c# - 小数未正确舍入

Question

我有一个名为“sum”的小数，其值为 5824088.999120m，但是当我尝试将其四舍五入到 3 位小数时，我得到的是 5824088.998m 而不是 >5824088.999m。它递减而不是留下 5824088.999m。

这是为什么呢？这是我的代码:

List<decimal> quantityList = QuantityList();
List<decimal> priceList = PriceList();

decimal destination = 5824088.999M;
decimal sum = 0M;
bool lifesaver = false;

for (int x = 0; x < priceList.Count; x++)
{
    sum = 0M;
    for (int i = 0; i < 3500; i++)
    {
        priceList[x] += 0.001M;
        sum = 0M;
        for (int y = 0; y < priceList.Count; y++)
        {
            decimal multipleProduct = priceList[y] * quantityList[y];
            sum = sum + multipleProduct;
            Console.WriteLine(priceList[y] + " " + quantityList[y]);

            sum = Math.Round(sum, 3);
            Console.WriteLine("Sum: " + sum);
            Console.ReadKey();
            Console.WriteLine();
        }

        if (sum == destination)
        {
            Console.WriteLine("The new number is " + priceList[x] + " and it is the {0} element!", x);
            lifesaver = true;
            break;
        }
        else if (sum > destination)
        {
            Console.WriteLine("Limit exceeded!");
        }

        if (i == 3499)
        {
            priceList[x] -= 3.500M;
        }
        if (lifesaver == true)
        {
            break;
        }
    }//Second for loop

    if (lifesaver == true)
    {
        break;
    }
}//Main for loop

列表采用另一种方法。

Answer 1

您似乎有舍入错误累积，因此总数是错误的:

  for (int y = 0; y < priceList.Count; y++) {
    ...
    sum = Math.Round(sum, 3); // <- this accumulates round up errors
    ...
  }

假设 priceList 包含

  priceList = new List<Decimal>() {
    1.0004M, 1.0004M, 1.0004M};

而quantityList全为1； 总和将为

1.000M, 2.000M, 3.000M

而实际总数为

Math.Round(1.0004M + 1.0004M + 1.0004M, 3) 

3.001M。 可能的补救措施是不提前四舍五入:

   for (int y = 0; y < priceList.Count; y++) {
      ...
      //DONE: comment out this: no premature rounding (within the loop)
      // sum = Math.Round(sum, 3);
      //DONE: but format out when printing out
      Console.WriteLine("Sum: {0:F3}", sum);
      ...
   } 

   // round up (if you want) after the loop
   sum = Math.Round(sum, 3);