### Introduction The `AggregatorV3Interface` provides a streamlined ABI for interacting with the Data Feed contract. In this lesson, we'll explore how to retrieve pricing information from it. ### Creating a New Contract Instance The `AggregatorV3Interface` includes the `latestRoundData` function, which retrieves the latest cryptocurrency price from the specified contract. We'll start by declaring a new variable, `priceFeed`, of type `AggregatorV3Interface`. This interface requires the address of the Data Feed contract deployed on the Sepolia Network. ```solidity AggregatorV3Interface priceFeed = AggregatorV3Interface(0x1b44F3514812d835EB1BDB0acB33d3fA3351Ee43); ``` ### The `latestRoundData` Function We can call the `latestRoundData()` function on this interface to obtain several values, including the latest price. ```solidity function latestRoundData() external view returns (uint80 roundId, int256 answer, uint256 startedAt, uint256 updatedAt, uint80 answeredInRound); ``` For now, we'll focus on the `answer` value and ignore the other returned values by using commas as placeholders for the unneeded variables. ```solidity function getLatestPrice() public view returns (int) { (,int price,,,) = priceFeed.latestRoundData(); return price; } ``` Our `getLatestPrice()` function now retrieves the latest ETH price in USD from the `latestRoundData()` function of the Data Feed contract. The returned price is an `int256` with a precision of 1e8, as indicated by the `decimals` function. ### Decimals - `msg.value` is a `uint256` value with 18 decimal places. - `answer` is an `int256` value with 8 decimal places (USD-based pairs use 8 decimal places, while ETH-based pairs use 18 decimal places). This means the `price` returned from our `latestRoundData` function isn't directly compatible with `msg.value`. To match the decimal places, we multiply `price` by 1e10: ```solidity return price * 1e10; ``` ### Typecasting Typecasting, or type conversion, involves converting a value from one data type to another. In Solidity, not all data types can be converted due to differences in their underlying representations and the potential for data loss. However, certain conversions, such as from `int` to `uint`, are allowed. ```solidity return uint(price) * 1e10; ``` We can finalize our `view` function as follows: ```solidity function getLatestPrice() public view returns (uint256) { (,int answer,,,) = priceFeed.latestRoundData(); return uint(answer) * 1e10; } ``` ### Conclusion This complete `getLatestPrice` function retrieves the latest price, adjusts the decimal places, and converts the value to an unsigned integer, making it compatible for its use inside other functions. ### 🧑‍💻 Test yourself 1. 📕 Why we need to multiply the latest ETH price by 1e10? 2. 📕 What's the result of the typecasting `uint256(-2)`? 3. 🧑‍💻 Create a contract with a `getLatestBTCPriceInETH()` function that retrieves the latest BTC price in ETH.