C＃模数方程返回错误值

Question

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

当position为66时，键为7964且Alphabet.length = 91，即使应该为19，newPosition仍为-72，为什么？

Answer 1

请注意，％实际上不是模数，而是余数

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

...

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

输出

19

Full Demo Here

Answer 2

将/用于模块

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

Answer 3

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

有花招