如何在LINQ方法语法中使用int和string类型交集两个选择？

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英文:

How to intersect two select with int and string type in LINQ Method syntax?

问题

  1. 我是新手程序员，希望你能帮我写出更好的代码。
  3. **描述**
  5. 我想编写一个查询，打印出加利福尼亚州活跃且名字中包含字母R或D的人的平均分数。
  7. **示例输入**
  9.     StudentName = {"Ben Wild", Score = 65, StudentCity = "CA", StudentActvity= true}
  10.     
  11.     StudentName = {"Sara Ride", Score = 81, StudentCity = "CA", StudentActvity = true}
  13. **我的问题**
  15. 我需要选择他们的姓名和分数，但我不能选择两个Lambda表达式并使用`&&`连接它们。
  16. 我得到了这个错误：`'string'不包含'Score'的定义，也没有可访问的扩展方法'Score'来接受类型为'string'的第一个参数`
  18. 我尝试了很多方法，我将在下面提到我的解决方案：
  20. **Program.cs 代码**

using System;
using System.Collections.Generic;
using System.Linq;

namespace _64_LINQ_Practice
{
class Program
{
static void Main(string[] args)
{
//访问类方法
var studentList = StudentDatabase.GetStudentsFromDb();
var studentsName6 = studentList.Where(x => (x.StudentActvity == true) && (x.StudentCity == "CA")).Select(x => x.StudentName);
List xstudent = new List(studentList.Select(x => x.StudentName));
IEnumerable final = xstudent.Where(
x => x.Contains("R") || x.Contains("r") || x.Contains("d") || x.Contains("D"));

  1.         foreach (string studentFinal in final)
  2.         {
  3.             Console.WriteLine("活跃，住在CA并且名字中包含r/R或D/d字母的学生：" + studentFinal);
  4.         }
  5.     }
  6. }

}

  2. **第二种方式：**
  4. var studentsName6 = studentList.Where(x => (x.StudentActvity == true) && (x.StudentCity == "CA")).Average(x => x.Score);
  5. List<string> xstudent = new List<string>(studentList.Select(x => x.StudentName));
  6. List<int> intstudent = new List<int>(studentList.Select(x => x.Score));
  7. IEnumerable<string> final = xstudent.Where(
  8.                     x => x.Contains("R") || x.Contains("r") || x.Contains("d") || x.Contains("D"));
  9. IEnumerable<int> final2 = intstudent;
  10. var convFinal = final.AsQueryable();
  11. var convFinal2 = final2.AsQueryable();
  12. var final3 = studentsName6.AsQueryable().Concat(convFinal.AsQueryable()).Concat(convFinal2);
  13. var final4 = final3.Average(x => x.Score);
  14. Console.WriteLine(final4);
英文:

I am new to programming I hope you can help me to write better code.

Description

I want to write a query that prints Average scores of Californians who are active and have the letter R or D in their name.

Example Input

  1. StudentName = {&quot;Ben Wild&quot;, Score = 65, StudentCity = &quot;CA&quot;, StudentActvity= true}
  3. StudentName = {&quot;Sara Ride&quot;, Score = 81, StudentCity = &quot;CA&quot;, StudentActvity = true}

My problem

I need to select their name and their score at the end but I can't select two Lambda expressions and &amp;&amp; them together.
I got this error : &#39;string&#39; does not contain a definition for &#39;Score&#39; and no accessible extension method &#39;Score&#39; accepting a first argument of type &#39;string&#39; could be

I tried so many ways that I will mention my solutions below :

Program.cs code

  1. using System;
  2. using System.Collections.Generic;
  3. using System.Linq;
  5. namespace _64_LINQ_Practice
  6. {
  7.     class Program
  8.     {
  9.         static void Main(string[] args)
  10.         {
  11.             //Access to the class method
  12.             var studentList = StudentDatabase.GetStudentsFromDb();
  13.             var studentsName6 = studentList.Where(x =&gt; (x.StudentActvity == true) &amp;&amp; (x.StudentCity == &quot;CA&quot;)).Select(x =&gt; x.StudentName);
  14.             List&lt;string&gt; xstudent = new List&lt;string&gt;(studentList.Select(x =&gt; x.StudentName));
  15.             IEnumerable&lt;string&gt; final = xstudent.Where(
  16.                                 x =&gt; x.Contains(&quot;R&quot;) || x.Contains(&quot;r&quot;) || x.Contains(&quot;d&quot;) || x.Contains(&quot;D&quot;));
  18.             foreach (string studentFinal in final)
  19.             {
  20.                 Console.WriteLine(&quot;The students that are active, live in CA and names with r/R or D/d letter :&quot; + studentFinal);
  21.             }
  22.         }
  23.     }
  24. }

and second way:

  1.         var studentsName6 = studentList.Where(x =&gt; (x.StudentActvity == true) &amp;&amp; (x.StudentCity == &quot;CA&quot;)).Average(x =&gt; x.Score);
  2.         List&lt;string&gt; xstudent = new List&lt;string&gt;(studentList.Select(x =&gt; x.StudentName));
  3.         List&lt;int&gt; intstudent = new List&lt;int&gt;(studentList.Select(x =&gt; x.Score));
  4.         IEnumerable&lt;string&gt; final = xstudent.Where(
  5.                             x =&gt; x.Contains(&quot;R&quot;) || x.Contains(&quot;r&quot;) || x.Contains(&quot;d&quot;) || x.Contains(&quot;D&quot;));
  6.         IEnumerable&lt;int&gt; final2 = intstudent;
  7.         var convFinal = final.AsQueryable();
  8.         var convFinal2 = final2.AsQueryable();
  9.         var final3 = studentsName6.AsQueryable().Concat(convFinal.AsQueryable()).Concat(convFinal2);
  10.         var final4 = final3.average(x =&gt; x.Score);
  11.         Console.WriteLine(final4);

答案1

得分: 0

以下是翻译好的内容：

"代码中没有尝试计算平均值，所以不清楚问题出在哪里。根据以下条件，输出学生的平均分数：住在加利福尼亚（CA）且活动为真，名字中包含 R/r 或 D/d 字母的学生的平均分数。

似乎只需要一个 Where 条件来筛选学生，然后使用 Avg 计算平均值。由于不需要计算多个分组的平均值，不需要使用 GroupBy。"

  1. var letters = new[] { "r", "d" };
  2. var average = studentList.Where(st =>
  3.     st.StudentActvity
  4.     && st.StudentCity == "CA"
  5.     && letters.Any(l => st.StudentName.Contains(l, StringComparison.OrdinalIgnoreCase))
  6. )
  7. .Average(st => st.Score);

String.Contains 可以接受一个参数，指定不区分大小写的比较，因此不需要同时指定大小写字母。

letters.Any(l => st.StudentName.Contains(l...)) 检查是否在 StudentName 中包含任何字母。

这个查询将适用于内存数据库和可能是实际数据库。不过，它将非常慢。name.Contains("r") 被转换成 SQL 条件 name LIKE '%r%'。无法使用索引快速查找具有 r 的名字，因此数据库将不得不搜索整个表。

英文:

There's no attempt to calculate averages in the code so it's unclear what the problem is. Going by

> prints the average score of students, with these conditions: Average score of students that live in CA and their activity is true and there are R/r or D/d letters contain in their name.

It seems a single Where is needed to filter the students, then Avg to calculate the average. Since there's no need to calculate averages for multiple groups, GroupBy isn't needed.

  1. var letters=new[]{&quot;r&quot;,&quot;d&quot;};
  2. var average=studentList.Where(st=&gt;
  3.         st.StudentActvity 
  4.         &amp;&amp; st.StudentCity == &quot;CA&quot;
  5.         &amp;&amp; letters.Any(l=&gt;st.StudentName.Contains(l,StringComparison.OrdinalIgnoreCase))
  6.      )
  7.     .Average(st=&gt;st.Score);

String.Contains can accept a parameter that specifies case-insensitive comparison, so there's no need to specify both small and capital letters.

letters.Any(l=&gt;st.StudentName.Contains(l...)) checks if any letter is contained inside StudentName

This query will work with an in-memory database and probably an actual database. It will be very slow though. name.Contains(&quot;r&quot;) is translated to the SQL condition name LIKE &#39;%r%&#39;. There's no way to use an index to quickly find which names have r in their middle, so the database will have to search the entire table

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  • 本文由 发表于 2023年2月16日 17:28:42
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