Question

Given the following function which takes two parameters (csv-data1 and csv-data2, vectors of vectors with only two lines and a guaranteed mismatch so will get cmp-val in my output).

(defn test-key-inclusion
    "Accepts csv-data param and an index, a second csv-data param and an index,
     and searches the second csv-data instances' rows (at index) to see if
     the first file's data is located in the second csv-data instance."

    [csv-data1 pkey-idx1 csv-data2 pkey-idx2 lnam-idx fnam-idx]

    (reduce
        (fn [out-log csv-row1]
            (let [cmp-val (nth csv-row1 pkey-idx1 nil)
                  lnam (nth csv-row1 lnam-idx nil)
                  fnam (nth csv-row1 fnam-idx)
                  temp-rc (first (key-pres? cmp-val pkey-idx2 csv-data2))]

                (if-not (nil? cmp-val)
                    (concat out-log (sorted-map cmp-val (vector lnam fnam))))))
         {}
         csv-data1))

With this conditional in the function,

(if-not (nil? cmp-val)
  (concat out-log (sorted-map cmp-val (vector lnam fnam))))))

reduce returns nil.

Why is that?

Answer 1

Because if cmp-val is nil then you return nil from your reduce function.

Conditional should be

(if cmp-val 
    (concat out-log (sorted-map cmp-val (vector lnam fnam))))) 
     out-log)

Note that if-not (nil? foo) is equivalent to if foo