Mappings

1. One way hash function.
2. Cannot loop through a mapping
3. On Demand Data Structure
4. Hash key then get results
5. A hash function is any function that can me bused to map data of arbitrary size to data of fixed size

contract MappingDemo {
struct Person {
 uint id;
 bytes32 name;
 bool isPerson;
}
mapping(address => Person) p;

function getPerson(address _a) view public returns(uint, bytes32, bool)
{
 var person = p[_a];
 // take the value of a and hash it using sha3(_a)
 // once it hashes it, it will use that value, as the location of where in memory or value is.
 // memory location 0x0a 
 return (person.id, person.name, person.isPerson);
 
}
function setPerson(address _a, uint _id, bytes32 _name) public{
 var person = p(_a); // this will get the memory location
 
}

}