[Solidity] How to initialize the mapping?

This section explains how to initialize mapping in Solidity.

Syntax

First, declare the mapping as follows.

mapping (key data type => value data type) access qualifier variable name;

Then, initialize the mapping by assigning a value to it.

variable[key] = value;

Sample Code

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.7.0 <0.9.0;

contract Test {
    // Declare mapping of "key: address type, value: uint type, access modifier: public"
    mapping (address => uint) public balances;
    
    function store(uint256 num) public {
        // Initialize mapping with argument num
        balances[msg.sender] = num;
    }

    function retrieve() public view returns (uint256){
        return balances[msg.sender];
    }

}

Mappings are what other programming languages call "associative arrays," which are composed of keys and values.