Solidity by Example
version 0.7.6


Maps are created with the syntax mapping(keyType => valueType).

keyType can be value types such as uint, address or bytes.

valueType can be any type including another mapping or an array.

Mappings are not iterable.

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.6;

contract Mapping {
    // Mapping from address to uint
    mapping(address => uint) public myMap;

    function get(address _addr) public view returns (uint) {
         // Mapping always returns a value.
         // If the value was never set, it will return the default value.
        return myMap[_addr];

    function set(address _addr, uint _i) public {
        // Update the value at this address
        myMap[_addr] = _i;

    function remove(address _addr) public {
        // Reset the value to the default value.
        delete myMap[_addr];

contract NestedMapping {
    // Nested mapping (mapping from address to another mapping)
    mapping(address => mapping(uint => bool)) public nested;

    function get(address _addr1, uint _i) public view returns (bool) {
         // You can get values from a nested mapping
         // even when it is not initialized
        return  nested[_addr1][_i];

    function set(address _addr1, uint _i, bool _boo) public {
        nested[_addr1][_i] = _boo;

    function remove(address _addr1, uint _i) public {
        delete nested[_addr1][_i];

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