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:

• eg 1 and
• eg 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.

PS: 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 (eg in python) and pass it to javascript

Answer 1

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.