Are there null like thing in solidity
<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>