Solidity – Mappings

Mapping in Solidity acts like a hash table or dictionary in any other language. These are used to store the data in the form of key-value pairs, a key can be any of the built-in data types but reference types are not allowed while the value can be of any type. Mappings are mostly used to associate the unique Ethereum address with the associated value type.

Syntax:

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

Creating a Mapping

Mapping is defined as any other variable type, which accepts a key type and a value type.

Example: In the below example, the contract mapping_example a structure is defined and mapping is created.

Solidity

// Solidity program to
// demonstrate mapping
pragma solidity ^0.4.18;

// Defining contract
contract mapping_example {

    //Defining structure
    struct student
    {
        // Declaring different
        // structure elements
        string name;
        string subject;
        uint8 marks;
    }

    // Creating a mapping
    mapping (
    address => student) result;
    address[] public student_result;
}

Output :

Adding values to mapping

As the mapping is created let’s try to add some values to the mapping for better understanding.

Example: In the below example, the contract mapping_example defines a structure, mapping is created and values are added to the mapping.

Solidity

// Solidity program to
// demonstrate adding
// values to mapping
pragma solidity ^0.4.18;

// Creating contract
contract mapping_example {

    //Defining structure
    struct student {

        //Declaring different
        // structure elements
        string name;
        string subject;
        uint8 marks;
    }

    // Creating mapping
    mapping (
      address => student) result;
    address[] public student_result;

    // Function adding values to
    // the mapping
    function adding_values() public {
        var student
          = result[0xDEE7796E89C82C36BAdd1375076f39D69FafE252];

        student.name = "John";
        student.subject = "Chemistry";
        student.marks = 88;
        student_result.push(
          0xDEE7796E89C82C36BAdd1375076f39D69FafE252) -1;

    }

}

Output :

Getting values

We have added values to the mapping, to retrieve the values we have to create a function that returns the values added to the mapping.

Example: In the below example, the contract mapping_example defines a structure, mapping is created, values are added to the mapping, and values are retrieved from the mapping.

Solidity

// Solidity program to
// demonstrate retrieve
// values from the mapping

pragma solidity ^0.4.18;

contract mapping_example {

    // Defining Structure
    struct student {

        // Declaring different data types
        string name;
        string subject;
        uint8 marks;
    }

    // Creating mapping
    mapping (
      address => student) result;
    address[] student_result;

    // Function adding values to the mapping
    function adding_values() public {
        var student
          = result[0xDEE7796E89C82C36BAdd1375076f39D69FafE252];

        student.name = "John";
        student.subject = "Chemistry";
        student.marks = 88;
        student_result.push(
          0xDEE7796E89C82C36BAdd1375076f39D69FafE252) -1;

    }

     // Function to retrieve
     // values from a mapping
     function get_student_result(
     ) view public returns (address[]) {
        return student_result;
    }
}

Output :

Counting Mappings

Mappings can be counted so that we can know how many values are stored in mapping.

Example: In the below example, the contract mapping_example defines a structure, mapping is created, values are added to the mapping, and values are retrieved from the mapping.

Solidity

// Solidity program to
// count number of
// values in a mapping
pragma solidity ^0.4.18;

contract mapping_example {

    // Defining structure
    struct student {

        // Declaring different
        // structure elements
        string name;
        string subject;
        uint8 marks;
    }

    // Creating mapping
    mapping (address => student) result;
    address[] student_result;

    //Function adding values to the mapping
    function adding_values() public {
        var student
          = result[0xDEE7796E89C82C36BAdd1375076f39D69FafE252];

        student.name = "John";
        student.subject = "Chemistry";
        student.marks = 88;
        student_result.push(
          0xDEE7796E89C82C36BAdd1375076f39D69FafE252) -1;

    }

     // Function to retrieve
     // values from the mapping
     function get_student_result(
     ) view public returns (address[]) {
        return student_result;
    }

    // Function to count number
    // of values in a mapping
    function count_students(
    ) view public returns (uint) {
        return student_result.length;
    }
}

Output :

Nested Mappings

If you need to keep track of multiple relationships, use nested mapping. Nested mapping is similar to regular mapping, but it has a different syntax.

Syntax:

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

Example : In the below example , nested mapping created for electing the manager for an organization. Employees used their Employee ID and Voting choice to cast their vote. For knowing the voting status of the Employees ,You can use their Employee ID and Employee Address. 

Solidity

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.16;
/// @title A contract for demonstrating Nested Mapping
/// @author Jitendra Gangwar
/// @notice For now, this contract defining the Voting choice of the Employees for electing their Manager
contract NestedMapping{
    //Nested Mapping
    mapping(uint => mapping(address => bool)) voteToManager;
    //function for Vote Registration of the Employees
    function voteRegistration(uint _empId, bool _voteChoice) external {
        voteToManager[_empId][msg.sender] = _voteChoice;
    }
    //function for knowing the Voting Status of the Employees
    function getVoteStatus(uint _empId, address _empAddr) external view returns(bool){
            return voteToManager[_empId][_empAddr];
        }
    }

Output : 

Output