Words On A Telephone Keypad

January 13, 2017

The digits on a telephone keypad are associated with letters; for instance, the digit 2 is associated with the letters A, B and C, and the digit 7 is associated with the letters P, Q, R and S. Thus, a word can be converted to its numeric equivalent; for instance, PRAXIS can be converted to the number 772947. The conversion is not necessarily unique, so ACT, BAT and CAT all convert to 228.

Your task is to write a program that takes a number, such as 228, and returns a list of all the words in a dictionary that are represented by that number. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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Posted by programmingpraxis
Filed in Exercises
3 Comments »

3 Responses to “Words On A Telephone Keypad”

  1. Jamie Hope said
    January 13, 2017 at 4:46 PM

    Here’s one that goes the other way, turning the provided number into a string regex, and then using grep to search /usr/share/dict/words (via a Kawa process quasi-literal).

    (import (srfi 13))                      ; for string-join
(require 'regex)                        ; for regex-split

(define letters
  ["[+]" "1" "[abc]" "[def]" "[ghi]" "[jkl]" "[mno]" "[pqrs]" "[tuv]" "[wxyz]"])

(define (digits n::integer)
  (let loop ((ds '()) (n n))
    (if (< n 10)
        (cons n ds)
        (loop (cons (remainder n 10) ds) (div n 10)))))

(define (num->regex n::integer)
  (string-join (map letters (digits n)) ""))

(define (paragraph->lines s::String)
  (regex-split #/\n/ (*:trim s)))

(define (lookup n::integer)
  (paragraph->lines
   &`{grep -i "^&[(num->regex n)]$" /usr/share/dict/words}))

(for-each
 (lambda (arg)
   (format #t "~a: ~a~%" arg (lookup (string->number arg))))
 (cdr (command-line)))

    $ kawa /tmp/words.scm 772947 228
    772947: (praxis)
    228: (Abu abu act Bat bat Cat cat)

  2. matthew said
    January 14, 2017 at 11:42 PM

    @Jamie: Great idea! Here’s a node.js version:

    const readline = require('readline');
const s = process.argv[2] || "43556";
const map = ['[+]','1','[abc]','[def]','[ghi]','[jkl]','[mno]','[pqrs]','[tuv]','[wxyz]'];
let restr = "";
for (const c of s.split('')) {
    restr += map[Number(c)] || "";
}
restr = "^" + restr + "$"; // Match whole line

const re = new RegExp(restr,"i"); // Case insensitive

const rl = readline.createInterface({
  input: process.stdin;
});

rl.on('line', function(line){
    if (re.test(line)) console.log(line);
})

    $ cat /usr/share/dict/words | nodejs --harmony phone.js
hello
$ cat /usr/share/dict/words | nodejs --harmony phone.js 666
Mon
Ono
mom
moo
non
  3. r. clayton said
    January 17, 2017 at 9:31 AM

    A solution in bash.

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