Question

class order {
    Guid employeeId;
    DateTime time;
}

I need to filter a list of orders into 4 lists based on the time range. 0-9AM to 1st list, 9AM-2PM to 2nd, 2-6PM to 3rd and 6-12PM to a 4th list.

I am curious if this can be achieved using lambda expressions in a efficient way? otherwise what would be the best way to split the list?

Answer 1

This should work:

var orders = list.OrderBy(o => o.time);
var first  = orders.TakeWhile(o => o.time.TimeOfDay.TotalHours <= 9);
var second = orders.SkipWhile(o => o.time.TimeOfDay.TotalHours <= 9)
                   .TakeWhile(o => o.time.TimeOfDay.TotalHours <= 14);
var third  = orders.SkipWhile(o => o.time.TimeOfDay.TotalHours <= 14)
                   .TakeWhile(o => o.time.TimeOfDay.TotalHours <= 18);
var fourth = orders.SkipWhile(o => o.time.TimeOfDay.TotalHours <= 18);

Here’s another, maybe more efficient, more flexible and concise approach which uses Enumerable.GroupBy:

var groups = list.Select(o => new
{
    Order = o,
    DayPart = o.time.TimeOfDay.TotalHours <= 9 ? 1
       : o.time.TimeOfDay.TotalHours > 9  && o.time.TimeOfDay.TotalHours <= 14 ? 2
       : o.time.TimeOfDay.TotalHours > 14 && o.time.TimeOfDay.TotalHours <= 18 ? 3 : 4
})
.GroupBy(x => x.DayPart)
.OrderBy(g => g.Key);

var first = groups.ElementAt(0);  
var second = groups.ElementAt(1);
// ...