title: Are there null like thing in solidity
body: <pre><code>    struct buyer{
         uint amount;
         Status status;
      }
  
      mapping(address=&gt;buyer) public buyers;
      mapping(uint=&gt;address) buyerIndex;
      uint public buyerNum;
      //Order a product.
      function(){
        uint doubleValue=value*2;
        uint amount=msg.value/doubleValue; 
        if(buyers[msg.sender]==null){ //Error in this line
        buyer abuyer=buyer({amount:amount,status:Status.Created}); //Error in this line
        buyerNum++;
        buyerIndex[buyerNum]=msg.sender;
        buyers[msg.sender]=abuyer;
      }else{
        buyers[msg.sender].amount+=amount;
      }
        Order(msg.sender,amount*doubleValue,amount);
  
   }
  </code></pre>
  
  <p>If a buyer is not recorded in the buyer mapping, then buyerNum++; 
  but I don't know how to tell whether a buyer is in the mapping</p>