Question

My question is how can I extract a number of elements from a sequence without knowing how many at compile time. Using partial came to mind, but I’ve been having difficulty pulling out elements instead of sequences.

I would like to achieve the sequence generated by the interleave below, but without coding in a finite number of map forms.

(def s1 [[:000-00-0000 "TYPE 1" "JACKSON" "FRED"]
         [:000-00-0001 "TYPE 2" "SIMPSON" "HOMER"]
         [:000-00-0002 "TYPE 4" "SMITH" "SUSAN"]])

(interleave (map #(nth % 0 nil) s1) 
            (map #(nth % 2 nil) s1) 
            (map #(nth % 3 nil) s1))

(:000-00-0000 "JACKSON" "FRED" 
 :000-00-0001 "SIMPSON" "HOMER" 
 :000-00-0002 "SMITH" "SUSAN")

If I do the following

(def cols [0 2 3])

(defn f1
    [s1 col]
    (nth s1 col nil))

(map (partial f1 s1) cols)

I get

([:000-00-0000 "TYPE 1" "JACKSON" "FRED"]\
 [:000-00-0002 "TYPE 4" "SMITH" "SUSAN"] nil)

I believe I know why this is happening. The cols param is acting like a sequence selector rather than an element selector within a sequence. I would like to pull several elements out of each sequence. How can I pull elements out of each sequence?

Thank You.

Answer 1

You can use the get-in function to access nested data structures, like so:

(def cols [0 2 3])

(defn f1 
  [s1 col] 
  (map #(get-in s1 [% col] nil) (range (count s1))))

(apply interleave (map (partial f1 s1) cols))

(:000-00-0000 "JACKSON" "FRED" 
 :000-00-0001 "SIMPSON" "HOMER" 
 :000-00-0002 "SMITH" "SUSAN")