How to update an array of struct?

Question

this is my code:

struct Entity{
    address _address;
    uint data;
}

   Entity[] entityArray;

function addEntityArray(uint _data)  public returns(uint){
    entityArray.push(Entity((msg.sender), _data));
    return entityArray[(entityArray.length) - 1].data;
}

to update a struct in this array at any point in time, i will need to keep track of the address and the index of it using MAPPING right? or i have to loop everytime which is costly in gas... how to tackle this issue in the best way possible?

Answer 1

Yes, the easiest way to update a single instance of the array is for every address that invokes addEntityArray() to store the index of the struct.

The mapping would look like this:

mapping(address => uint) public addressesEntityIndex;

Note: it is better to store the index to the struct, not a whole struct because it is cheaper.

The updated function would look like this:

function addEntityArray(uint _data)  public returns(uint){
    Entity storage newEntity = Entity(msg.sender, _data);
    uint entityIndex = entityArray.length;
    entityArray.push(newEntity);
    addressesEntityIndex[msg.sender] = entityIndex;
    return entityArray[entityIndex].data;
}

Note: I slightly modified your function's design, it will cost a bit more gas but it is more human-readable.

And finally, you define a function that will update:

function updateEntityArray(uint _data) public {
    uint addressEntityIndex = addressesEntityIndex[msg.sender];
    entityArray[addressEntityIndex].data = _data
}