## Using `getAmountIn` with Foundry Tests In this lesson, we'll run a test using the `getAmountIn` function from the Uniswap v2 library. We'll use Foundry to execute the test, which allows us to call smart contract functions and check the output. First, we'll write a Foundry test that calls the `getAmountIn` function. We'll use the same path as our previous example: WETH, DAI, and MKR. ```javascript function test_getAmountIn() public { address[] memory path = new address[](3); path[0] = WETH; path[1] = DAI; path[2] = MKR; uint256 amountOut = 1e18; uint256[] memory amounts = router.getAmountIn(amountOut, path); console2.log("WETH", amounts[0]); console2.log("DAI", amounts[1]); console2.log("MKR", amounts[2]); } ``` We need to set the `amountOut` and the `path` parameters for this function. Our `amountOut` will be 1 MKR, or 1e18. This function will return an array of `uint` values representing the amount of each token we'll need to get 1 MKR. Next, we'll set up our environment by creating a `.env` file. ```bash FORK_URL=https://eth-mainnet.g.alchemy.com/v2/Kz7pzZcHojHs4Jn_0SgMzM4AJsJQ5Ok4 ``` The `FORK_URL` variable is our connection to the Ethereum mainnet via Alchemy. Now we can run the test using the following command: ```bash forge test --fork-url $FORK_URL --match-path test/uniswap-v2/UniswapV2SwapAmountsTest.sol --vvv ``` The output of the test will show us the amounts of WETH and DAI we'll need to swap to get 1 MKR. We'll be able to use these values to run an example in our UniswapV2 library.