如何获取for循环JavaScript中所有值的总和？

Question

I tried calculate distance of truck marker on google map.

var Total=0;
var json = result.d;
obj = JSON.parse(json);
for (var i = 0; i < obj.length; i++) 
{
var LatLng1 = new google.maps.LatLng(obj[i].Lat, obj[i].Lng);
var LatLng2 = new google.maps.LatLng(obj[i+1].Lat, obj[i+1].Lng);
var Distance=(google.maps.geometry.spherical.computeDistanceBetween(LatLng1, LatLng2) / 1000).toFixed(2);

 Total+=Distance;
  alert(Total); //It is working but.Like this : 0.01 0.22 0.03 0.045
 // I want to be 0.01+0.022+0.03+....=Total
  }

  alert(Total);  //Does not working !!

I get this error at console : TypeError: obj[(i + 1)] is undefined.

Where is my wrong ?

Answer 1

Your loop condition is i < obj.length

However, later in your code, you're using obj[i+1] .

As a consequence, on your last interation, you'll be using obj[obj.length] .

To solve your problem, you either have to do one less iteration or to have a special case for the end of your array. The choice depends on what you seek to achieve (eg : you may want to have the distance between the last and the first).

Answer 2

Try this code:

var Total=0;
var json = result.d;
obj = JSON.parse(json);
for (var i = 0; i < obj.length -1; i++) 
{
   var LatLng1 = new google.maps.LatLng(obj[i].Lat, obj[i].Lng);
   var LatLng2 = new google.maps.LatLng(obj[i+1].Lat, obj[i+1].Lng);
   var Distance=(google.maps.geometry.spherical.computeDistanceBetween(LatLng1, LatLng2) / 1000);

   Total+=Distance;
}

  alert(Total);  //It will work

The reason it wasn't working earlier is that you got an exception everytime you accessed element at obj.length index. I guess you didn't look into developer tools in your browser to find the exception.

Answer 3

As said fxm you loop one extra time that breaks for the upper boundary of your loop

So I suggest you change this

for (var i = 0; i < obj.length; i++)

to this

for (var i = 0, l = obj.length - 1; i < l; i++)

Answer 4

Your are trying to access obj[i+1] but i+1 = length in the last iteration. You should iterate from 0 to obj.lenght-1.

Your code must be:

var Total = 0;
var json = result.d;
obj = JSON.parse(json);
for (var i = 0; i < obj.length-1; i++) { // Max is length-1 !
    var LatLng1 = new google.maps.LatLng(obj[i].Lat, obj[i].Lng);
    var LatLng2 = new google.maps.LatLng(obj[i + 1].Lat, obj[i + 1].Lng);
    var Distance = (google.maps.geometry.spherical.computeDistanceBetween(LatLng1, LatLng2) / 1000).toFixed(2);

    Total += Distance;
    alert(Total);
}

alert(Total);