_You can follow along with the video course from here._ ### Introduction In this recap, we'll review how to interact with an external contract and utilize its functions, understand Chainlink Price Feeds, perform Solidity math, and explore global properties. ### Interacting with an External Contract To interact with any external contract, you need the contract's _address_ and _ABI_ (Application Binary Interface). Think of the `address` as a _house number_ that identifies the specific contract on the blockchain, while the `ABI` serves as a _manual_ that explains how to interact with the contract. To obtain the contract ABI, you can compile a Solidity **interface** that the target contract implements. Then, create a new instance of the interface pointing to the specific address of the deployed contract. ### Chainlink Price Feeds [Chainlink Price Feeds](https://docs.chain.link/docs/using-chainlink-reference-contracts/) provide a reliable way to access real-world data, such as pricing data, and inject it into smart contracts. This is particularly useful for executing mathematical operations in Solidity and the Ethereum Virtual Machine (EVM), where floating-point numbers are not used. ### Solidity Global Properties The [Solidity documentation](https://docs.soliditylang.org/en/latest/cheatsheet.html#block-and-transaction-properties) provides several global properties that are essential for interacting with the Ethereum blockchain. Here are two key properties: - `msg.sender`: this property refers to the address of the account that **initiated the current function call** - `msg.value`: this property represents the **amount of Wei** sent with a function call ```js function updateValue() public payable { require(msg.value >= 1 ether, "Not enough Ether provided."); } ``` By understanding these concepts, you can effectively interact with external contracts, leverage Chainlink Price Feeds for real-world data, and utilize Solidity's global properties for more robust smart contract development. ### 🧑‍💻 Test yourself 1. 🏆 Attempt to answer all the theoretical questions from lessons 1 through 12, and then go back again to complete all the coding tasks.