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Daily Challenge #9 - What's Your Number?

Can I have your number?

Write a function that accepts an array of 10 integers (between 0 and 9), that returns a string of those numbers in the form of a phone number.

The returned format must be correct in order to complete this challenge.
Don't forget the space after the closing parentheses!

Today's challenge comes from user xDranik on Codewars

Thank you to CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

Want to propose a challenge for a future post? Email with your suggestions!

Top comments (26)

_bigblind profile image
Frederik 👨‍💻➡️🌐 Creemers

I think this challenge description is very country specific, as every country has a different way of formatting these.

coreyja profile image
Corey Alexander

I agree with the other commentors that we need an example in the post to make it easier to understand! Especially with all the different ways telephone numbers are handled country to country.

Luckily the linked Codewars page has an example!

createPhoneNumber([1, 2, 3, 4, 5, 6, 7, 8, 9, 0]) // => returns "(123) 456-7890"
alvaromontoro profile image
Alvaro Montoro • Edited


I assumed the US phone number format that is (XXX) XXX-XXXX

const formatNumber = numbers => {

  let phoneNumber = "";

  // prerequisites:
  //   - input must be an array
  //   - with 10 elements
  //   - each element will be a single digit number
  if (
    Array.isArray(numbers) &&
    numbers.length === 10 &&
    numbers.every(n => n > -1 && n < 10)
  ) {
    // break the phone number into parts and generate the formmated string
    const areaCode = numbers.slice(0,3).join('');
    const firstPart = numbers.slice(3,6).join('');
    const secondPart = numbers.slice(6).join('');
    phoneNumber = `(${areaCode}) ${firstPart}-${secondPart}`;

  return phoneNumber;

Live demo on CodePen (with an alternative version in one line).

sturzl profile image

Why the one line solution?

alvaromontoro profile image
Alvaro Montoro • Edited

No real reason. Both do the same, the first one is more verbose and easier to understand. I deleted the one-line one to avoid any confusion.

ganderzz profile image
Dylan Paulus


import strutils
from sequtils import map

const phone_number = @[1, 2, 3, 4, 5, 6, 7, 8, 9, 0]

proc createPhoneNumber(numbers: seq[int]): string =
  if len(numbers) < 10:
    raise newException(ValueError, "invalid length provided to createPhoneNumber(), requires 10 digits")

  return "($#$#$#) $#$#$#-$#$#$#$#" % (p: int): string = intToStr(p))

# Run
echo createPhoneNumber(phone_number)
yzhernand profile image
Yozen Hernandez • Edited

Well I think I've found my new language I want to learn.

cecilelebleu profile image
Cécile Lebleu

Could you provide an example? The length and format tends to differ in different countries.
In Costa Rice, for example, it might be 8 or 11 numbers long; as in 8765-4321, or, with the country code, (+506) 8765-4321. Either way it’s not 10 characters.

jaloplo profile image
Jaime López

That's not the first time a challenge is not well explained. You need to grab the whole problem description here to let people give possible solutions. I like these challenges but need a good and clear description of the problem, why don't connect to codewars directly?

choroba profile image
E. Choroba
#! /usr/bin/perl
use warnings;
use strict;

sub phone_num {
    local $" = "";
    "(@_[0 .. 2]) @_[3..5]-@_[6..9]"

use Test::More tests => 1;
is phone_num(0 .. 9), '(012) 345-6789';

The special variable $" is used to separate arrays interpolated in double quotes. By default, it contains a space, but we need the numbers to be adjacent, so we set it locally (i.e. in a dynamic scope) to an empty string.

Another possible short solution is

sub phone_num {
    "(012) 345-6789" =~ s/(\d)/$_[$1]/gr

The substitution replaces each digit in the template string with the corresponding element of the @_ array which keeps the list of the subroutine arguments. /g means "global", it's needed to replace all the digits, not just the first one. The /r means "return" - normally, a substitution changes the bound left-hand side value, but with /r, it just returns the value.

yzhernand profile image
Yozen Hernandez • Edited

Nice, basically what I got. sprintf works as well.

#!/usr/bin/env perl

use strict;
use warnings;

sub createPhoneNumber {
    sprintf "(%s%s%s) %s%s%s-%s%s%s%s", @_;
    # local $" = "";
    # "(@_[0..2]) @_[3..5]-@_[6..9]";

print createPhoneNumber(1, 2, 3, 4, 5, 6, 7, 8, 9, 0) . "\n";

The regex solution was pretty interesting. Thanks for the explanation!

stvnwlsn profile image


#!/usr/bin/env python3

def createPhoneNumber(array_of_integers):
    return "(%s%s%s) %s%s%s-%s%s%s%s" % tuple(array_of_integers)

if __name__ == '__main__':
    print(createPhoneNumber([1, 2, 3, 4, 5, 6, 7, 8, 9, 0]))
stevemoon profile image
Steve Moon

Erlang: (in the REPL)

Num = [8,0,0,5,5,5,1,2,1,2].
io:format("(~B~B~B) ~B~B~B-~B~B~B~B~n", Num).
(800) 555-1212

Or you can use io_lib:format and assign the result to a variable instead.

Checking for valid input is fairly trivial, but the Erlang convention is to assume that something this far into the system is okay, and correct for the error by dying and letting the supervisor deal with it.

maskedman99 profile image
Rohit Prasad


var = input("Enter 10 integers: ")
var = list(var)

if len(var) != 10:
        print("Invalid Entry")
        print('(',var[0],var[1],var[2],') ',var[3],var[4],var[5],'-',var[6],var[7],var[8],var[9], sep='')

anyone's skin crawling?

peter279k profile image

Here is the simple solution with PHP:

function createPhoneNumber($numbersArray) {
    $format = "(%s%s%s) %s%s%s-%s%s%s%s";

    return sprintf(
      $numbersArray[0], $numbersArray[1], $numbersArray[2], $numbersArray[3],
      $numbersArray[4], $numbersArray[5], $numbersArray[6], $numbersArray[7],
      $numbersArray[8], $numbersArray[9]
margo1993 profile image
package utils

import (

func FormatPhoneNumber(phoneNumberArray []int) (string, error) {

    if len(phoneNumberArray) != 10 {
        return "", errors.New("Array length must be 10")

    return fmt.Sprintf("(%s) %s-%s", intArrayToString(phoneNumberArray[:3]), intArrayToString(phoneNumberArray[3:6]), intArrayToString(phoneNumberArray[6:])), nil

func intArrayToString(intArray []int) string {
    number := ""

    for _, num := range intArray {
        number += strconv.Itoa(num)

    return number
thefern profile image
Fernando B 🚀 • Edited

I am not a big fan of code challenges in this format. For one people can see other answers. That can skew your thinking before even attempting the challenge. Secondly I think you should submit your own answers on codewars, and then link it here. Only people that has passed the challenge would be able to see it.

@ben definitely need a spoiler tag.

craigmc08 profile image
Craig McIlwrath

Haskell (US formatting):

formatNumber :: [Int] -> Maybe String
formatNumber xs
  | length xs /= 10 = Nothing
  | not $ and $ (map (<10) xs) ++ (map (>=0) xs) = Nothing 
  | otherwise = Just $ "(" ++ (showSlice 0 3 xs) ++ ") " ++ (showSlice 3 3 xs) ++ "-" ++ (showSlice 6 4 xs) 
  where showSlice from len = concat . map show . take len . drop from
wolverineks profile image
Kevin Sullivan • Edited

const iterative = (input: number[], schema: string = "(###) ###-####") => {
  let result = schema;
  input.forEach(number => (result = result.replace("#", String(number))));

  return result;

const recursive = (
  input: number[],
  schema: string = "(###) ###-####",
  result: string = schema
): string => {
  const [head,] = input;
  if (head === undefined) return result;

  return recursive(rest, schema, result.replace("#", String(head)`));

export const createPhoneNumber = {