Collecting a list of items from an ArrayList?

There's the recursive approach with . . .

A source object:

class MyObj
{
    public int AnInteger;
}

A generic recursive sort function:

private static void GetValues<T>(List<T> sourceList, int iter, List<T> resultList, Func<T, bool> compareFunc)
{
    if (iter >= sourceList.Count)
    {
        return;
    }
    if (compareFunc(sourceList[iter]))
    {
        resultList.Add(sourceList[iter]);
    }
    GetValues(sourceList, iter + 1, resultList, compareFunc);
}

and a usage case:

List<MyObj> myList = new List<MyObj>();
for (int i = 0; i < 10; i++)
{
    myList.Add(new MyObj() { AnInteger = i });
}
List<MyObj> results = new List<MyObj>();
GetValues(myList, 0, results, obj => obj.AnInteger < 5);
foreach (MyObj obj in results)
{
    Trace.WriteLine("Obj: " + obj.AnInteger);
}

Hopefully this is a homework assignment or some kind of exercise because limiting yourself to no looping and no linq in this case is kind of silly. This recursive example is by far not the best solution performance wise. Also, Lists are preferred over ArrayLists now.

If you want an easy way, you could keep an ArrayList that you know is sorted by grade. Then, you'd only have to make a loop from start up to the end of the grade you're searching for instead of the whole list. The algorithm would still be O(n) since you'd have to loop on every item.

Another solution is to keep an ArrayList for every grade. This way, finding all the student of a grade could be done in O(1) but showing them on the screen is O(n) anyway.

Instead of storing stuff in an ArrayList, you could put them in a Database.. like an Access Database. Then, you could use some simple SQL to rapidly get any information you want. This is definitely the best solution. ArrayList were never created to rapidly filter or sort data.

You could try the BinarySearch[^] method.

Change to a List (as opposed to an ArrayList), and either write the appropriate sort delegates, or use the code from this tip/trick:

A Generic Comparison Class for Collection Items[^]

BTW, this smells like a homework assignment. Only college instructors would impose moronic don't-use-these-features requirements on a project.

I mean, seriously...