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Mapping in Solidity

mapping( _KeyType => _ValueType ) mapName

  • Described as a hashtable (in Java) | Associative array in Javascript
  • _KeyType can be almost any type except for a mapping
  • _ValueType can be any type, including mapping
  • Returns 0, empty string for non existent keys
  • keccak256(Key data) hash is stored
  • Allowed only as a storage or state variable
  • CANNOT be used as a local variable in a function
  • DOES NOT provide length function/attribute & a way to iterate

Enums in Solidity

Enums are one way to create a user-defined type in Solidity. They are explicitly convertible to and from all integer types but implicit conversion is not allowed. The explicit conversions check the value ranges at runtime and a failure causes an exception. Enums needs at least one member.

Struct in Solidity

Solidity provides a way to define new types in the form of structs. Struct types can be used inside mappings and arrays and they can itself contain mappings and arrays. Struct is kind of like an object. It has an object name and a bunch of properties inside.

  • Struct are not part of the public interface
  • External functions can not have struct type arguments
  • Struct type cannot contain member of its own type

Examples of Mapping and Struct

From https://www.youtube.com/watch?v=gfXewa4xmYE contract Courses {

struct Instructor { uint age; string fName; string lName; }

mapping (address => Instructor) instructors; address[] public instructorAccts;

function setInstructor(address _address, uint _age, string _fName, string _lName) public { var instructor = instructors[_address];

instructor.age = \_age;
instructor.fName = \_fName;
instructor.lName = \_lName;

instructorAccts.push(\_address) -1;

}

function getInstructors() view public returns(address[]){ return instructorAccts; }

function getInstructor(address _address) view public returns (uint, string, string){ return (instructors[_address].age, instructors[_address].fName, instructors[_address].lName); }

function countInstructors() view public returns (uint) { return instructorAccts.length; } }