如何从D中的数组有条件地创建类参数数组？

假设我有一个关联数组，其中包含一组类实例。我想找到一种惯用的方法来创建一个数组（或范围），其中包含属于数组中表示一些布尔条件的类实例的属性

请参见下面的示例，在本例中，我希望创建一个数组或范围，其中包含五年级学生的年龄

我知道如何使用循环和条件实现这一点，但如果在D中有内置函数或惯用方法实现这一点，那将非常有用

import std.stdio;

class Student {
    private:
        uint grade;
        uint age;
        uint year;

    public:
        this(uint g, uint a, uint y) {
            grade = g;
            age = a;
            year = y;
        }

        uint getAge() {
            return age;
        }

        uint getGrade() {
            return grade;
        }

        uint getYear() {
            return year;
        }
}

void main() {
    Student[uint] classroom;

    Student s1 = new Student(1, 5, 2);
    Student s2 = new Student(2, 6, 1);
    Student s3 = new Student(3, 7, 2);
    Student s4 = new Student(4, 8, 9);

    classroom[1] = s1;
    classroom[2] = s1;
    classroom[3] = s1;
    classroom[4] = s1;

    // I want to generate an array or range here containing the age of students who are in the X'th grade
}

---

std.algorithm有你的支持：

import std.algorithm, std.array;
auto kids = classroom.values
    .filter!(student => student.grade == 5)
    .array;

如果你想一次对每个年级都这样做，你需要排序，然后按chunkBy排序，比如：

classroom.values
    .sort!((x, y) => x.grade < y.grade)
    .chunkBy((x, y) => x.grade == y.grade)

class.values
分类（（x，y）=>x.gradex.grade==y.grade）

---

这为您提供了一系列[同年级学生的范围]。

您只需要借助std.algorithm模块进行一点函数编程：

import std.stdio;
import std.algorithm, std.array;


class Student {
    private:
        uint grade;
        uint age;
        uint year;

    public:
        this(uint g, uint a, uint y) {
            grade = g;
            age = a;
            year = y;
        }

        uint getAge() {
            return age;
        }

        uint getGrade() {
            return grade;
        }

        uint getYear() {
            return year;
        }
}

void main() {
    Student[uint] classroom;

    Student s1 = new Student(1, 5, 2);
    Student s2 = new Student(2, 6, 1);
    Student s3 = new Student(3, 7, 2);
    Student s4 = new Student(4, 8, 9);

    classroom[1] = s1;
    classroom[2] = s2;
    classroom[3] = s3;
    classroom[4] = s4;
    classroom[5] = new Student(3, 8, 3);

    // I want to generate an array or range here containing the age of students who are in the X'th grade
    uint grd = 3;

    auto ages = classroom.values
        .filter!(student => student.getGrade() == grd)
        .map!(student => student.getAge());
    writeln(ages);

    uint[] arr = ages.array;  // if you need to turn the range into an array
    writeln(arr);             // prints the same as above
}

---

谢谢，但这两个例子都返回了一系列的学生。我想要的不是一系列学生，而是属于这些学生的一系列财产，例如这些学生的年龄范围。