Question

Hey guys – I’m stuck with this problem and im just wondering what the best way to handle it is.

Foreach (var customer in CustomerList)
{
      Foreach (var appointment in AppointmentList)
      {
        if(appointment.customerId == customer.id)
           customer.appointments.add(appointment)


       }

}

this is the simplest way I can think to do it but im not sure if it’s the most efficient!

Any help would be great-

Thanks.

Answer 1

You could perhaps pre-group the shorter list; this should give you better performance – I can’t find citations to the exact big-O ratings since MSDN doesn’t cite them, but it could be O(n + m) instead of O(n * m).

var apptsByCustomer = AppointmentList.ToLookup(appt => appt.customerId);

then you can use:

foreach (var customer in CustomerList) {
    foreach(var appointment in apptsByCustomer[customer.id]) {
        customer.appointments.add(appointment);
    }
}

---

Or without LINQ (from comments):

// this bit is **broadly** comparable to ToLookup...
Dictionary<int, List<Appointment>> apptsByCustomer =
     new Dictionary<int, List<Appointment>>();
List<Appointment> byCust;
foreach(Appointment appt in AppointmentList) {            
    if (!apptsByCustomer.TryGetValue(appt.customerId, out byCust)) {
        byCust = new List<Appointment>();
        apptsByCustomer.Add(appt.customerId, byCust);
    }
    byCust.Add(appt);
}

foreach (Customer cust in CustomerList) {
    if (apptsByCustomer.TryGetValue(cust.id, out byCust)) {
        foreach (Appointment appt in byCust) cust.appointments.Add(appt);
    }
}