Why is this function returnin nil? Is it because the D.S is lazy?

Question

(defn get-doc-list [a-term]
     (map #(Integer/parseInt %)(take-nth 3 (take (* 3 3)(rest (rest a-term))))))

This function works on small lists but returns and empty sequence on larger ones. What is the problem?

Answer 1

This is certainly an input issue, as your function will parse at most 3 integers:

user=> (get-doc-list (repeat "1"))
(1 1 1)

And only return empty sequence if less than 3 strings are supplied:

user=> (get-doc-list ["1" "1"])
()

And throw an exception if a non string is given:

user=> (get-doc-list [1 1 1])
java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

I suggest checking what the input it "fails" with really is... maybe you are passing in something like [["1" "1" "1" ...]]

user=> (get-doc-list [["1" "1" "1"]])
()

This is a vector of vectors, as you can see nothing is processed so there is no error or sequence to generate.

Answer 2

The issue is your use of the take function, which returns only the first n items of the collection. Regardless of how big a collection you pass get-doc-list, only the first 9 get passed on to take-nth.