nested arrays in d3.js

Question

I have seen questions about dealing with “nested json” in d3.js and have seen that the proposed solutions are mostly to “flatten” the data (meaning creating a new variable and pushing in the data in desired level) Here two examples:

e.g. 1 and
e.g. 2

My question is : can one use one of the various array functions to access the level of the data without creating a new variable ? Handling arrays

I have an array which looks like that:

var dataset = 
 {"directed": false, 
  "graph": [], 
  "nodes": 
 [
 {"Region": "X15", "Group": "EU", "id": "BE"}, 
{"id": "FR"}, 
{"id": "BG"}, 
{"id": "DK"}, 
{"id": "HR"}, 
{"id": "DE"}
], 
"links": 
 [
 {"source": 0, "target": 0, "weight": 0}, 
 {"source": 0, "target": 1, "weight": 130}, 
 {"source": 0, "target": 2, "weight": 1}, 
 {"source": 0, "target": 3, "weight": 36}, 
 {"source": 0, "target": 4, "weight": 4}, 
 {"source": 0, "target": 5, "weight": 117}
 ], 
 "multigraph": false}

I have tried the following

and Iwould like to access the weight (calculating its maximum with d3.max)

dataset.links // gives me link 
dataset.links[0].weight // gives me the weight of first but can't get the index to work

when using the array functions I can't get elements lower than that

 d3.entries(dataset)
 d3.values(dataset.links)

what am I missing ?

I hope the question makes sense.

P.S: if one has to flatten the array, is there any advantage of doing it in javascript or is it the same if I create the array (e.g. in python) and pass it to javascript

dataset is not an array, it is an object. Array manipulation won't work on it. What you're doing on the bottom is turning it into an array, but I'm not sure what you want to do with it. — ckersch
– ckersch, Commented Jun 17, 2013 at 21:10
thanks. I see now the difference. I just want to access the weight for example to calculating its maximum. any idea how I can do it ? — user1043144
– user1043144, Commented Jun 17, 2013 at 21:21

ckersch · Accepted Answer · 2013-06-17 21:32:07Z

3

After reading over your post again, I think I know what your trying to do.

If you want to access the weights column with something like d3.max, you can use an accessor function.

An accessor function specifies how you want to look through an item in the array. In your case, you want a function that looks at the weight field in each link in dataset.links. With the link as an input, this function is quite simple:

function getWeight(link){return link.weight}

We can then use this function as the accessor function to get the max of the weight fields in dataset.links, as follows:

var maxWeight = d3.max(dataset.links, getWeight)

This uses the general form of the d3.max command, d3.max(array [,accessorFcn]).

I've set up a fiddle that does this and then alerts the max weight.

edited Jun 17, 2013 at 21:32

answered Jun 17, 2013 at 21:27

ckersch

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2 Comments

user1043144 Over a year ago

how can I use the function to retrieve the actual values of weights (for example in another variable?)

ckersch Over a year ago

It would depend on the structure of your variables. For example, if links gives the IDs of a bunch of variables, and the weights are in some database corresponding to the IDs, you can create an accessor function to look for the IDs in the database, and then grab the weight associated with the ID.

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