Javascript 将对象数组操纵为具有平均数的新数组。

我有一个我想要操作的数组，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},
]

在本例中，我想创建一个新数组，其中两个数组的平均长度为一个月。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

我有一些问题，结合在一个好的方式。我可以创建长度之和，但要划分长度之和很困难。我的密码是：

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;
                }
        }
     }
   }
});

data.forEach（函数（el）{
对于（var j=0；j您可以使用d3.js本身来简化您的工作并使您的代码更具可读性。使用
d3.nest（）

这是一个有效的
所以原始数据的

日部分无关紧要？输入数据总是按月份和年份“排序”吗？输入数据从哪里来（可以修改该部分以立即提供所需的格式）？太棒了！我不知道nest（）函数。谢谢。当我需要处理其他数据集时，这将非常有用。

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