Question

我试图在d3区域元素中返回渐变元素的平均值。如果我只是在循环中运行console.log(dataGroup[i].gradient)，它会很好地记录渐变。当我尝试+=时，它会告诉我cannot read property gradient of undefined。

我已将此功能撤出并粘贴下面的完整代码。问题调用和函数已在完整代码中标记。

// PROBLEM FUNCTION
function dataGroupGradient(dataGroup)
{
  var sum = dataGroup[0].gradient;
  for (var i = 1; i <= dataGroup.length; i++)
  {
    sum += parseFloat(dataGroup[i].gradient);  
  }
  return sum/dataGroup.length;
}

这里是Plunk

<!DOCTYPE html>
<meta charset="utf-8">
<style>

body {
    font: 12px Arial;
}

text.shadow {
  stroke: #fff;
  stroke-width: 2.5px;
  opacity: 0.9;
}

path { 
    stroke: steelblue;
    stroke-width: 2;
    fill: none;
}

.axis path,
.axis line {
    fill: none;
    stroke: grey;
    stroke-width: 1;
    shape-rendering: crispEdges;
}

.grid .tick {
    stroke: lightgrey;
    stroke-opacity: 0.7;
    shape-rendering: crispEdges;
}
.grid path {
          stroke-width: 0;
}

.area {
    stroke-width: 0;
}

</style>
<body>
<script src="http://d3js.org/d3.v3.min.js"></script>

<script>

var margin = {top: 30, right: 20, bottom: 35, left: 50},
    width = 600 - margin.left - margin.right,
    height = 270 - margin.top - margin.bottom;

var color = d3.scale.ordinal()
    .domain([-10,-5,0,5,10])
    .range(['#a1d99b','#c7e9c0','#fdd0a2','#fdae6b','#fd8d3c','#e6550d']);

var x = d3.scale.linear().range([0, width]);
var y = d3.scale.linear().range([height, 0]);

var xAxis = d3.svg.axis()
    .scale(x)
    .orient("bottom")
    .ticks(5);

var yAxis = d3.svg.axis()
    .scale(y)
    .orient("left")
    .ticks(5);

var area = d3.svg.area()
    .x(function(d) { return x(d.distance); })
    .y0(height)
    .y1(function(d) { return y(d.elevation); });

var valueline = d3.svg.line()
    .x(function(d) { return x(d.distance); })
    .y(function(d) { return y(d.elevation); });

var svg = d3.select("body")
    .append("svg")
        .attr("width", width + margin.left + margin.right)
        .attr("height", height + margin.top + margin.bottom)
    .append("g")
        .attr("transform", 
              "translate(" + margin.left + "," + margin.top + ")");

// function for the x grid lines
function make_x_axis() {
    return d3.svg.axis()
        .scale(x)
        .orient("bottom")
        .ticks(5)
}

// function for the y grid lines
function make_y_axis() {
  return d3.svg.axis()
      .scale(y)
      .orient("left")
      .ticks(5)
}

// PROBLEM FUNCTION
function dataGroupGradient(dataGroup)
{
  var sum = dataGroup[0].gradient;
  for (var i = 1; i <= dataGroup.length; i++)
  {
    sum += parseFloat(dataGroup[i].gradient);  
  }
  return sum/dataGroup.length;
}

// Get the data
d3.csv("data.csv", function(error, data) {
    data.forEach(function(d) {
        d.distance = +d.distance;
        d.elevation = +d.elevation;
        d.gradient = +d.gradient;
    });

    var dataGroup = d3.nest()
        .key(function(d) {
            return d.group;
        })
        .entries(data);

    dataGroup.forEach(function(group, i) {
      if(i < dataGroup.length - 1) {
        group.values.push(dataGroup[i+1].values[0])
      }
    })

    // Scale the range of the data
    x.domain(d3.extent(data, function(d) { return d.distance; }));
    y.domain([0, d3.max(data, function(d) { return d.elevation; })]);

    dataGroup.forEach(function(d, i){
        svg.append("path")
            .datum(d.values)
            .attr("class", "area")
            .attr("d", area);
        });

    // PROBLEM CALL
    svg.selectAll(".area")
        .style("fill", function(d) { return color(dataGroupGradient(d)); });

    // Add the valueline path.
    svg.append("path")
        .attr("d", valueline(data));

    // Add the X Axis
    svg.append("g")
        .attr("class", "x axis")
        .attr("transform", "translate(0," + height + ")")
        .call(xAxis);

    // Add the Y Axis
    svg.append("g")
        .attr("class", "y axis")
        .call(yAxis);

    // Add the text label for the X axis
    svg.append("text")
        .attr("transform",
              "translate(" + (width/2) + " ," + 
                             (height+margin.bottom) + ")")
        .style("text-anchor", "middle")
        .text("Distance");

    // Add the text label for the Y axis
    svg.append("text")
        .attr("transform", "rotate(-90)")
        .attr("y", 6)
        .attr("x", margin.top - (height / 2))
        .attr("dy", ".71em")
        .style("text-anchor", "end")
        .text("");

    // Add the title
    svg.append("text")
        .attr("x", (width / 2))     
        .attr("y", 0 - (margin.top / 2))
        .attr("text-anchor", "middle")
        .style("font-size", "16px")
        .style("text-decoration", "underline")
        .text("Elevation Graph");

});

</script>
</body>

Answer 1

您也可以使用更短的纯 d3方式，让您不必自己处理访问阵列：

function dataGroupGradient(dataGroup) {
    var sum = d3.sum(dataGroup, function(value) {
        return parseFloat(value.gradient);
    });

  return sum/dataGroup.length;
}

由于您似乎在计算渐变属性的平均值，因此可能会采用更简洁的方法：

function dataGroupGradient(dataGroup) {
    return d3.mean(dataGroup, function(value) {
        return parseFloat(value.gradient);
    });
}

Answer 2

这只是'<=而不是<问题。

对于任何数组，arr[arr.length]未定义。（arr[arr.length - 1]是最后一个元素。）

将您的功能更改为使用<，如下所示：

function dataGroupGradient(dataGroup)
{
  var sum = dataGroup[0].gradient;
  for (var i = 1; i < dataGroup.length; i++)
  {
    sum += parseFloat(dataGroup[i].gradient);  
  }
  return sum/dataGroup.length;
}

在d3对象数组上求和元素

2 个答案: