input: Write a function to retrieve customer information based on customer ID.
  assert get_customer(1) == {
      'id': 1,
      'name': 'John Doe',
      'email': 'john.doe@example.com',
      'address': '123 Elm Street'
  }
  assert get_customer(99) == {'error': 'customer not found'}, 404
output: def get_customer(customer_id):
      customers = {
          1: {
              'id': 1,
              'name': 'John Doe',
              'email': 'john.doe@example.com',
              'address': '123 Elm Street'
          }
      }
      if customer_id in customers:
          return customers[customer_id]
      else:
          return {'error': 'customer not found'}, 404