C#数组排序程序中FindIndex返回-1引发越界异常的排查

问题描述 #

尝试用C#编写数组数字降序排序程序，代码如下：

//basic number sorter. every number should be on one line and separated by a " " (space).
int[] numbers = Console.ReadLine().Split(" ").Select(int.Parse).ToArray();
int[] bigToSmall = new int[numbers.Length];
int biggestNum = int.MinValue;
for (int y = 0; y < numbers.Length; y++)
{
    for (int i = 0; i < numbers.Length; i++)
    {
        if (numbers[i] > biggestNum)
        {
            biggestNum = numbers[i];
        }
        else if (numbers[i] == biggestNum)
        {
            numbers[i] = biggestNum;
        }
    }
    var bigIndx = Array.FindIndex(numbers, row => row == biggestNum);
    numbers[bigIndx] = 0;
    bigToSmall[y] = biggestNum;
}
for (int i = 0; i < bigToSmall.Length; i++)
{
    Console.WriteLine(bigToSmall[i]);
}

输入1 2 3时触发System.IndexOutOfRangeException: Index was outside the bounds of the array.异常，使用IndexOf也无法解决。

错误原因 #

1. 索引越界核心诱因：每次找到最大值后，你将对应位置的元素设为0。当循环到最后一轮时，原数组numbers中的所有元素都被替换成了0，但此时biggestNum仍保留上一轮的最大值（比如输入1 2 3时，最后一轮biggestNum是3）。Array.FindIndex在数组中找不到等于3的元素，会返回-1，随后访问numbers[-1]直接触发索引越界异常。
2. 冗余代码无意义：else if (numbers[i] == biggestNum)分支里的numbers[i] = biggestNum完全多余，只是给元素赋值自身，对逻辑没有任何帮助。

修复方案 #

方案一：修复原手动排序逻辑 #

把“找到最大值后设为0”改为“设为int.MinValue”，这样下一轮循环时，这个位置的元素不会被误判为最大值，且最后一轮循环时，biggestNum会被更新为int.MinValue，此时能在数组中找到对应的索引，避免越界。同时移除冗余的else if分支：

//basic number sorter. every number should be on one line and separated by a " ".
int[] numbers = Console.ReadLine().Split(" ").Select(int.Parse).ToArray();
int[] bigToSmall = new int[numbers.Length];

for (int y = 0; y < numbers.Length; y++)
{
    int biggestNum = int.MinValue;
    // 每轮循环重新初始化最大值，避免残留上一轮的值
    for (int i = 0; i < numbers.Length; i++)
    {
        if (numbers[i] > biggestNum)
        {
            biggestNum = numbers[i];
        }
    }
    var bigIndx = Array.FindIndex(numbers, row => row == biggestNum);
    numbers[bigIndx] = int.MinValue; // 替换为最小值，而非0
    bigToSmall[y] = biggestNum;
}

foreach (int num in bigToSmall)
{
    Console.WriteLine(num);
}

方案二：使用内置排序方法（更简洁高效） #

如果不需要手动实现排序逻辑，直接用C#内置的排序API，代码更简洁且不易出错：

方法A：Array.Sort + Array.Reverse

int[] numbers = Console.ReadLine().Split(" ").Select(int.Parse).ToArray();
Array.Sort(numbers);
Array.Reverse(numbers);
foreach (int num in numbers)
{
    Console.WriteLine(num);
}

方法B：LINQ OrderByDescending

int[] numbers = Console.ReadLine().Split(" ").Select(int.Parse).ToArray();
var sortedNumbers = numbers.OrderByDescending(n => n).ToArray();
foreach (int num in sortedNumbers)
{
    Console.WriteLine(num);
}

内容的提问来源于stack exchange，提问作者Kaloyan056