What is the difference between "memory" and "storage" keyword

Question

pragma solidity >=0.5.0 <0.6.0;

contract ZombieFactory {

    uint dnaDigits = 16;
    uint dnaModulus = 10 ** dnaDigits;

    struct Zombie {
        string name;
        uint dna;
    }

    Zombie[] public zombies;

    function createZombie (string memory _name, uint _dna) public {
        // start here
    }

}

Here I am confused because as per this post https://ethereum.stackexchange.com/questions/1701/what-does-the-keyword-memory-do-exactly?newreg=743a8ddb20c449df924652051c14ef26

"the local variables of struct are by-default in storage, but the function arguments are always in memory". So does it mean that in this code when we pass string _name as a function argument, it will be assigned to memory or will it remain in the storage like all other state variables?

Answer 1

All the state variables are stored in storage permanently. It is like hard disk storage.

Memory is like RAM. When a contract finishes its code execution, the memory is cleared.

Sometimes after you declared a state variable, you want to modify it inside a function. For example you defined

 Zombie[] public zombies;

function createZombie (string memory _name, uint _dna) public {
        Zombie storage firstZombie=zombies[0]
        // mutate the name of the firstZombie
        firstZombie.name=_name
        // you have actually mutated state variable zonbies
    }

If you used memory keyword, you would be actually copying the value to the memory:

function createZombie (string memory _name, uint _dna) public {
            // firstZombie is copied to the memory
            // New memory is allocated.
            Zombie memory firstZombie=zombies[0]
            // mutate the name of the firstZombie
            firstZombie.name=_name
            return firstZombie
            // this did not mutate the state variable zombies
            // after returning allocated memory is cleared
        }

In solidity, function parameter variables are stored in memory.