another "invalid opcode" error

Question

I am stuck on this little problem in my chat app smart contract(this is only the faulty piece of code)

contract ChatApp {
        struct user{
            bool acceptReq;
            uint nreq;
            address[] requests;
            address[] friends;
        }
        mapping(address  => user)  _users;
            
        function sendreq(address friend) public returns(string memory){
            //_users[friend].requests[_users[friend].nreq]=msg.sender;
            _users[friend].requests[0]=msg.sender;
            return "this function has been executed properly";
        }
}

The problem that I'm facing is that when I execute the function sendreq(), the IDE throws this error

VM error: invalid opcode. invalid opcode

I think that the problem is somewhere in the mapping but I can't find it. Can you guys help me?

Answer 1

Your issue is that you are trying to read an array slot that does not yet exist.

You need to push the value to _users[friend].requests instead of setting it as you are doing now.

To resolve your issue, you can change:

_users[friend].requests[0]=msg.sender;

to

_users[friend].requests.push(msg.sender);