按课程列出

本课属于 Coddy Ruby 学习旅程的“逻辑与流程”单元。第 52 课，共 56 课。

添加一个方法，用来查找选修指定课程的所有学生，并以数组形式返回他们的姓名。这就是你已经练习过的 select/map 链式调用。

挑战

简单

添加一个方法 list_by_course(course)，返回 :courses 数组中包含 course 的学生姓名数组。姓名应按插入顺序排列；如果没有学生选修该课程，则返回 []。

将测试代码块替换为：

add_student("Alice", 20, ["Math", "Physics"])
add_student("Bob",   22, ["Biology", "Math"])
add_student("Cara",  19, ["Physics"])
puts list_by_course("Math").inspect
puts list_by_course("Physics").inspect
puts list_by_course("Art").inspect

预期输出：

Student 'Alice' added.
Student 'Bob' added.
Student 'Cara' added.
["Alice", "Bob"]
["Alice", "Cara"]
[]

亲手试试

$student_records = {}

def add_student(name, age, courses)
  if $student_records.key?(name)
    puts "Student '#{name}' already exists."
    return
  end
  $student_records[name] = { age: age, grades: [], courses: courses }
  puts "Student '#{name}' added."
end

def add_grade(name, grade)
  unless $student_records.key?(name)
    puts "Student '#{name}' not found."
    return
  end
  $student_records[name][:grades] << grade
  puts "Grade #{grade} added for '#{name}'."
end

def average_grade(name)
  unless $student_records.key?(name)
    puts "Student '#{name}' not found."
    return nil
  end
  grades = $student_records[name][:grades]
  return 0 if grades.empty?
  grades.sum.to_f / grades.length
end

# TODO：定义 list_by_course(course)，返回选修 `course` 的学生姓名数组

add_student("Alice", 20, ["Math", "Physics"])
add_student("Bob",   22, ["Biology", "Math"])
add_student("Cara",  19, ["Physics"])
puts list_by_course("Math").inspect
puts list_by_course("Physics").inspect
puts list_by_course("Art").inspect