Building Blockchain Projects

Mappings

A mapping data type is a hash table. Mappings can only live in storage, not in memory. Therefore, they are declared only as state variables. A mapping can be thought of as consisting of key/value pairs. The key is not actually stored; instead, the keccak256 hash of the key is used to look up for the value. Mappings don't have a length. Mappings cannot be assigned to another mapping.

Here is an example of how to create and use a mapping:

contract sample{     mapping (int => string) myMap;     function sample(int key, string value){         myMap[key] = value;         //myMap2 is a reference to myMap         mapping (int => string) myMap2 = myMap;     } } 

Remember that if you try to access an unset key, it gives us all 0 bits.