pragma solidity ^0.4.20;
contract Error{
    uint a = 30;
    function deposit() public payable{}
    function kill(address addr) public{
        selfdestruct(addr);//销毁当前合约，并把它所有资金发送到给定的地址。
    }

    function testRequire() view public returns (uint){
        require(a == 30);//用于判断输入或外部组件错误，条件不满足时抛出异常
        return a;
    }
    function testAssert() view public returns (uint){
        assert(a == 30);//用于判断内部错误，条件不满足时抛出异常
        // uint a = addmod(10,21,3);
        return a;
    }
    function testRevert() pure public returns (uint){
        revert();//终止执行并还原改变的状态
    }
}