Mappings in Solidity

The key-value store; missing keys read as zero.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.24;

// A mapping is a key-value store; missing keys read as zero.
contract ShareLedger {
    mapping(address => uint256) public shares;
    mapping(address => mapping(uint256 => bool)) public votedOn;

    function issue(address member, uint256 amount) external {
        shares[member] += amount;
    }

    function castVote(uint256 proposal) external {
        votedOn[msg.sender][proposal] = true;
    }

    function hasShares(address member) external view returns (bool) {
        return shares[member] > 0;
    }
}

How it works

1. mapping(address => uint256) has no length and no iteration.
2. Mappings nest: address to proposal to bool.
3. Reading an absent key returns the type's zero, never reverts.

Keywords and builtins used here

• address
• bool
• contract
• external
• function
• mapping
• public
• return
• returns
• true
• uint256

The run, in numbers

Lines

20

Characters to type

519

Tokens

97

Three-star pace

55 tpm

At the three-star pace of 55 tokens a minute, this run takes about 106 seconds.

Type this snippet

Step 1 of 3 in Storage, step 5 of 28 in Language basics.

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