. The <mappingName> is the name that you give to your mapping variable.

Accessing and Modifying Values in a Mapping

To access or modify values in a mapping, you need to use the corresponding key. Here’s an example:

// Declare a mapping of addresses to integers
mapping(address => uint) public balances;

// Set the balance of an address
balances[msg.sender] = 100;

// Retrieve the balance of an address
uint myBalance = balances[msg.sender];

In the above code snippet, we declare a mapping called balances. The keys are Ethereum addresses (address type) and the values are unsigned integers (uint type).

We then set the balance of the current user’s address to 100. Finally, we retrieve the balance of the current user’s address and store it in the myBalance variable.

Mapping Properties and Limitations

Mappings have some interesting properties and limitations worth noting:

  • No Iteration: Unlike arrays, mappings do not have a built-in way to iterate over their keys or values. If you need to iterate, you may need to maintain an additional data structure.
  • No Length Property: Mappings do not have a length property like arrays.

    To check if a key exists in a mapping, you can compare its value with a default value (e.g., 0 for integers).

  • No Default Value: Mappings do not have default values. If you access a non-existent key, it will return the default value for that data type (e.

Nested Mappings

You can also create nested mappings, where the value of one mapping is another mapping. This allows you to create complex data structures. Here’s an example:

// Declare a nested mapping
mapping(address => mapping(uint => bool)) public permissions;

// Set permission for an address and an ID
permissions[msg.sender][42] = true;

// Retrieve permission for an address and an ID
bool hasPermission = permissions[msg.sender][42];

In this example, we declare a nested mapping called permissions. The keys of the outer mapping are Ethereum addresses, and the values are inner mappings. The keys of the inner mappings are unsigned integers, and the values are booleans indicating permission.


Mappings in Solidity provide a powerful way to store and retrieve data using unique keys. They offer flexibility in terms of data types, including nested mappings.

However, they also come with some limitations, such as no built-in iteration or length property. Understanding how to effectively use mappings will contribute to writing efficient and organized Solidity contracts.

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