在Coffeescript中，如何将一个对象的数组按一个属性分组？

Question

Let's say I have an array of objects like this:

var array = [
    {name:"aaa", height:"20"},
    {name:"bbb", height:"100"},
    {name:"ccc", height:"20"},
    {name:"ddd", height:"20"},
    {name:"eee", height:"100"},
]

I want to group it by height in this case, so I end up with a different array that has the group name and then each item in that group like so:

var grouped_by_height = [
    [{name:"aaa", height:"20"}, {name:"ccc", height:"20"}, {name:"ddd", height:"20"}],
    [{name:"bbb", height:"100"}, {name:"eee", height:"100"}]
]

I've written a long solution in JS/jQuery but I was wondering if there was a quick and easy way to do this is Coffeescript.

Answer 1

I don't think there's anything special in CoffeeScript that would help you (or at least nothing that takes care of the hard parts). I'd probably use Array.prototype.reduce to do the heavy lifting with something like this:

group_by_height = (groups, obj) ->
    groups[obj.height] ?= [ ]
    groups[obj.height].push(obj)
    groups
grouped_obj = array.reduce(group_by_height, { })
grouped_by_height = (v for k, v of grouped_obj)

That won't guarantee any particular order in grouped_by_height though but that can be remedied without much effort by adding a sorting step:

by_height = (a, b) -> +a[0].height - +b[0].height
group_by_height = (v for k, v of grouped_obj).sort(by_height)

You'd be sorting an array-of-arrays so a and b would be arrays, hence the [0] to look at the first elements. The unary + operators are there to convert your string heights to numeric heights so that they'll compare the way you probably expect them to.

Answer 2

This is what I was able to come up with. As mu is too short stated, there is no magic in coffeescript for it but there are some built in things that help it out. Only issue is that the compiled js version ends up being longer than if you just wrote it in plain js.

groupByKey = (array, key) ->
  grouped = {}
  for obj in array
    grouped[obj[key]] ?= []
    grouped[obj[key]].push obj
  Object.keys(grouped).map (group) ->
    grouped[group]

Try it out for yourself:

array = [
  {
    name: 'aaa'
    height: '20'
  }
  {
    name: 'bbb'
    height: '100'
  }
  {
    name: 'ccc'
    height: '20'
  }
  {
    name: 'ddd'
    height: '20'
  }
  {
    name: 'eee'
    height: '100'
  }
]

groupByKey = (array, key) ->
  grouped = {}
  for obj in array
    grouped[obj[key]] ?= []
    grouped[obj[key]].push obj
  Object.keys(grouped).map (group) ->
    grouped[group]

console.log groupByKey(array, 'height')

The output is:

[
  [
    { 
      name: 'aaa',
      height: '20' 
    },
    { 
      name: 'ccc',
      height: '20' 
    },
    { 
      name: 'ddd',
      height: '20' 
    } 
  ],
  [ 
    { 
      name: 'bbb',
      height: '100'
    },
    { 
      name: 'eee',
      height: '100' 
    }
  ]
]

Answer 3

array = [
    {name:"aaa", height:"20"},
    {name:"bbb", height:"100"},
    {name:"ccc", height:"20"},
    {name:"ddd", height:"20"},
    {name:"eee", height:"100"},
]

# unique heights
uniq = array.reduce (memo, el) ->
  memo.push(el.height) if memo.indexOf(el.height) is -1
  memo
, []

# output grouped by height
out = [[obj for obj in array when obj.height is height][0] for height in uniq][0]