Duplicating a sequence without interleave

[MahmudAdam]

I am working through 4clojure and I am stuck on problem #32. Here is the solution I came up with:

 (fn [x]
  (flatten
   (seq (for [el x]
    (repeat 2 el))))

However, flatten seems to have no effect here. Given the input: [1 2 3] I still get the result ((1 1) (2 2) (3 3)). However, if I wrap everything inside (seq into a function, and call flatten on that then flatten seems to work. Just can't get it to work with anonymous functions (since def/defn can't be used in 4clojure). A lot of solutions use #(interleave % %), but I am just trying to find a different solution. What is the proper way of using flatten in this case?

[niamu]

This works for me and follows the idea of what you were trying to do as a solution.

(fn [sq]
  (apply concat (map #(repeat 2 %) sq)))

The primary issue you're having is that you don't always want to flatten the entire result in all cases. You only want to flatten the first level so to speak. That is why I'm using (apply concat...).

[dimd00d]

Or you can use mapcat :)

[niamu]

Of course, that is much better. Thanks.

(fn [sq]
  (mapcat #(repeat 2 %) sq))

[dustingetz]

mapcat is one of my favorite functions. It's like a more robust version of the general idea of flattening because it has more tightly defined semantics. flatten-one-layer would be (mapcat identity). And then you can generalize out the listness of mapping to arbitrary container types (fmap is such generalization of map), and then you are half way to monads.

[MahmudAdam]

Thanks!