Accessing mapped values to save gas in Solidity

Originally posted on CryptoGuide.dev

Instead of accessing the same mapped value multiple times, use a local variable for reduced gas

Every time you access a mapped value, there is a fresh lookup of where that value is stored. This involves generating keccak256 hashes and accessing the storage.

It can save gas if you create a variable for the object you're accessing, for example:

pragma solidity >=0.8.4;

contract GasOptimizer {
    struct Person {
      uint age;
      uint highScore;
      uint numVisits;
    }
    mapping (address => Person) people;

    // 60449 gas
    function badExample() public {
      people[msg.sender].age = 20;
      people[msg.sender].highScore = 30;
      people[msg.sender].numVisits = 40;
    }

    // 60306 gas - slightly less, and it is doing the same thing
    function goodExample() public {
      Person storage person = people[msg.sender];
      person.age = 20;
      person.highScore = 30;
      person.numVisits = 40;
    }
}

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