c# - 列表<>太多循环

Question

我的 foreach 循环次数过多，我不明白为什么。该程序应该遍历 1-100 并对每个数字的四次方求和。

using System;
using System.Collections.Generic;    

public class Program
{
    public static void Main()
    {
        int sum = 0;            
        string temp = "";           
        List<int> digits = new List<int>();

        for (long i = 2; i < 100; i++)
        {
            temp = i.ToString();

            for(int y = 0; y < temp.Length; y++)
            {
                digits.Add(Convert.ToInt32(temp.Substring(y,1)));

                foreach(int j in digits)
                {
                    sum += Convert.ToInt32(Math.Pow(j,4));                      
                    Console.WriteLine("foreach loop: i = {0}, y = {1}, sum = {2}, j = {3}, digits count = {4}",i,y,sum,j,digits.Count);
                }                   
            }

            Console.WriteLine("i = {0}, sum = {1}", i, sum);                
            sum = 0;                    
            digits.Clear();
        }
    }
}

下面是输出示例

foreach loop: i = 10, y = 0, sum = 1, j = 1, digits count = 1
foreach loop: i = 10, y = 1, sum = 2, j = 1, digits count = 2
foreach loop: i = 10, y = 1, sum = 2, j = 0, digits count = 2
i = 10, sum = 2
foreach loop: i = 11, y = 0, sum = 1, j = 1, digits count = 1
foreach loop: i = 11, y = 1, sum = 2, j = 1, digits count = 2
foreach loop: i = 11, y = 1, sum = 3, j = 1, digits count = 2
i = 11, sum = 3
foreach loop: i = 12, y = 0, sum = 1, j = 1, digits count = 1
foreach loop: i = 12, y = 1, sum = 2, j = 1, digits count = 2
foreach loop: i = 12, y = 1, sum = 18, j = 2, digits count = 2
i = 12, sum = 18

为什么 foreach 在一个 2 位数字上循环 3 次？循环后清空数字List

以10为例。

temp = "10";
temp.Length = 2;
second for loop (y) runs twice. 1 < 2
digits gets filled twice
foreach runs three times

Answer 1

您的 foreach 循环似乎创建了额外的循环。每次添加下一个数字时，它都会循环列表中的所有数字。

在这里你甚至不需要 foreach 循环和数字列表。

public static void Main()
{
    int sum = 0;
    string temp = "";

    for (long i = 2; i < 100; i++)
    {
        temp = i.ToString();
        for(int y = 0; y < temp.Length; y++)
        {
            int digit = Convert.ToInt32(temp.Substring(y,1));
            sum += Math.Pow(digit,4);
        }
        Console.WriteLine("i = {0}, sum = {1}", i, sum);
        sum = 0;
    }
}

我不喜欢使用字符串来处理整数，下面是不在字符串和整数之间转换的解决方案

public static void Main()
{
    int sum = 0;
    for (long i = 2; i < 100; i++)
    {
        for(int temp = i; temp > 0; temp /= 10)
        {
            int digit = temp % 10;
            sum += Math.Pow(digit,4);
        }
        Console.WriteLine("i = {0}, sum = {1}", i, sum);
        sum = 0;
    }
}