Select from array / struct by not using the index

Question

Here is the code on how to grab from and array:

struct Collection {
    uint collectionId;
    uint collectionType;
    address collectionOwnerAddress;
    uint value;
}

Collection[] public collections;

function getCollection (uint _collectionId) view public returns (uint) {
     return collections[_collectionId].value;
}

My question is: I'm curious if I can grab the collection by the collection owners address. Something like this? Can I select by both msg.sender and collectionType?

function getCollection (uint _collectionType) view public returns (uint) {
     // select by _collectionType and msg.sender here
     return collections[msg.sender].value;
}

I'm trying to incrementally increase the value. So during addCollection if the array doesn't exist with that member then it creates a new array.push row. But, if the user already has an index in the array it collections[_collectionId].value++ instead of adding another array index row for that user. This above question relates because I cant loop through the whole collections array to see if that member already started a collection.

Answer 1

What you want to do is basically the functionnality proposed by a mapping. Why not use a mapping then?

struct Collection {
    uint collectionId;
    uint value;
}
mapping (address => Collection) collections;

Then you can check in O(1) if an address already has a collection.