## Mappings to track funders In this lesson, we'll learn how to keep track of who sent funds to our contract, and how much they sent. We'll use a mapping. Mappings are a fundamental concept in smart contracts. Think of them as dictionaries in Python or JavaScript. They pair a key with a value. We can make mappings public to allow anyone to read them. We'll create a new mapping to keep track of funders and how much they sent. ```python # # Keep track of who sent us # # How much they sent us ``` We'll call this new mapping `_funders_to_amount_funded`. ```python # funder -> how much they funded _funders_to_amount_funded: public(HashMap[address, uint256]) ``` This mapping will store the address of the funder as a key and the amount they sent as a value. In our `fund` function, we'll track the amount each person sent us. ```python @payable def fund(): """ Allows users to send $ to this contract Have a minimum $ amount to send """ # ... previous code self._funders_to_amount_funded[msg.sender] += msg.value # ... ``` Here, we access the hash map with the `msg.sender` address as the key and add the amount sent by the `msg.sender` (`msg.value`) to the value associated with the `msg.sender` in the hash map. We can also use the `+=` operator as a shorthand for adding to a value in a mapping. ```python self._funders_to_amount_funded[msg.sender] = self._funders_to_amount_funded[msg.sender] + msg.value ``` We can also use the `+=` operator as a shorthand for adding to a value in a mapping. ```python self._funders_to_amount_funded[msg.sender] += msg.value ``` This is the same as writing the line above.