有很多關于計算陣列中唯一值的帖子，但我想根據第二個陣列計算陣列中的唯一值。具體來說：我想計算內的不同出現次數arr，但計算內的所有值grade

arr = [
  "12D","12D","12D",
  "12C","12C","12C","12C","12C",
  "12B","12B","12B","12B","12B",
  "12B","12B",
  "12A","12A","12A","12A","12A",
  "12A","12A","12A","12A","12A",
  "12A","12A"
]

grade = ["13B", "13A", "12D", "12C", "12B", "12A"]

result = arr.filter(item => grade.includes(item))
  .reduce((acc, val) => {
    acc[val] = acc[val] === undefined ? 1 : acc[val]  = 1;
    return acc;
  }, {});

當前結果

我目前的結果確實計算了獨特的出現次數

{ '12D': 3, '12C': 5, '12B': 7, '12A': 12 }

期望輸出

但我想在我的最終物件中包含等級內的所有值（并按grade]

desired_result = {
    '13B': 0,
    '13A': 0,
    '12D': 3,
    '12C': 5,
    '12B': 7,
    '12A': 12
}

是否可以使用第二個向量來執行這種減少？

uj5u.com熱心網友回復：

另一種方法是從成績開始，并建立一個頻率表，如下所示。然后，您可以Object.fromEntries()在生成的陣列對陣列上呼叫：

const result = Object.fromEntries(
    grade.map(grade => [grade, arr.filter(g => g === grade).length])
);

演示

顯示代碼片段

const arr = ["12D","12D","12D","12C","12C","12C","12C","12C","12B","12B","12B","12B","12B","12B","12B","12A","12A","12A","12A","12A","12A","12A","12A","12A","12A","12A","12A"];

const grade = ["13B", "13A", "12D", "12C", "12B", "12A"];

const result = Object.fromEntries(
    grade.map(grade => [grade, arr.filter(g => g === grade).length])
);

console.log( result );

uj5u.com熱心網友回復：

您可以將成績映射為條目并將物件作為reduce 的起始值。

const
    array = ["12D", "12D", "12D", "12C", "12C", "12C", "12C", "12C", "12B", "12B", "12B", "12B", "12B", "12B", "12B", "12A", "12A", "12A", "12A", "12A", "12A", "12A", "12A", "12A", "12A", "12A", "12A"],
    grade = ["13B", "13A", "12D", "12C", "12B", "12A"],
    result = array.reduce(
        (acc, val) => {
            if (val in acc) acc[val]  ;
            return acc;
        },
        Object.fromEntries(grade.map(g => [g, 0]))
    );

console.log(result);

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標籤：javascript 数组

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使用第二個陣列計算陣列中的唯一值

當前結果

期望輸出