gistfile1.txt

pragma solidity ^0.5.3;

contract Mapping {
    
    mapping(address => uint) balances;
    
    function manipulateMap() external {
        balances[msg.sender] = 100;             //assign a value; 
        
        balances[msg.sender]                    //read a value;
        
        balances[unknown_key]                   //will return the default value of the type, i.e 0 for uint
    }
    
}