-------Getting this issue:Please guide:::I have started with some basics----------
"System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, countcontatcs: execution of BeforeInsert

caused by: System.DmlException: Update failed. First exception on row 0; first error: MISSING_ARGUMENT, Id not specified in an update call: []

Trigger.countcontatcs: line 10, column 1: []"


--------------Trigger:-----------------------------------
trigger countcontatcs on Contact (before insert) {
    
    for (contact c : trigger.new)
    { integer i =0;
        account a = new account();
        if (c.accountid == a.id)
        { i++;
        a.contactcounter__c=i;
        }
        update a;
    }

}


--------------test clsss----------------------
@isTest
public class countcontactstest {
    static testmethod  void validatecountcontacts()
    { list<account> acclist = new list<account>();
     list<contact> conlist= new list<contact>();
     
        for (integer i = 0 ; i<2; i++)
        {
            account a = new account (name ='test' + i, First_Name__c= 'testfirstname'+i, Last_Name__c='testlastname'+i);
            
            for (integer j = 0 ; j<1 ; j++)
            {
                contact c = new contact();
                c.LastName= 'lastcontactname'+i;
                c.AccountId = a.id;
            conlist.add(c); 
            }
        acclist.add(a);
        }
     test.startTest();
     insert acclist;
     insert conlist;
        test.stoptest();
     
    }

}