LINQ 查询中的对象初始值设定项 - 是否可以重用计算数据？

发布于 2024-11-15 16:54:35 字数 697 浏览 2 评论 0原文

我正在使用一个 linq 查询，它看起来（经过一些简化）如下所示：

List<UserExams> listUserExams = GetUserExams();

var examData = 
from userExam in listUserExams
group by userExam.ExamID into groupExams
select new ExamData()
{
    ExamID = groupExams.Key,
    AverageGrade = groupExams.Average(e => e.Grade),
    PassedUsersNum = groupExams.Count(e => /* Some long and complicated calculation */),
    CompletionRate = 100 * groupExams.Count(e => /* The same long and complicated calculation */) / TotalUsersNum
};

令我困扰的是出现两次的计算表达式，即 PassedUsersNum 和 CompletionRate。

假设CompletionRate = (PassedUsersNum / TotalUsersNum) * 100，如何通过重用PassedUsersNum的计算来编写它，而不是再次编写该表达式？

原文

I'm using a linq query which looks (after some simplification) something like the following:

List<UserExams> listUserExams = GetUserExams();

var examData = 
from userExam in listUserExams
group by userExam.ExamID into groupExams
select new ExamData()
{
    ExamID = groupExams.Key,
    AverageGrade = groupExams.Average(e => e.Grade),
    PassedUsersNum = groupExams.Count(e => /* Some long and complicated calculation */),
    CompletionRate = 100 * groupExams.Count(e => /* The same long and complicated calculation */) / TotalUsersNum
};

What bothers me is the calculation expression which appears twice, for PassedUsersNum and CompletionRate.

Assuming that CompletionRate = (PassedUsersNum / TotalUsersNum) * 100, how can I write it by reusing the calculation of PassedUsersNum, instead of writing that expression again?

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℡Ms空城旧梦 2024-11-22 16:54:35

最简单的方法是首先使用 let 注入另一个选择步骤：

List<UserExams> listUserExams = GetUserExams();

var examData = 
    from userExam in listUserExams
    group by userExam.ExamID into groupExams
    let passCount = groupExams.Count( /* long expression */)
    select new ExamData()
    {
        ExamID = groupExams.Key,
        AverageGrade = groupExams.Average(e => e.Grade),
        PassedUsersNum = passCount,
        CompletionRate = 100 * passCount / TotalUsersNum
    };

当然，每个组只会对表达式求值一次。

The simplest way would be to use let to inject another selection step first:

List<UserExams> listUserExams = GetUserExams();

var examData = 
    from userExam in listUserExams
    group by userExam.ExamID into groupExams
    let passCount = groupExams.Count( /* long expression */)
    select new ExamData()
    {
        ExamID = groupExams.Key,
        AverageGrade = groupExams.Average(e => e.Grade),
        PassedUsersNum = passCount,
        CompletionRate = 100 * passCount / TotalUsersNum
    };

The expression will only be evaluated once per group, of course.

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檐上三寸雪 2024-11-22 16:54:35

您还可以将 Count 函数提取到另一个返回 Func 的方法中（如果需要），或者采用 double 并返回 bool 的方法。

List<UserExams> listUserExams = GetUserExams();

var examData = 
from userExam in listUserExams
group by userExam.ExamID into groupExams
select new ExamData()
{
    ExamID = groupExams.Key,
    AverageGrade = groupExams.Average(funcMethod()),
    PassedUsersNum = groupExams.Count(e => traditionalMethod(e)),
    CompletionRate = 100 * groupExams.Count(e => /* The same long and complicated calculation */) / TotalUsersNum
};

// later...
private Func<double,bool> funcMethod(){ return e=> /* your calculation */ }

private bool traditionalMethod(double d){ return /* your calculation */ }

You can also just extract your Count func into another method which returns a Func if you want, or a method that takes a double and returns a bool.

List<UserExams> listUserExams = GetUserExams();

var examData = 
from userExam in listUserExams
group by userExam.ExamID into groupExams
select new ExamData()
{
    ExamID = groupExams.Key,
    AverageGrade = groupExams.Average(funcMethod()),
    PassedUsersNum = groupExams.Count(e => traditionalMethod(e)),
    CompletionRate = 100 * groupExams.Count(e => /* The same long and complicated calculation */) / TotalUsersNum
};

// later...
private Func<double,bool> funcMethod(){ return e=> /* your calculation */ }

private bool traditionalMethod(double d){ return /* your calculation */ }

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