Question

I’m looking to try to make an efficient delete method to work on a list. This situation is as follows:

Say for e.g. I have a (potentially) massive list of names:

Alex Smith
Anna Hobb
Bertie Blackman
Bill Clinton
David Smith
David Warner
George Jung
George Washington
William Wobbits

Lets say that this is a List<Person> with Person having properties FirstName and LastName. As in the example, there is the possibility for two people sharing same FirstName. What I need to do is go through the list and delete all Davids, for example.

I was looping through finding all davids, adding to a list DeletePerson, then looping through again for each DeletePerson and removing. I’m sure there would be a more efficient method? Efficiency isn’t critical in this app but it just seems a long winded way to do it, also I guess as after the letter D in the first name we know we won’t be adding any more to the DeletePerson list (assuming the list is sorted alphabetically)

Thanks!

Answer 1

For a concise approach, use RemoveAll.

list.RemoveAll(person => person.FirstName == "David")

for an (possibly? I haven’t run any benchmarks) efficient approach based on the order in a list, use List.RemoveRange — but you’ll have to find the indexes.

class NameComparer : IComparer<Name>
{
    public int Compare(Name x, Name y) { 
        return x.First.CompareTo(y.First); 
    }
}
...
...
var comparer = new NameComparer();
var david = new Name { First = "David" };
int guess = list.BinarySearch(david, comparer);
int start, end;
start = list.FindLastIndex(guess, person => person.First != "David") + 1;
if (start > 0 && list[start].First == "David") {
    end = list.FindIndex(guess, person => person.First != "David");
    list.RemoveRange(start, end - start);
}