# Change quote feed

> Change block-latency quote feeds using Solidity.

> For the complete documentation index, see [llms.txt](/llms.txt). Markdown versions of documentation pages are available by appending `.md` to the page URL.

Source: https://dev.flare.network/ftso/guides/change-quote-feed

This guide will show you how to fetch the latest feed values for two feeds and convert them to a new quote feed while retaining the number of decimals. For example, if you need the price of `BTC/ETH`, you can fetch the latest feed values for `BTC/USD` and `ETH/USD` and calculate the price of `BTC/ETH = (BTC/USD) / (ETH/USD)`.

FtsoV2ChangeQuoteFeed.sol

```
// SPDX-License-Identifier: MITpragma solidity >=0.8.0 <0.9.0;import {ContractRegistry} from "@flarenetwork/flare-periphery-contracts/coston2/ContractRegistry.sol";/* THIS IS A TEST IMPORT, in production use: import {FtsoV2Interface} from "@flarenetwork/flare-periphery-contracts/coston2/FtsoV2Interface.sol"; */import {TestFtsoV2Interface} from "@flarenetwork/flare-periphery-contracts/coston2/TestFtsoV2Interface.sol";/** * THIS IS AN EXAMPLE CONTRACT. * DO NOT USE THIS CODE IN PRODUCTION. */contract FtsoV2ChangeQuoteFeed {    /**     * @dev Internal function to scale the base feed value to match the decimals of the final feed.     */    function _scaleBaseFeedValue(        uint256 _baseFeedValue,        uint8 _baseFeedDecimals,        uint8 _finalDecimals    ) internal pure returns (uint256) {        if (_baseFeedDecimals < _finalDecimals) {            // Scale up if base feed decimals are less than final feed decimals            return                _baseFeedValue *                10 ** uint256(_finalDecimals - _baseFeedDecimals);        } else if (_baseFeedDecimals > _finalDecimals) {            // Scale down if base feed decimals are more than final feed decimals            return                _baseFeedValue /                10 ** uint256(_baseFeedDecimals - _finalDecimals);        } else {            // No scaling needed if decimals are equal            return _baseFeedValue;        }    }    /**     * @dev Function to compute the new quote feed value based on the base and quote feed values.     * @param _baseAndQuoteFeedIds Array containing the IDs of the base and quote feeds.     * @return The computed new quote feed value in the original quote decimals.     */    function getNewQuoteFeedValue(        bytes21[] calldata _baseAndQuoteFeedIds    ) external view returns (uint256) {        require(            _baseAndQuoteFeedIds.length == 2,            "Invalid feed indexes. Please provide exactly two indexes."        );        /* THIS IS A TEST METHOD, in production use: ftsoV2 = ContractRegistry.getFtsoV2(); */        TestFtsoV2Interface ftsoV2 = ContractRegistry.getTestFtsoV2();        // Fetch current feeds        (uint256[] memory feedValues, int8[] memory decimals, ) = ftsoV2            .getFeedsById(_baseAndQuoteFeedIds);        // The base feed gets scaled by:        // - comparison with the quote feed        // - being returned in the quote feed decimals        uint8 finalDecimals = 2 * uint8(decimals[1]);        // Scale the base feed value to match the final feed decimals        // Scaling is done all at once for smaller calculation errors        uint256 scaledBaseFeedValue = _scaleBaseFeedValue(            feedValues[0],            uint8(decimals[0]),            finalDecimals        );        // Prevent division by zero        require(feedValues[1] != 0, "Division by zero");        // Compute the new quote feed value        uint256 newQuoteFeedValue = scaledBaseFeedValue / feedValues[1];        return newQuoteFeedValue;    }    /**     * @dev Function to compute the new quote feed value based on the base and quote feed values.     * @param _baseAndQuoteFeedIds Array containing the IDs of the base and quote feeds.     * @param _newQuoteDecimals Number of decimals for the returned value.     * @return The computed new quote feed value in the parameter decimals.     */    function getNewQuoteFeedValueSetDecimals(        bytes21[] calldata _baseAndQuoteFeedIds,        uint8 _newQuoteDecimals    ) public view returns (uint256) {        require(            _baseAndQuoteFeedIds.length == 2,            "Invalid feed indexes. Please provide exactly two indexes."        );        /* THIS IS A TEST METHOD, in production use: ftsoV2 = ContractRegistry.getFtsoV2(); */        TestFtsoV2Interface ftsoV2 = ContractRegistry.getTestFtsoV2();        // Fetch current feeds        (uint256[] memory feedValues, int8[] memory decimals, ) = ftsoV2            .getFeedsById(_baseAndQuoteFeedIds);        // The base feed gets scaled by:        // - comparison with the quote feed        // - being returned in the "_newQuoteDecimals" decimals        uint8 finalDecimals = uint8(decimals[1]) + _newQuoteDecimals;        // Scale the base feed value to match the final feed decimals        // Scaling is done all at once for smaller calculation errors        uint256 scaledBaseFeedValue = _scaleBaseFeedValue(            feedValues[0],            uint8(decimals[0]),            finalDecimals        );        // Prevent division by zero        require(feedValues[1] != 0, "Division by zero");        // Compute the new quote feed value        uint256 newQuoteFeedValue = scaledBaseFeedValue / feedValues[1];        return newQuoteFeedValue;    }    /**     * @dev Function to compute the new quote feed value based on the base and quote feed values.     * @param _baseAndQuoteFeedIds Array containing the IDs of the base and quote feeds.     * @return The computed new quote feed value in wei (with 18 decimals).     */    function getNewQuoteFeedValueInWei(        bytes21[] calldata _baseAndQuoteFeedIds    ) external view returns (uint256) {        uint256 newQuoteFeedValue = getNewQuoteFeedValueSetDecimals(            _baseAndQuoteFeedIds,            18        );        return newQuoteFeedValue;    }}
```

[Open in Remix](https://remix.ethereum.org/#url=https://github.com/flare-foundation/developer-hub/blob/main/examples/developer-hub-solidity/FtsoV2ChangeQuoteFeed.sol&version=builtin&evmVersion=cancun&optimize=true&runs=200)

  

Don't use test interfaces in production

The `TestFtsoV2Interface` is for **testing only**, with all methods as `view` to allow rapid development without gas costs or state changes. For production, use [`FtsoV2Interface`](/ftso/solidity-reference/FtsoV2Interface), which includes `payable` methods required for real transactions and state modifications. Make the following changes to `FtsoV2ChangeQuoteFeed.sol`:

```
import {ContractRegistry} from "@flarenetwork/flare-periphery-contracts/coston2/ContractRegistry.sol";import {FtsoV2Interface} from "@flarenetwork/flare-periphery-contracts/coston2/FtsoV2Interface.sol";contract FtsoV2ChangeQuoteFeed {  FtsoV2Interface internal ftsoV2;  constructor() {    ftsoV2 = ContractRegistry.getFtsoV2();  }  //...}
```

Set EVM Version to Cancun

-   **Using Remix:** Set EVM version to `cancun` in the **Advanced Configurations** section of the **Solidity Compiler** tab:
    
    ![](/assets/images/set-evm-version-remix-69f5d8d1405ae172504ea23012d0bf09.png)
-   **Using Hardhat or Foundry:** Set EVM version to `cancun` in [hardhat.config.ts](https://github.com/flare-foundation/flare-hardhat-starter/blob/master/hardhat.config.ts#L34) or [foundry.toml](https://github.com/flare-foundation/flare-foundry-starter/blob/master/foundry.toml).
    
-   **Using Standard Solidity JSON:** Set `evmVersion` to `cancun`:
    
    ```
    {  "settings": {    "optimizer": {      /* ... */    },    "evmVersion": "cancun"  }}
    ```
    
-   **Using `solc` CLI:** Set `--evm-version` to `cancun`:
    
    ```
    solc --evm-version cancun <args>
    ```