C# - (int)Math.Round((double)(3514 + 3515)/2) =351

Question

Helo everyone.

int[] ai1=new int[2] { 3514,3515 };

    void average1()
    {
        List<int> aveList = new List<int> { ai1[0],ai1[1]};
        double AveragLI = aveList.Average();
        int AverLI = (int)Math.Round((double)AveragLI);
        label1.Text = AverLI.ToString();
    }

Returns 3514; should not be 3515?

Answer 1

The default rounding scheme for Math.Round is what's known as banker's rounding (which is the standard in financial and statistical areas), where midpoint values are rounded to the nearest even number. It looks like you were expecting midpoint values to be rounded away from zero (which is the kind you were probably taught in grade school: if it ends in 5, round up).

If you were just concerned that it wasn't working in an acceptable way, don't worry. If you'd like it to be rounded away from zero, you can do this:

int AverLI = (int)Math.Round((double)AveragLI, MidpointRounding.AwayFromZero);

Answer 2

Math.Round is the culprit

int AverLI = (int)Math.Round((double)AveragLI);

Its what we call Banker's Rounding or even rounding.

Info on Math.Round says

The integer nearest a. If the fractional component of a is halfway between two integers, one of which is even and the other odd, then the even number is returned.

3514.5 is rounded to 3514 and 3515.5 will also be rounded to 3514.

Read this

To avoid do this

int AverLI = (int)Math.Ceiling((double)AveragLI);