Maps in any form are usually are core feature of any language or its standard library. No matter whether they are called HashMap (backed by a hash table) or TreeMap (backed by a tree), they are an essential part of many algorithms. They are the goto data structure when you want to map a key to a value.

Solidity also has a map type, but it's not a complex type like in many other languages, it's actually a primitive and is called mapping.

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The Mapping Type

Solidity's mapping type is straightforward to declare and use:

mapping(keyType => valueType) variableIdentifier;

You don't even need to assign anything. The default value of a variable declared as mapping is already a fully initialized map.

If you want to add a value to your map, you can use the same syntax you might already be comfortable with from handling JavaScript objects:

myMapping[key] = value;

Yep, that's it. It's that simple.

And reading from it is equally as straightforward:

valueType value = myMapping[key];

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A Mapping Use Case

You might ask yourself what you can actually use Solidity's map for. Well, whenever you want to store the relation between a key and a value.

ERC20 tokens, for example, store the balance of their coin holders in mappings, like in the example below:

// SPDX-License-Identifier: MIT

pragma solidity >=0.7.0 <0.9.0;

contract MyToken {
    mapping(address => uint) private _balances;

    function balance() public view returns (uint) {
        return _balances[msg.sender];
    }

    function addFunds(address to, uint amount) public {
        _balances[to] += amount;
    }
}

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