Stratify > Hierarchy

[DDDgfx]

Just a comment here. Spent a long time today trying to make a layout from flat data by using d3.stratify but running into trouble when I then ran the output through d3.hierarchy. Its not clear in the docs that d3.stratify's output is a legit hierarchy object that you can run .count() and .sum() on. Since sum and count were not listed under stratify, I assumed I had to go through hierarchy to get them. The result was double nested data objects.
It seem like in fact, stratify is the same as hierarchy, with a preceeding d3.nest stage. Is that true?

[mbostock]

d3.stratify and d3.hierarchy do the same thing: they return a root node. The difference is that d3.hierarchy constructs a hierarchy (a root node) from a hierarchical input data structure (typically derived from JSON), whereas d3.stratify constructs a hierarchy from a tabular input data structure (typically derived from CSV or TSV).

Both d3.stratify and d3.hierarchy are documented in the Hierarchy section of the README, which includes an example of converting a table to a hierarchy:

var root = d3.stratify()
    .id(function(d) { return d.name; })
    .parentId(function(d) { return d.parent; })
    (table);

And it explicitly says “This hierarchy can now be passed to a hierarchical layout, such as d3.tree, for visualization.” The constructor also says it “generates a new hierarchy.”

Apologies for the confusion, but I’m not sure how I can make it clearer…

[DDDgfx]

Yes. I see now. Also - realize that even after a hierarchy has been pushed through a layout such as d3.tree you could still call .count() or .sum(). But I guess one way that a hierarchy from d3.stratify would be different than a hierarchy from d3.hierarchy would be the presence of reserved ID property. Thank you as always Mike.

[mbostock]

Thanks for the feedback; related #61 on that node.id issue.

[DDDgfx]

#62 Understanding this would also have helped.