Hello, World!

In this smart contract i used the Chainlink Api (https://docs.chain.link/docs/using-chainlink-reference-contracts/) to get the ether price, but how can i convert usd in eth? 

Ethereum doesn't work with decimals, but it has different units: Eth, Gei and Wei, so we need have all these values in same unit. 

The Api return ether price in 8 decimals,  and we need 18 decimals:

uint adjust_price = uint(price) * 1e10;

Usd to wei:

uint usd = _amount * 1e18;

To get the final result in unit wei, i multiply usd * (10 ** 18), then i divide for eth adjust_price:

uint rate = (usd * 1e18) / adjust_price;

Math check using $50 usd:

• Result in ether:

usd = 50 * (10**18) 
eth = 4696022434670000000000 
result= usd / eth 
print(result) 
0.01064730858840405

• Result in wei:

usd = 50 * (10**18)
eth = 4696022434670000000000
result = (usd * (10**18)) / eth
print(result)
1.0647308588404052e+16

You can check  convert Wei->Gei->Eth in this page: https://eth-converter.com/

Full code here down:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.7;

import "@chainlink/contracts/src/v0.8/interfaces/AggregatorV3Interface.sol";

contract PriceConsumerV3 {

    AggregatorV3Interface internal priceFeed;

    constructor() {
        //Rinkeby address
        priceFeed = AggregatorV3Interface(0x8A753747A1Fa494EC906cE90E9f37563A8AF630e);
    }

    function getPriceRate(uint _amount) public view returns (uint) {
        (, int price,,,) = priceFeed.latestRoundData();
        uint adjust_price = uint(price) * 1e10;
        uint usd = _amount * 1e18;
        uint rate = (usd * 1e18) / adjust_price;
        return rate;
    }
}

If you have any question, you can ask me: 

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