Angular：如何获取具有特定值的Object的计数？

Question

我有一个带对象的JSON数据库。每个属性都具有特定指定值的属性：a，b或c。

[
  {
    "id": 1,
    "category": "a"
  },
  {
    "id": 2,
    "category": "b"
  },
  {
    "id": 3,
    "category": "c"
  },
  {
    "id": 4,
    "category": "a"
  },
  {
    "id": 5,
    "category": "a"
  },
  {
    "id": 5,
    "category": "b"
  }
]

我希望显示如下内容：

  

共有 6 项：ax 3 ，bx 2 和cx 1 。

我知道我必须使用objectsinmyjsondatabase.length来获得总数。

我想知道如何获得具有特定值的对象的长度（数量）？

Answer 1

解决问题的一种方法是使用map-reduce。这是一个快速的解决方案。希望能解决你的问题。

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var data = [
  {
    "id": 1,
    "category": "a"
  },
  {
    "id": 2,
    "category": "b"
  },
  {
    "id": 3,
    "category": "c"
  },
  {
    "id": 4,
    "category": "a"
  },
  {
    "id": 5,
    "category": "a"
  },
  {
    "id": 5,
    "category": "b"
  }
];

// First get list of all categories
var categories = data.map(function(x) { return x["category"]; });

// Count no of items in each category
var countByCategories = categories.reduce(function(x, y) {
    if (typeof x !== "object") {
       var reduced = {};
       reduced[x] = 1;
       reduced[y] = 1;
       return reduced;
    }
    
    x[y] = (x[y] || 0) + 1;
    return x;
});

// Final build string that you want
var categoryStrings = [];
for (var category in countByCategories) {
   categoryStrings.push(category + ' x ' + countByCategories[category]); 
}

var msg = 'There is a total of ' + categories.length + ' items: ';
if (categoryStrings.length > 2) {
   msg = categoryStrings.slice(0, -1).join(', '); 
   msg += ' and ' + categoryStrings.slice(-1);
} else {
   msg = categoryStrings.join(', '); 
}

// print results
console.log(msg);

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Answer 2

定义一个函数：

getCount(character) {
  return this.objects.filter(obj => obj.category === character).length;
}

并将其称为： this.getCount('a');

Answer 3

您可以轻松计算具有此特定值的对象

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let objects = [
    {
        "id": 1,
        "category": "a"
    },
    {
        "id": 2,
        "category": "b"
    },
    {
        "id": 3,
        "category": "c"
    },
    {
        "id": 4,
        "category": "a"
    },
    {
        "id": 5,
        "category": "a"
    },
    {
        "id": 5,
        "category": "b"
    }
];
var filtered = new Array();
objects.filter(function (t) {
    var found = filtered.some(function (el, index) {
        if (false == (t.category === el.category)) {
            return false;
        }
        filtered[index].count = filtered[index].count + 1;
        return true;

    }, filtered);
    if (false === found) {
        filtered.push({category: t.category, count: 1});
    }
}, filtered);
console.log('There is a total of ' + objects.length + ' items: '
    + filtered.map(function (t) {
        return t.category + ' x ' + t.count;
    })
);

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