C# modulus equation return wrong value

Question

var newPosition = (position - key) % alphabet.Length;

当position为66时，键为7964，并且Alphabet.length = 91时，newPosition以某种方式为-72，即使应该为19。为什么？

Answer 1

Be aware % is actually not a modulus its a remainder

static decimal modulus(decimal a, decimal b)
{
  return a - b * Math.Floor(a / b);
}

...

Console.WriteLine( nfmod(66-7964 , 91));

Ouput

19

Full Demo Here

Answer 2

使用/用于模块

var newPosition = (position - key) / alphabet.Length;

Answer 3

((position - key) % alphabet.Length + alphabet.Length) % alphabet.Length

绝招