Solidity Assembly – How to Extend Storage Array Length in Assembly?

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Is it possible to just "extend" an array length directly in Assembly?

Ideally that would fill the new values with equivalent "0" values.

Example:

  address[] internal owners;
  
  function mint(to, amount) {
    owners.push(to);

    assembly {
      // owners.length += amount - 1... :-S
    }
  }

I am trying to avoid/optimize this:

  function mint(to, amount) {
    owners.push(to);

    for (uint256 i; i < amount - 1; i++) {
      owners.push(address(0));
    }
  }

Best Answer

Is it possible to just "extend" an array length directly in Assembly?

Yes, you need to increment the length of the array which is located at the array's storage slot:

  function mint(to, amount) {
    owners.push(to);

    assembly {
     sstore(owners.slot, add(sload(owners.slot), sub(amount, 1)))
    }
  }

As per the documentation: https://docs.soliditylang.org/en/develop/internals/layout_in_storage.html#mappings-and-dynamic-arrays

Assume the storage location of the mapping or array ends up being a slot p after applying the storage layout rules. For dynamic arrays, this slot stores the number of elements in the array (byte arrays and strings are an exception, see below).

The value of the items in the array are then by default already set to 0x0000...000 etc.

Related Solutions

[Ethereum] Can’t get length of array? (not querying for property)

In the code above userList is not an array, it is a mapping. Mapping's do not have a length property because we can't know what the size of a mapping is.

If you convert mapping(uint256 => address) public userList; to address[] public userList; then it should work as expected.

Also, regarding your last point. It's fine for you to use userList.length to determine a user's ID, even if two users are added in the same block/at the same time, as the transactions are processed sequentially, so they will still get different ID's.

Solidity: How to Access the Length of a Public Array?

Solidity creates getter functions for public state variables automatically. For example, the following will work:

contract A {
  uint256 public foo = 1;
}

contract B {
  A a;

  function getFoo() public view returns (uint256) {
    return a.foo();
  }
}

In the case of arrays however, you can only get a single element from the array, rather than the entire array. From the Solidity documentation:

If you have a public state variable of array type, then you can only retrieve single elements of the array via the generated getter function. This mechanism exists to avoid high gas costs when returning an entire array. You can use arguments to specify which individual element to return, for example data(0). If you want to return an entire array in one call, then you need to write a function.

Basically, while you can do the following:

contract A {
  uint256[] public foo;
}

contract B {
  A a;

  function getFoo() public view returns (uint256) {
    return a.foo(0); // Get item at index 0 from the array
  }
}

You cannot use a.foo() to read the entire array, and you will have to use a getter function like this for example:

contract A {
  uint256[] public foo;

  function getFooLength() public view returns (uint256) {
    return foo.length;
  }
}

contract B {
  A a;

  function getFoo() public view returns (uint256) {
    return a.getFooLength();
  }
}

Best Answer

Related Solutions

[Ethereum] Can’t get length of array? (not querying for property)

Solidity: How to Access the Length of a Public Array?

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