Mapping Types

Last Updated on 12. June 2023 by Mario Oettler

Mapping types assign a value to a key. The key can be any Solidity built-in value type. The value can be any type.

The syntax of a mapping declaration looks like that:

mapping(key_type => value_type) mapping_name;

assigning a value to a key is done like that:

mapping_name[key] = value;

Retrieving a value from a mapping is done like that:

retrieved_value = mapping_name[key];

Mappings always have the data location storage. They cannot be used as function parameters or return parameters for functions that are publicly visible.

If you want to retrieve a value from a mapping, you have to provide the key.

A simple representation of a mapping would be like a table, where each row has a unique key column and a value column.

In reality, this is a bit more complicated.

Limitations of Mappings

In a mapping, the key is represented as the hash of the key. If a mapping is initialized, all possible keys exist, but their associated values are zero. The consequence is that mappings do not have a length, and it is impossible to iterate through them. It is also not possible to retrieve the keys since every key already exists.

If you want to iterate over a mapping (more precisely, values that are non-zero), you need to implement a second data structure on top of it that stores the keys. An array can serve this purpose.

You can find an example here: https://docs.soliditylang.org/en/v0.8.7/types.html#iterable-mappings

Code Example

pragma solidity 0.8.20;


contract mappingTest{
    
    mapping(uint256 => string) public names;
    
    function fillMapping(uint256 _key, string memory _name) public{
        names[_key] = _name;
    }
}

If you make a mapping public, the compiler automatically adds a getter-function. It requires the key as an input parameter. Try some “existing” keys and some non-existing keys.