Question

I’ve defined the following 3 functions using the leiningen REPL:

(defn rand-int-range [floor ceiling] (+ floor (rand-int (- ceiling floor))))

(defn mutate-index
  "mutates one index in an array of char and returns the new mutated array"
  [source idx]
  (map
    #(if (= %1 idx)
       (char (+ (int %2) (rand-int-range -3 3)))
       %2)
    (iterate inc 0)
    source))

(defn mutate-string
  [source]
  (str
    (mutate-index
      (.toCharArray source)
      (rand-int (alength (.toCharArray source))))))

When I run (mutate-string "hello"), instead of the REPL printing out the mutated string, it is printing out clojure.lang.LazySeq@xxxxxx where the ‘xxxxx’ is a random sequence of numbers and letters. I would expect it to instead print something like “hellm” Is this really giving me a string back like I think? If it is, how can I get the REPL to show me that string?

Answer 1

1) To convert a sequence of characters (which is what mutate-index returns) to a string, use apply str rather than just str. The latter operates on objects, not sequences.

(str [\a \b \c])
=> "[\\a \\b \\c]"

(apply str [\a \b \c])
=> "abc"

2) Strings are seqable, meaning you can use sequence functions like map and filter on them directly without stuff like .toCharArray.

3) Consider using map-indexed or StringBuilder to accomplish what you’re trying to do:

(apply str (map-indexed (fn [i c] (if (= 3 i) \X c)) "foobar"))
=> "fooXar"

(str (doto (StringBuilder. "foobar") (.setCharAt 3 \X)))
=> "fooXar"