将对象数组操作为具有平均数的新数组。

Question

我有一个要操作的数组，因为我需要将它用作 D3.js 的数据源。此数据集的一个示例是：

var data =  [
 {day: 1, month: 1,  length: 100,  year: 2010},
 {day: 2, month: 1,  length: 125,  year: 2010},
 {day: 3, month: 1,  length: 150,  year: 2010},
 {day: 4, month: 1,  length: 175,  year: 2010},
 {day: 1, month: 2,  length: 225,  year: 2010},
 {day: 2, month: 2,  length: 250,  year: 2010},
 {day: 3, month: 2,  length: 325,  year: 2010},

 {day: 1, month: 1,  length: 225,  year: 2011},
 {day: 1, month: 1,  length: 150,  year: 2011},
 {day: 1, month: 1,  length: 190,  year: 2011},
 {day: 1, month: 2,  length: 210,  year: 2011},
 {day: 2, month: 2,  length: 110,  year: 2011},
 {day: 3, month: 2,  length: 160,  year: 2011},
 {day: 4, month: 2,  length: 190,  year: 2011},
]

在这种情况下，我想创建一个新数组，其中两个数组保存一个月的平均长度。例如：

var newData = [ [137.5, 266.7], [183.33, 167.5] ]

其中 newData[0][1] 是 2010 年第 1 个月的平均长度。

我在以一种很好的方式合并它时遇到了一些问题。我可以创建长度的总和，但除以总和很困难。我的代码是：

data.forEach(function (el) {
   for (var j = 0; j <= 3; j++) {
     if (el.year === 2010 + j) {
       for (var i = 1; i <= 2; i++) {
                if (el.month === i) {
                    var oldLength = dataNew[j][i - 1] || 0;
                    var newLength = el.length + oldLength;
                    dataNew[j][i - 1] = newLength;
                }
        }
     }
   }
});

如何调整此函数，使其在 newData 中保存平均值而不是总和。

Answer 1

您可以使用 d3.js 本身来方便您的工作并使您的代码更具可读性。使用 d3.nest()

var data =  [
 {day: 1, month: 1,  length: 100,  year: 2010},
 {day: 2, month: 1,  length: 125,  year: 2010},
 {day: 3, month: 1,  length: 150,  year: 2010},
 {day: 4, month: 1,  length: 175,  year: 2010},
 {day: 1, month: 2,  length: 225,  year: 2010},
 {day: 2, month: 2,  length: 250,  year: 2010},
 {day: 3, month: 2,  length: 325,  year: 2010},

 {day: 1, month: 1,  length: 225,  year: 2011},
 {day: 1, month: 1,  length: 150,  year: 2011},
 {day: 1, month: 1,  length: 190,  year: 2011},
 {day: 1, month: 2,  length: 210,  year: 2011},
 {day: 2, month: 2,  length: 110,  year: 2011},
 {day: 3, month: 2,  length: 160,  year: 2011},
 {day: 4, month: 2,  length: 190,  year: 2011},
]

var nest = d3.nest()
    .key(function(d){return d.year})
    .key(function(d){return d.month})
    .rollup(function(d){
        return d3.mean(d, function(g){return g.length});
    })
    .entries(data)

console.log(nest[0].values[0]) // 137.5

这是一个有效的 fiddle

将对象数组操作为具有平均数的新数组。

Manipulate array of objects to new array with average number.

javascript

arrays

d3.js