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# ericnormand/00 Phone to letter translations.md

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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.

### jaihindhreddy commented Aug 3, 2021

```;; deps.edn coord {org.clojure/math.combinatorics {:mvn/version "0.1.6"}}
(require '[clojure.math.combinatorics :as comb])

(let [mapping {\1 "1"    \2 "abc" \3 "def"
\4 "ghi"  \5 "jkl" \6 "mno"
\7 "pqrs" \8 "tuv" \9 "wxyz"
\0 " "}]
(defn digits->letters [msg]
(->> (map #(or (mapping %) [%]) msg)
(apply comb/cartesian-product)
(mapv #(apply str %)))))```

### kolstae commented Aug 3, 2021

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

(defn digits->letters [s]
(->> s
(map #(digit->letters % (str %)))
(reduce #(for [a %1 b %2] (str a b)) [""])))```

I'm relatively new to Clojure, so it's quite likely I'm missing something obvious.

I thought I had a solution, until I realized clojure.core/for was a macro. So then I thought I had I bad solution, to make a macro that allows me to have a "variadic for" (not really the best name, just couldn't think of a better one for now). I think I might be close, but maybe it's not actually possible like this. I know you wouldn't actually want to use a macro for this problem as there are other ways of achieving the same result, but I am curious how and if it can be done for the sake of learning.

```(def keypad-letters
{:1 nil    :2 "abc" :3 "def"
:4 "ghi"  :5 "jkl" :6 "mno"
:7 "pqrs" :8 "tuv" :9 "wxyz"
:0 " "})

(defn vector-for-the-for-macro [message]
(into []
(mapcat
message
(range))))

(comment

y (:3 keypad-letters)] (str x y))

z (:4 keypad-letters)] (str x y z))

a (:2 keypad-letters)] (str x y z a))

a (:2 keypad-letters)] (apply str [x y z a]))

(for [digit "223"]
(for [x ((keyword (str digit)) keypad-letters)]
x))

(map #(keypad-letters % (keyword (str %))) "23")
(map #(keypad-letters % [(str "i" %2)]) "23" (range))
(mapcat #(keypad-letters % [(str "i" %2) ((keyword (str %)) keypad-letters)]) "23" (range))
(mapcat #(keypad-letters % [(symbol (str "i" %2)) ((keyword (str %)) keypad-letters)]) "23" (range))
(into []
(mapcat
"23"
(range)))

(vector-for-the-for-macro "262")
(filter symbol? (vector-for-the-for-macro "262"))

[]
'(for ~(vector-for-the-for-macro "262")
(apply str (~(filter symbol? (vector-for-the-for-macro "262"))))))

[binding-vector]
'(for ~binding-vector
(apply str ((filter symbol? ~binding-vector)))))

,)

#_(digits->letters "222")
```

### mchampine commented Aug 4, 2021

```(defn combo [n items]
(cond
(= n 0) '(())
(empty? items) '()
:else (concat (map #(cons (first items) %)
(combo (dec n) (rest items)))
(combo n (rest items)))))

(defn digits->letters [ds]
(let [ds' (s/replace ds "1" "")
dial {\1 nil \2 "abc" \3 "def" \4 "ghi" \5 "jkl"
\6 "mno" \7 "pqrs" \8 "tuv" \9 "wxyz" \0 " "}
pstr (apply str (map (fn [v] (get dial v v)) ds'))]
(vec (distinct (map #(apply str %) (combo (count ds') pstr))))))```

### ZaymonFC commented Aug 5, 2021

```;; combo is clojure.math.combinatorics
(def phone-letter-mapping
{\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 number-to-list [xs]
(->> xs
(map (fn [x] (get phone-letter-mapping x [x])))
(apply combo/cartesian-product)
(map (partial apply str))))
```

### mcuervoe commented Aug 5, 2021

```(defn digits->letters [digits-str]
(let [mapping {\1 [\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 [\space]}]
(->>
(loop [digits digits-str
results [nil]]
(if (= 0 (count digits))
results
(let [chars (get mapping (first digits))]
(recur
(next digits)
(for [c chars, result results]
((fnil conj []) result c))))))
(map (fn [chars] (apply str chars))))))```

```(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 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)))

(map #(list letter %)  letters))

(defn permutate2 [l1 l2]

(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))))
``````

```(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)))))))))))```