22

如何使用 LINQ Lambda 表达式/Statemene 表达式编写“// 使用 Foreach 显示”循环实现？

我想简化我的开发并尽可能避免嵌套的 foreach 循环。我试图在第二个 foreach 语句中包含更多逻辑，并且我想使用 Lambda / Statement 表达式。

 internal class Program
{
    internal class Country
    {
        public string CountryName { get; set; }
        public int CountryCode { get; set; }
    }
    static void Main(string[] args)
    {
        List<Country> countries = new List<Country>()
        {
            new Country{CountryName = "India", CountryCode=1},
            new Country{CountryName = "Andaman Nicobar", CountryCode=1},
            new Country{CountryName = "United States of America",  CountryCode=2},
            new Country{CountryName = "Alaska",  CountryCode=2},
            new Country{CountryName = "Hawaii",  CountryCode=2},
            new Country{CountryName = "United Kingdom", CountryCode=3},
            new Country{CountryName = "Australia", CountryCode=4}
        };

        Dictionary<int, List<Country>> countriesDictionary = new Dictionary<int, List<Country>>();
        foreach (Country country in countries)
        {
            if (!countriesDictionary.ContainsKey(country.CountryCode))
                countriesDictionary.Add(country.CountryCode, new List<Country>());
            countriesDictionary[country.CountryCode].Add(country);                
        }

       // Display using Foreach

        foreach (int key in countriesDictionary.Keys)
        {                             
            List<Country> dCountries = countriesDictionary[key];

            foreach (Country country in dCountries)
            {
                if (country.CountryCode.Equals(key))
                {
                    Console.WriteLine(country.CountryName);
                }                
            }
            Console.WriteLine();            
        }
        Console.ReadLine();
    }

请建议。

4

6 回答 6

21

这是另一种选择：

countriesDictionary.ToList().ForEach
(
    pair =>
    {
        pair.Value.ForEach(country => Console.WriteLine(country.CountryName));
        Console.WriteLine();
    }
);

此外，这个基于Romoku 的答案（答案已被删除）：

var countriesDictionary = countries.ToLookup(x => x.CountryCode, x => x);

foreach(var countryGroup in countriesDictionary)
{
    foreach(var country in countryGroup)
    {
        Console.WriteLine(country.CountryName);
    }
    Console.WriteLine();
}
于 2013-07-05T13:58:08.097 回答
6

如果您想按代码对国家/地区进行分组，那么您不需要两个字典。使用Enumerable.GroupBy

foreach(var codeGroup in countries.GroupBy(c => c.CountryCode))
{
    foreach(var country in codeGroup)
       Console.WriteLine(country.CountryName);

    Console.WriteLine();
}

或者只使用您的countriesDictionary（它已经有按代码分组的国家/地区）：

foreach(var kvp in countriesDictionary)
{
    foreach(var country in kvp.Value)
       Console.WriteLine(country.CountryName);

    Console.WriteLine();
}
于 2013-07-05T13:53:47.033 回答
2

尽管您暂时可能有足够的答案，但此 LINQ 会将您的字典压缩为国家名称列表，从而使 foreach 可以轻松显示它们。

    List<string> countryNames = countriesDictionary.SelectMany(
        pair=>pair.Value.Where(
            country=>country.CountryCode == pair.Key
        ).Select(x=>x.CountryName)).ToList();

    foreach (var name in countryNames)
        Console.WriteLine(name);

但是您的字典设置方式，键应始终与值中的国家代码匹配，对吗？

于 2013-07-05T14:36:45.960 回答
1

或者只是让它变得非常容易......如前所述，您已经按国家代码分组......

        foreach (var country in countriesDictionary.SelectMany(pair => pair.Value))
        {
            Console.WriteLine(country.CountryName);
        }
于 2013-07-05T14:02:51.993 回答
1

一种方法可能是这样，避免使用 GroupBy 的第一个 foreach，只有 1 个 foreach 逻辑用指定的代码打印每个国家/地区名称：

Dictionary<int, List<Country>> countriesDictionary = countries.GroupBy(g => g.CountryCode).ToDictionary(k => k.Key, k => k.ToList());

foreach (int key in countriesDictionary.Keys)
{
      Console.WriteLine("****Countries with code {0}****",key);
      int count = 0;
      while (count < countriesDictionary[key].Count)
      {
            Console.WriteLine(countriesDictionary[key][count].CountryName);
            count++;
      }
      Console.WriteLine();
}

Console.ReadLine();
于 2013-07-05T14:22:29.090 回答
0

我将使用扩展来做到这一点。

 static class CountryExtension
    {
        public static void WriteCountriesGroupyCountryCode(this IEnumerable<Country> list)
        {
            int currentCountry=int.MinValue;
            list.OrderBy(c => c.CountryCode).ThenBy(c=>c.CountryName).ToList().ForEach(c =>
            {
                if (currentCountry == int.MinValue)
                {
                    currentCountry = c.CountryCode;
                }
                else if (currentCountry != c.CountryCode)
                {
                    Console.WriteLine();
                    currentCountry = c.CountryCode;
                }
                Console.WriteLine(c.CountryName);
            });
        }
    }
于 2013-07-05T14:03:36.450 回答