Illegal assignment from Account to String

Question

I wrote a trigger which attempts to grab the LastModifiedById and populate it to another field. When I use trigger.NewMap to get the LastModifiedById, it throws this error:

Error: Compile Error: Illegal assignment from Account to String

This is the line throwing the error:

String userIdString = trigger.newmap.get(acc.LastModifiedById);

Here's the complete trigger:

trigger Test on Account (after insert) 
{
    system.debug('test::'+trigger.new);
    for (account acc:trigger.new)
    {

        account dd = new account(id=acc.id);
        String userIdString = trigger.newmap.get(acc.LastModifiedById);

        dd.Last_Modified_By_End_User__c =userIdString.substring(0, 15);

        update dd;
    }
}

Why am I getting this error? How can I fix it?

Answer 1

Change:

String userIdString = trigger.newmap.get(acc.LastModifiedById);

to:

String userIdString = trigger.newmap.get(acc.Id).LastModifiedById;

The description of error is quite obvious.