TypeError: Data location must be "storage", "memory" or "calldata" for variable, but none was given!

Question

I am very new in Solidity programming. I was practicing a code by myself and faced this problem. Everything seems okay from the example except when i try to compile, it is giving me this error in solidity "TypeError: Data location must be "storage", "memory" or "calldata" for variable, but none was given". can u guys please help me understand and also tell me how can i write this part more accurately? Thank you very much for taking time! the code is below!

    function issueCertificate(
    address _issuer, address _prover, string memory _status,
    uint meatBatchId, bytes memory signature) public returns (uint) {
        Entity memory issuer = entities[_issuer];
        require (issuer.mode == Mode.ISSUER);

        Entity memory prover = entities[_prover];
        require (prover.mode == Mode.PROVER);

        Status status = unmarshalStatus(_status);

        uint[] id = certificates.length;
        Certificate memory certificate = Certificate(id, issuer, prover, signature, status);

        certificateIds.push(certificateIds.length);
        certificates[certificateIds.length-1] = certificate;

        emit IssueCertificate(_issuer, _prover, certificateIds.length-1);

        return certificateIds.length-1;
    }

Answer 1

In your code you have at least two declarations without data location while it's required. Reference types require the data location notation, according to solidity document: "reference types comprise structs, arrays and mappings":

1. Status status = unmarshalStatus(_status); should become: Status memory status = unmarshalStatus(_status);

2. uint[] id = certificates.length; should become: uint[] memory id = certificates.length;

For the second one, I don't think you mean to use array because the length could be integer. So you may use uint id = certificates.length; depends on the logic of your contract.

Read more : Data location