Question

I need fastest way to find the values in array A that their mods is array B when divided by 4:

A = {4,5,6,7,8,9,10}

B={2,3}

result={6,7,10}

mine:

  foreach (int b in B)
       {
        S=  A.Where(n => n % 4 == b).ToArray();

        foreach (int s in S)
        {
            newlist.Add(s); 

        }
       }
   newlist.Distinct().ToArray();

Answer 1

Why not:

var query = A.Where(a => B.Contains(a % 4))
             .ToArray();

If B could be large, you could create a HashSet<int> instead, but if it’s really going to be mod 4, there’s no point.

Alternatively, assuming you know that every element in A and B is positive (and that B doesn’t have any silly values, like 5), you can map B to a bool[4] and use & 3 instead of % 4:

var truth = new bool[4];
foreach (var b in B)
{
    truth[b] = true;
}
var query = A.Where(a => truth[a & 3]).ToArray();