Trying to check if mapping exists, TypeError: Type uint256 is not implicitly convertible to expected type bool

Question

I am checking if a struct exists in vehicleList and returning true if it exists and false if it does not.

However, the function produces an error and I can't understand why it does not work.

TypeError: Type uint256 is not implicitly convertible to expected type bool. if (bytes(vehicleList[_registrationNo].vehicleRegistrationNo).length =! 0) {

function checkVehicleExists (string memory _registrationNo) public view returns (bool) {
        bool vehicleExists = false;
        if (bytes(vehicleList[_registrationNo].vehicleRegistrationNo).length =! 0) {
            vehicleExists = true;
        }
        return (vehicleExists);
    }

Answer 1

The ! should come before the =, so:

if (bytes(vehicleList[_registrationNo].vehicleRegistrationNo).length != 0) {
  // ...
}

length =! val basically means you're assigning length to !val (not val). If you do that in an if-statement (if (length = !val)), it's the equivalent of writing this:

uint256 length = !val;
if (length) {
  // ...
}

Both the assignment and if-statement won't work here because you cannot convert from uint256 to bool.