how to paritally flatten a list in clojure?

3
votes

Let's say I have a data structure like so:

[[1 2 3] [4 5 6] [[7 8 9] [10 11 12]]]

And what I want to end up with is:

[[1 2 3] [4 5 6] [7 8 9] [10 11 12]]

Is there any function that does this automatically?

Basically I'm converting/transforming a SQL result set to CSV, and there are some rows that will transform to 2 rows in the CSV. So my map function in the normal case returns a vector, but sometimes returns a vector of vectors. Clojure.data.csv needs a list of vectors only, so I need to flatten out the rows that got pivoted.

3
clojure
My preferred solution would be always to produce a vector of vectors; the single row results would then be singleton vectors. Whatever time / LOCs you lose in performing the extra wrapping, you'll get right back in being able to process the data more simply for output. Plus it feels cleaner and more robust. Just my €.02. - Michał Marczyk

3 Answers

2
votes

Mapcat is useful for mapping where each element can expand into 0 or more output elements, like this:

(mapcat #(if (vector? (first %)) % [%]) data)

Though I'm not sure if (vector? (first %)) is a sufficient test for your data.

0
votes

A different approach using tree-seq:

(def a [[1 2 3] [4 5 6] [[7 8 9] [10 11 12]]])

(filter (comp not vector? first) 
   (tree-seq (comp vector? first) seq a))

I am stretching to use tree-seq here. Would someone with more experience care to comment on if there is a better way to return only the children other than using what is effectively a filter of (not branch?)

0
votes

Clojure: Semi-Flattening a nested Sequence answers your question, but I don't want to mark this question as a duplicate of that one, since you're really asking a different question than he was; I wonder if it's possible to move his answer over here.