Angular：如何獲得具有特定值的對象的計數？

Question

我有一個JSON數據庫與對象。每個人都有一個具有特定分配值的屬性：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" 
    } 
] 

我想顯示類似：

共有項是：一X ，B X 和c X 。

我知道我必須用objectsinmyjsondatabase.length才能得到總數。

我想知道如何獲得具有特定值的對象的長度（數量）？

Answer 1

您可以輕鬆地指望它有這樣

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; 
 
    }) 
 
);

Answer 2

特定的值對象定義一個函數：

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

，並調用它： this.getCount('a');

Answer 3

解決您的問題的一種方法是使用map-reduce。這是一個快速解決方案。希望能解決你的問題。

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);