GroupBy 并以无点样式减少对象数组

Question

我最近开始使用 Ramda 并试图找到一种无点方式来编写减少对象数组的方法。

这是对象数组：

const someObj = [
    {
        name: 'A',
        city: 1,
        other: {
            playtime: 30
        }
    },
    {
        name: 'B',
        city: 2,
        other: {
            playtime: 20
        }
    },
    {
        name: 'c',
        city: 1,
        other: {
            playtime: 20
        }
    }
];

我正在尝试的是使用 ramda 以像

这样的 poinfree 风格来减少对象

{
    '1': {
        count: 2,
        avg_play_time: 20 + 30 / count
    },
    '2': {
        count: 1,
        avg_play_time: 20 / count
    }
}

我可以使用数组 reduce 方法来完成，但不确定如何以 ramda pointfree 样式编写相同的方法。任何建议将不胜感激。

Answer 1

一个解决方案是做这样的事情：

// An optic to extract the nested playtime value
// Coupled with a `lift` operation which allows it to be applied over a collection
// Effectively A -> B => A[] -> B[]
const playtimes = R.lift(R.path(['other', 'playtime']))

R.pipe(
  // Group the provided array by the city value
  R.groupBy(R.prop('city')),
  // Return a body specification which computes each property based on the 
  // provided function value.
  R.map(R.applySpec({
    count: R.length,
    average: R.pipe(playtimes, R.mean)
  }))
)(someObj)

Answer 2

Ramda 还有另一个名为 R.reduceBy 的函数，它提供介于 reduce 和 groupBy 之间的功能，允许您将值与匹配的键折叠在一起。

因此您可以创建如下所示的数据类型来计算平均值。

const Avg = (count, val) => ({ count, val })
Avg.of = val => Avg(1, val)
Avg.concat = (a, b) => Avg(a.count + b.count, a.val + b.val)
Avg.getAverage = ({ count, val }) => val / count
Avg.empty = Avg(0, 0)

然后使用R.reduceBy将它们组合在一起。

const avgCities = R.reduceBy(
  (avg, a) => Avg.concat(avg, Avg.of(a.other.playtime)),
  Avg.empty,
  x => x.city
)

然后将 Avg 的平均值拉出最终对象的形状。

const buildAvg = R.applySpec({
  count: x => x.count,
  avg_play_time: Avg.getAverage
})

最后通过管道将两者结合起来，将 buildAvg 映射到对象中的值。

const fn = R.pipe(avgCities, R.map(buildAvg))
fn(someObj)

Answer 3

这是另一个使用 reduceBy 并在每个属性结果对象上映射一个 applySpec 函数的建议：

想法是使用getPlaytimeByCity将someObj转换成这个对象：

{ 1: [30, 20],
  2: [20]}

然后您可以在该对象的每个属性上映射 stats 函数：

stats({ 1: [30, 20], 2: [20]});
// { 1: {count: 2, avg_play_time: 25}, 
//   2: {count: 1, avg_play_time: 20}}

const someObj = [
    { name: 'A',
      city: 1,
      other: { playtime: 30 }},
    { name: 'B',
      city: 2,
      other: { playtime: 20 }},
    { name: 'c',
      city: 1,
      other: { playtime: 20 }}
];

const city = prop('city');
const playtime = path(['other', 'playtime']);
const stats = applySpec({count: length, avg_play_time: mean});
const collectPlaytime = useWith(flip(append), [identity, playtime]);
const getPlaytimeByCity = reduceBy(collectPlaytime, [], city);

console.log(

  map(stats, getPlaytimeByCity(someObj))
  
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.26.1/ramda.min.js"></script>
<script>const {prop, path, useWith, flip, append, identity, applySpec, length, mean, reduceBy, map} = R;</script>

Answer 4

我喜欢到目前为止给出的所有其他答案。所以很自然地我想添加我自己的。 ;-)

这是一个使用 reduceBy 来跟踪运行计数和平均值的版本。如果您正在寻找中值或其他一些统计数据，这将不起作用，但给定一个计数、一个平均值和一个新值，我们可以直接计算新的计数和平均值。这允许我们以每次迭代都做一些算术为代价只迭代数据一次。

const transform = reduceBy(
  ({count, avg_play_time}, {other: {playtime}}) => ({
    count: count + 1,
    avg_play_time: (avg_play_time * count + playtime) / (count + 1)
  }),
  {count: 0, avg_play_time: 0},
  prop('city')
)
const someObj = [{city: 1, name: "A", other: {playtime: 30}}, {city: 2, name: "B", other: {playtime: 20}}, {city: 1, name: "c", other: {playtime: 20}}]

console.log(transform(someObj))

<script src="https://bundle.run/ramda@0.26.1"></script>
<script>
const {reduceBy, prop} = ramda
</script>

这不是没有意义的。虽然我是无点样式的忠实拥护者，但我只在适用时才使用它。我认为为了自己的利益而寻求它是错误的。

请注意，可以轻松修改 Scott Christopher 的答案以使用此类计算

Answer 5

我就这样写，希望对你有帮助！

const stats = R.pipe(
  R.groupBy(R.prop('city')),
  R.map(
    R.applySpec({
      count: R.length,
      avg_play_time: R.pipe(
        R.map(R.path(['other', 'playtime'])),
        R.mean,
      ),
    }),
  ),  
);

const data = [
  { name: 'A', city: 1, other: { playtime: 30 } },
  { name: 'B', city: 2, other: { playtime: 20 } },
  { name: 'c', city: 1, other: { playtime: 20 } },
];

console.log('result', stats(data));

<script src="https://cdnjs.cloudflare.com/ajax/libs/ramda/0.26.1/ramda.min.js"></script>

GroupBy 并以无点样式减少对象数组

GroupBy and reduce an array of object in a pointfree style

javascript

reduce

ramda.js