In Lesson 4: fund-me-fcc (library Q&A)

[KarthickSakthi]

in the File PriceConverter.sol , function getPrice():

function getPrice() internal view returns (uint256) {
        // Sepolia ETH / USD Address
        // https://docs.chain.link/data-feeds/price-feeds/addresses#Sepolia%20Testnet
        AggregatorV3Interface priceFeed = AggregatorV3Interface(
            0x694AA1769357215DE4FAC081bf1f309aDC325306
        );
        (, int256 answer, , , ) = priceFeed.latestRoundData();
        // ETH/USD rate in 18 digit
        return uint256(answer * 10000000000);
        // or (Both will do the same thing)
        // return uint256(answer * 1e10); // 1* 10 ** 10 == 10000000000
    } 

in return statement : why

uint256(answer * 10000000000) // 1e10

in FunMe.sol, for MINIMUM_USD:

uint256 public constant MINIMUM_USD = 50 * 10 ** 18; 50 * ; // 1e18

need explanation!

[alymurtazamemon]

@KarthickSakthi This is all because solidity does not work with decimal numbers. So this logic is for correct math.

Price return by price feed contains 8 decimals, i.e if the price is 1840 then the value will be 184000000000, now as we know solidity's smallest unit is wei which contains 18 decimals, i.e 1 ether will be equal to 1000000000000000000. So we added 10 more decimals to 8 decimals so the price will be in the 18 decimal system.

Now for comparing the value so we can identify whether the user has deposited the correct amount or not we do all this math;

first, we declared the minimum price for the deposit MINIMUM_USD = 50 * 10 ** 18; 50 * which is 50, so what value will be in wei? it will be 50 with 18 decimals 50000000000000000000 or we can write it like this 50 * 10e18

Now both values are in the 18 decimal system, we can now do the conversion.

[KarthickSakthi]

Thanks @alymurtazamemon , but why Price feed is Returning the value with 8 decimal places?

[alymurtazamemon]

@KarthickSakthi This is a design choice.

[KarthickSakthi]

OK @alymurtazamemon