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437 PurelyFunctional.tv Newsletter

Phone to letter translation

Phone keypads and rotary dials have little letters on them. Most numbers translate into letters. Of course, with only 10 digits, but 26 letters, there is a problem: the translation from letters to numbers is lossy. When translating from numbers to letters, there is always more than one possible interpretation.

Write a function that takes a string of digits and returns a collection of the possible strings of letters it corresponds to.

Examples

(digits->letters "22") ;=> ["aa" "ab" "ac" "ba" "bb" "bc" "ca" "cb" "cc"]

Here are the mappings you should use:

1: no letters
2: abc
3: def
4: ghi
5: jkl
6: mno
7: pqrs
8: tuv
9: wxyz
0: space

If a character appears in the input that does not have a mapping, it will appear in the output untranslated.

Thanks to this site for the problem idea, where it is rated Expert in JavaScript. The problem has been modified.

Please submit your solutions as comments on this gist.

To subscribe: https://purelyfunctional.tv/newsletter/

@KubqoA
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KubqoA commented Aug 5, 2021

(require '[clojure.math.combinatorics :as combo])

(def digit->letters {\2 "abc"
                     \3 "def"
                     \4 "ghi"
                     \5 "jkl"
                     \6 "mno"
                     \7 "pqrs"
                     \8 "tuv"
                     \9 "wxyz"
                     \0 " "})

(defn digits->letters [digits] (->> digits
                                    (map digit->letters)
                                    (filter seq)
                                    (apply combo/cartesian-product)
                                    (mapv #(apply str %))))

@igajsin
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igajsin commented Aug 7, 2021

There are 3 steps here:

  1. deconstruct a digit string into a list of letters.
  2. do a permutation on lists.
  3. construct strings from lists.
(def dict {
           :1 [""]
           :2 ["a" "b" "c"]
           :3 ["d" "e" "f"]
           :4 ["g" "h" "i"]
           :5 ["j" "k" "l"]
           :6 ["m" "n" "o"]
           :7 ["p" "q" "r" "s"]
           :8 ["t" "u" "v"]
           :9 ["w" "x" "y" "z"]
           :0 [" "]
           })

(defn letter->list [letter]
  ((keyword letter) dict))

(defn string->list [s]
  (str/split s #""))

(defn get-letters [digits]
  (map letter->list (string->list digits)))

(defn add1 [letter letters]
  (map #(list letter %)  letters))

(defn permutate2 [l1 l2]
  (mapcat #(add1 % l2) l1))

(defn msg [s l]
  (print (str s ": ")) (clojure.pprint/pprint l))

(defn permutate [letters]
  (cond
    (< (count letters) 2) letters
    (= (count letters) 2) (permutate2 (first letters) (second letters))
    :else (permutate2 (first letters) (permutate (rest letters)))))

(defn list->string [permutted]
  (map clojure.string/join
       (map flatten permutted)))

(defn digits->letters [digits]
  (list->string (permutate (get-letters digits))))

@KingCode
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KingCode commented Oct 6, 2021

(def dial {\1 "1", \2 "abc", \3 "def", \4 "ghi", \5 "jkl", \6 "mno", 
           \7 "pqrs", \8 "tuv", \9 "wxyz", \0 " "})

(defn digits->letters [digits]
  (map 
   (partial apply str)
   (loop [digs (-> digits reverse rest),
          acc  (->> digits last dial
                    (map list))]
     (if (empty? digs)
       acc
       (recur (rest digs) 
              (->> (dial (first digs))
                   (mapcat (fn [ltr]
                             (->> acc
                                  (map #(conj % ltr)))))))))))

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