Obtain the index of the maximum element

Question

Given such a list:

        List intList = new List();
        intList.Add(5);
        intList.Add(10);
        intList.Add(15);
                


        

Answer 1

Here's how to do it in one (long) line using LINQ, with just a single pass through the collection. It should work for any IEnumerable, not just lists.

int maxIndex = intList
    .Select((x, i) => new { Value = x, Index = i })
    .Aggregate
        (
            new { Value = int.MinValue, Index = -1 },
            (a, x) => (a.Index < 0) || (x.Value > a.Value) ? x : a,
            a => a.Index
        );

Here's the non-LINQ equivalent of the above, using a foreach loop. (Again, just a single pass through the collection, and should work for any IEnumerable.)

int maxIndex = -1, maxValue = int.MinValue, i = 0;
foreach (int v in intList)
{
    if ((maxIndex < 0) || (v > maxValue))
    {
        maxValue = v;
        maxIndex = i;
    }
    i++;
}

If you know that the collection is an IList then a plain for loop is probably the easiest solution:

int maxIndex = -1, maxValue = int.MinValue;
for (int i = 0; i < intList.Count; i++)
{
    if ((maxIndex < 0) || (intList[i] > maxValue))
    {
        maxValue = intList[i];
        maxIndex = i;
    }
}