如何從具有相同鍵的嵌套散列和數組中分組和添加值？

Question

我試圖在這種散列和數組的組合中獲得每個學生的總分和平均成績，但我所有的嘗試只返回所有條目的總和。 有任何想法嗎？

student_data = 
  {"ST4"=>[{:student_id=>"ST4", :points=> 5, :grade=>5}, 
           {:student_id=>"ST4", :points=>10, :grade=>4}, 
           {:student_id=>"ST4", :points=>20, :grade=>5}], 
   "ST1"=>[{:student_id=>"ST1", :points=>10, :grade=>3}, 
           {:student_id=>"ST1", :points=>30, :grade=>4}, 
           {:student_id=>"ST1", :points=>45, :grade=>2}], 
   "ST2"=>[{:student_id=>"ST2", :points=>25, :grade=>5}, 
           {:student_id=>"ST2", :points=>15, :grade=>1}, 
           {:student_id=>"ST2", :points=>35, :grade=>3}], 
   "ST3"=>[{:student_id=>"ST3", :points=> 5, :grade=>5}, 
           {:student_id=>"ST3", :points=>50, :grade=>2}]}

Answer 1

可以這樣獲得所需的散列。

student_data.transform_values do |arr|
  points, grades = arr.map { |h| h.values_at(:points, :grade) }.transpose
  { :points=>points.sum, :grades=>grades.sum.fdiv(grades.size) }
end
  #=> {"ST4"=>{:points=>35, :grades=>4.666666666666667},
  #    "ST1"=>{:points=>85, :grades=>3.0},
  #    "ST2"=>{:points=>75, :grades=>3.0},
  #    "ST3"=>{:points=>55, :grades=>3.5}} 

傳遞給塊的第一個值是第一個鍵的值'ST4'並且塊變量arr被分配了該值：

a = student_data.first
  #=> ["ST4",
  #    [{:student_id=>"ST4", :points=> 5, :grade=>5},
  #     {:student_id=>"ST4", :points=>10, :grade=>4},
  #     {:student_id=>"ST4", :points=>20, :grade=>5}]
  #   ] 
arr = a.last
  #=> [{:student_id=>"ST4", :points=> 5, :grade=>5},
  #    {:student_id=>"ST4", :points=>10, :grade=>4},
  #    {:student_id=>"ST4", :points=>20, :grade=>5}]

塊計算如下。 map傳遞給內部塊的arr的第一個值是

h = arr.first
  #=> {:student_id=>"ST4", :points=>5, :grade=>5} 
h.values_at(:points, :grade)
  #=> [5, 5] 

在將arr的其余兩個元素傳遞給塊后，我們有

b = arr.map { |h| h.values_at(:points, :grade) }
  #=> [[5, 5], [10, 4], [20, 5]] 

然后

points, grades = b.transpose
  #=> [[5, 10, 20], [5, 4, 5]] 
points
  #=> [5, 10, 20] 
grades
  #=> [5, 4, 5] 

我們現在簡單地形成哈希值，即'ST4'的值。

c = points.sum
  #=> 35 
d = grades.sum
  #=> 14 
e = grades.size
  #=> 3 
f = c.fdiv(d)
  #=> 4.666666666666667 

因此， student_data中'ST4'的值映射到哈希

{ :points=>c, :grades=>f }
  #=> {:points=>35, :grades=>4.666666666666667} 

student_data其余鍵的映射也是類似計算的。

參見Hash#transform_values 、 Enumerable#map 、 Hash#values_at 、 Array#transpose 、 Array#sum和Integer#fdiv 。

Answer 2

無論您期望什么都可以實現，如下所示，

student_data.values.map do |z|
  z.group_by { |x| x[:student_id] }.transform_values do |v|
    { 
      points: v.map { |x| x[:points] }.sum, # sum of points
      grade: (v.map { |x| x[:grade] }.sum/v.count.to_f).round(2) # average of grades
    }
  end
end

由於沒有指定確切的預期輸出格式，通過以下方式獲得，

=> [
  {"ST4"=>{:points=>35, :grade=>4.67}},
  {"ST1"=>{:points=>85, :grade=>3.0}},
  {"ST2"=>{:points=>75, :grade=>3.0}},
  {"ST3"=>{:points=>55, :grade=>3.5}}
]

Answer 3

對於Ruby 2.6使用Object#then或Object#yield_self for Ruby 2.5

student_data.transform_values { |st| st
  .each_with_object(Hash.new(0)) { |h, hh|  hh[:sum_points] += h[:points]; hh[:sum_grade] += h[:grade]; hh[:count] += 1.0 }
  .then{ |hh| {tot_points: hh[:sum_points], avg_grade: hh[:sum_grade]/hh[:count] } }
}

---

這個怎么運作？

給定每個學生的數組：

 st = [{:student_id=>"ST4", :points=> 5, :grade=>5}, {:student_id=>"ST4", :points=>10, :grade=>4}, {:student_id=>"ST4", :points=>20, :grade=>5}]

首先使用Enumerable#each_with_object構建哈希添加和計數， Hash#default Enumerable#each_with_object設置為零(Hash.new(0) )

 step1 = st.each_with_object(Hash.new(0)) { |h, hh| hh[:sum_points] += h[:points]; hh[:sum_grade] += h[:grade]; hh[:count] += 1.0 } #=> {:sum_points=>35, :sum_grade=>14, :count=>3.0}

那就用吧！ （ Ruby 2.5 的yield_self ）

 step2 = step1.then{ |hh| {tot_points: hh[:sum_points], avg_grade: hh[:sum_grade]/hh[:count] }} #=> {:tot_points=>35, :avg_grade=>4.666666666666667}

使用Hash#transform_values將所有內容放在一起，就像在第一個代碼片段中一樣