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Solidity Fundamentals

Counter Contract and Solidity Basics

In our last video you learned how to use Remix IDE to create a new contract, compile it and deploy it. In this video you are going to learn about the Solidity basics by learning the Counter contract’s code.

Down below, you will find the code that is already commented out for you. First watch the video and after watching the video, copy and paste this contract, and try changing some little parts and experiment with it a little.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;


contract Example {


    // This is the owner of the contract
    address owner;


    // This is our counter struct. It will hold necessary information about the counter which are
    // number and description
    struct Counter {
        uint number;
        string description;
    }


    // Here we create an instance of our Counter.
    // It is empty for now, but we will initialize it in the constructor.
    Counter counter;


    // We will use this modidifer with our execute functions.
    // This modifiers make sure that the caller of the function is the owner of the contract.
    modifier onlyOwner() {
        require(msg.sender == owner, "Only the owner can increment or decrement the counter");
        _;
    }


    // This is our constructor function. It only runs once when we deploy the contract.
    // Since it takes two parameters, initial_value and description, they should be provided
    // when we deploy our contract.
    constructor(uint initial_value, string memory description) {
        owner = msg.sender;
        counter = Counter(initial_value, description);
    }


    // Below, we have two execute functions, increment_counter and decrement_counter
    // Since they modify data on chain, they require gas fee.
    // They are external functions, meaning they can only be called outside of the contract.
    // They also have the onlyOwner modifier which we created above. This make sure that
    // only the owner of this contract can call these functions.


    // This function gets the number field from the counter struct and increases it by one.
    function increment_counter() external onlyOwner {
        counter.number += 1;
    }


    // This function is similar the one above, but instead of increasing we deacrease the number by one.
    function decrement_counter() external onlyOwner {
        counter.number -= 1;
    }


    // The function below is a query function.
    // It does not change any data on the chain. It just rerieves data.
    // We use the keyword view to indicate it retrieves data but does not change any.
    // Since we are not modifying any data, we do not pay any gas fee.


    // This function returns the number field of our counter struct.
    // Returning the current state of our counter.
    function get_counter_value() external view returns(uint) {
        return counter.number;
    }
}
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