Recursive pattern match on string

Questions & Help Troubleshooting
1

Hello,

This is my first post here in the Clojure Forum.
I am currently trying to do my first steps in Clojure Programming by solving Clojure Koans.
Currently I ran into a problem while trying to complete a recursive pattern match on a string/ character list.
The main problem here seem to be type mismatches. However, the compiler output is not really helpful.
Any input or suggestions are appreciated!

    (ns rna-transcription
    (:require [clojure.string :as str]))

    (defn to-rna [dna] 
      (loop [dna dna rna[]]
        (if (empty dna)
           (str rna))
       (recur 
         (rest dna)
         (conj rna (    
           (str/replace (str(first dna)) "C" "G")     
           (str/replace (str(first dna)) "G" "C") 
           (str/replace (str(first dna)) "A" "U") 
           (str/replace (str(first dna)) "T" "A"))))))
2

There are couple of problems here:

  1. Format code properly
  2. Your if doesn’t have the else branch (check parentheses)
  3. Check precisely what do you conj to rna (check parentheses, currently it looks like: ((str/replace...) (....) ...))
  4. You intend to call str/replace 4 times in a row on the same input (remember, the last call will be returned). Possibly you want conditional replacement, check condp ((condp = (first dna) ...))
  5. And last… there is a function for that! :slight_smile: Try to find it in clojure.str namespace. One function to replace characters in given string: (str/... "CATG ...") ;; => "GUAC"

Good luck!

3

Ok, thank you!
I was partially able to follow your instructions.

  1. Placing the parenthesis correctly around the else clause: Yes, I also had the idea this is wrong (double checked Russ Olson’s book “Getting Clojure”)
  2. Instead of condp, I just made use of cond to achieve a simple pattern match.
  3. I still would like to stick to the idea of recursion instead of jumping to a map function.
    So here is an improved version, but there still seems to be a major pain-point:

(ns rna-transcription)

(defn to-rna [dna] 
   (loop [dna dna rna[]]
    (if (empty dna)
    (rna)
   (recur 
    (rest dna)
    (str (
      [first dna]    
      (cond 
       (= (first dna) \G) "C" 
       (= (first dna) \C) "G" 
       (= (first dna) \A) "U" 
       (= (first dna) \T) "A" 
       :else)
       (rna)))))))
4

A couple more comments… You are trying to call [first dna] as a function, with the results of your cond expression as arguments. That is almost certainly not what you want, and the compiler will complain, too.
Instead of repeating the (= (first dna) \...) bit, you could just do

(case (first dna)
  \G  "C"
  \C  "G"
  \A "U"
  \T  "A"
  (rna))

and so on.

5

I’m not entirely sure what you’re trying to achieve, but would something along this lines be close?

(defn dna->rna [dna]
  (let [transc {\G \C
                \C \G
                \A \U
                \T \A}]
    (apply str
           (reduce (fn[acc e]
                     (conj acc (transc e)))
                   [] dna))))

(dna->rna "GCCGTCCAAC")
;; => "CGGCAGGUUG"
6

Thank you for your reply!
Using a map / reduce function is of course another approach that is much more practicable. I will make use of these functions in the future. This time, I just wanted to stick to the recursive approach and finally got it to work.

7

I was finally able to solve it with a purely recursive approach!
Besides the wrong parenthesis around the else clause, my main problem was indeed proper handling of the string / character types.
The final fix I had to apply was a type conversion of the function input argument to string at the beginning of the loop (no idea why it was not a string to begin with)!
Now it looks like this:

(ns rna-transcription)

(defn to-rna [dna] 
 (loop [dna (str dna) rna ""]
  (if (empty? dna)
    rna
  (recur 
    (rest dna) 
        (str rna (cond 
              (= (first dna) \G) "C" 
              (= (first dna) \C) "G" 
              (= (first dna) \A) "U" 
              (= (first dna) \T) "A" 
              :else (throw (AssertionError. "Wrong input"))))))))
8

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