# Daily Challenge #50 - Number Neighbor

Daily Challenge (273 Part Series)

Given a phone number of integer length N (1 ≤ N ≤ 10) as a number string (e.g. 555-555-5555 would be passed as "5555555555"), return an array of all phone numbers as number strings that would be considered neighbors of that phone number (which is any numbers ±1).

For example: someone with the phone number 555-555-5555 has neighbors 555-555-5554 and 555-555-5556.

Want to propose a challenge idea for a future post? Email yo+challenge@dev.to with your suggestions!

This challenge comes from SaladFork on CodeWars. Thank you to CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

### Discussion Huh, this one was harder than I expected it to be... that probably means I missed something 🤣

Also, TIL that setting a character of a string in JS with square brackets doesn't throw an error - but also doesn't do anything! example:

let myStr = "abc123"
myStr = 'X'
// myStr is unchanged here!


Solution in javascript:


const numberNeighbor = (number) => {
return [...number].reduce((arr, num, i) => {
num = Number(num)

if(num - 1 >= 0) {
arr.push([
number.slice(0, i),
num - 1,
number.slice(i+1)
].join(""))
}

if(num + 1 <= 9) {
arr.push([
number.slice(0, i),
num + 1,
number.slice(i+1)
].join(""))
}

return arr
}, [])
}


Strings in javascript are immutable, so even if you change a character using the [] notation, it always returns a new string.

This instead work as you expected

// The string is transformed into an Array
const arrString = [..."abc123"]
arrString = "X"
// arrString is now ["X", "b", "c", "1", "2", "3"]


Makes sense - thanks! (forgot (or never really knew?) that strings in javascript were immutable)

// areaCode :: String -> String
const areaCode = str => str.substring(0,3);
// threeCode :: String -> String
const threeCode = str => str.substring(3,6);
// fourCode :: String -> String
const fourCode = str => str.substring(6);
// formatPhone :: String -> String
const formatPhone = str => ${areaCode(str)}-${threeCode(str)}-${fourCode(str)}; // getNeighbors :: String -> [Number] -> [String] const getNeighbors = number => [parseInt(number) + 1, parseInt(number) - 1] .map(neighbor => formatPhone(${neighbor}));


Attempting to learn and understand more functional paradigms. Is this right way to solve this problem functionally?

Way to go buddy!

From my perspective, to format the phone number in 3 segments, I would have used this instead of the 3 calls you made.

"use strict"

const phoneNumber = "123456789"

const formatPhoneNumber = (string, glue) => string.match(/.{1,3}/g).join(glue)

console.log(formatPhoneNumber(phoneNumber, "-")) // 123-456-789
console.log(formatPhoneNumber(phoneNumber, " ")) // 123 456 789
console.log(formatPhoneNumber(phoneNumber, ".")) // 123.456.789


This leverage the regular expressions to match three characters at most. I know regular expressions is not the easiest thing to use but in this case it can be quite handy. As it returns an array, joining the items will produce a string back so the signature of the function can be written as follow in a purely functional programming language like PureScript.

formatPhoneNumber :: String -> String -> String


Unfortunately, currying (partially applying parameters) is not built-in in JavaScript. You could implement your own curry function and use it on all your future function definitions. But this would be overkill for this challenge. But if you would ask me how I would implement a currying function in JavaScript, here is my take.

"use strict"

function curry(callable, ...initialParameters) {
if (typeof callable !== "function") {
throw new TypeError("Function expected as first argument")
}

if (parameters.length >= callable.length) {
return callable(...parameters)
}

return curry(callable, ...parameters)
}
}

// format :: String -> Number -> String -> String
const format = curry(function(glue, segments, phone) {
return phone.match(new RegExp(.{1,${segments}}, "g")).join(glue) }) // usFormat :: String -> String const usFormat = format("-", 3) // frenchFormat :: String -> String const frenchFormat = format(".", 2) console.log(usFormat("123456789")) // 123-456-789 console.log(frenchFormat("1234567890")) // 12.34.56.78.90  Last tip, when creating a function, try to put the data structure at the end, that way you can construct partial application of your function without pain, even in JavaScript. In this case, this means puting the phone number to format at the very end of the parameter's list in your function definition. Hope that helps you in your functional programing journey pal. Hey thanks for the response I will look at regexes next time for this kind of problem. Also I thought currying was supported in ES6? via const f = a => b=> a + b  What you did is indeed currying but it is not transparent in the sense that if you want to pass all three arguments at once you will have to make three calls. The curry implementation allows you to either pass all arguments at once like a regular function call or partially calling the function. I kinda dislike calling my function multiple times and the only two reasons are aesthetic and the fact that it is less natural to define functions like that but arrow functions syntax makes it a little bit cleaner. Here goes some ugly code! f=a=>{p=parseInt,l=(''+a).length,i=0,r=[a];while(i<l){r.push(...[('0'+(p(a)-(10**i))).slice(-l),''+(p(a)+(10**i))]);++i}return r}  It works like that: • Take the input's length • Add and subtract 10n (where n goes from 0 to the input's length - 1) to the input number • Add a trailing 0 if necessary • Return the built array! f('5555555555').join('\n'); "5555555555 5555555554 5555555556 5555555545 5555555565 5555555455 5555555655 5555554555 5555556555 5555545555 5555565555 5555455555 5555655555 5554555555 5556555555 5545555555 5565555555 5455555555 5655555555 4555555555 6555555555"  One line JS generate all possible neighbor number for any position. const allNumberNeighbor = number => [...number] .map(Number) .flatMap((n, i) => Number(n) && [ n + 1 <= 9 && Object.values({ ...number, [i]: n + 1 }), n - 1 >= 0 && Object.values({ ...number, [i]: n - 1 }), ]) .filter(Boolean) .map(n => n.join('')); allNumberNeighbor('555-555-555'); // return /* [ '655-555-555', '455-555-555', '565-555-555', '545-555-555', '556-555-555', '554-555-555', '555-655-555', '555-455-555', '555-565-555', '555-545-555', '555-556-555', '555-554-555', '555-555-655', '555-555-455', '555-555-565', '555-555-545', '555-555-556', '555-555-554', ]; */  My take on this challenge with JS const neighbors = number => { const format = n => { n = ${n}.padStart(10, '0');
return ${${n}.substr(0,3)}-${${n}.substr(3, 3)}-${${n}.substr(6, 4)};
};

const nArray = [...number].reduce((t, d, i) => {
const n1 = Number(number) + Math.pow(10, i);
const n2 = number - Math.pow(10, i);

t.push(format(n1));
t.push(format(n2));

return t;
}, [])

return nArray;
}

/**

neighbors("5555555555");

[
"555-555-5556",
"555-555-5554",
"555-555-5565",
"555-555-5545",
"555-555-5655",
"555-555-5455",
"555-555-6555",
"555-555-4555",
"555-556-5555",
"555-554-5555",
"555-565-5555",
"555-545-5555",
"555-655-5555",
"555-455-5555",
"556-555-5555",
"554-555-5555",
"565-555-5555",
"545-555-5555",
"655-555-5555",
"455-555-5555"
]

**/


My solution in js

const numberNeighbour = (num) => {
const neighbours = [];
num.split('').forEach((value, index) => {
if(parseInt(value) - 1 > 0) {
let prevNum = num.split('');
prevNum[index] = parseInt(value) - 1;
prevNum = prevNum.join('');
neighbours.push(prevNum.slice(0, 3) + "-" + prevNum.slice(3, 6) + "-" + prevNum.slice(6));
}
if(parseInt(value) + 1 < 10) {
let nextNum = num.split('');
nextNum[index] = parseInt(value) + 1;
nextNum = nextNum.join('');
neighbours.push(nextNum.slice(0, 3) + "-" + nextNum.slice(3, 6) + "-" + nextNum.slice(6));
}
});
return neighbours;
};

numberNeighbour('5555555555');
/*[
"455-555-5555",
"655-555-5555",
"545-555-5555",
"565-555-5555",
"554-555-5555",
"556-555-5555",
"555-455-5555",
"555-655-5555",
"555-545-5555",
"555-565-5555",
"555-554-5555",
"555-556-5555",
"555-555-4555",
"555-555-6555",
"555-555-5455",
"555-555-5655",
"555-555-5545",
"555-555-5565",
"555-555-5554",
"555-555-5556"
]*/
numberNeighbour('1111111111');
/*[
"211-111-1111",
"121-111-1111",
"112-111-1111",
"111-211-1111",
"111-121-1111",
"111-112-1111",
"111-111-2111",
"111-111-1211",
"111-111-1121",
"111-111-1112"
]*/
numberNeighbour('9999999999');
/*[
"899-999-9999",
"989-999-9999",
"998-999-9999",
"999-899-9999",
"999-989-9999",
"999-998-9999",
"999-999-8999",
"999-999-9899",
"999-999-9989",
"999-999-9998"
]*/


Simple ruby solution(with edge cases):

def neighbors str
num = str.to_i
res = case num
when 0
[num + 1]
when 9999999999
[num - 1]
when -Float::INFINITY..0, 9999999999..Float::INFINITY
raise 'Not a phone number'
else
[num - 1, num + 1]
end

res.map(&:to_s)
end


I liked the "any number can differ by 1" approach more - it's a but more work :)

Rust:

pub fn number_neighbors(number: u32) -> Vec<u32> {
let len = number.to_string().len() as u32;
(0..len)
.map(|delta| 10u32.pow(delta))
.flat_map(|delta| vec![number - delta, number + delta])
.collect()
}


And a test (I usually omit them from my submissions, but I think they have a place here):

#[test]
fn test_number_neighbors() {
assert_eq!(vec![221, 223, 212, 232, 122, 322], number_neighbors(222));
}


My take at the challenge written in Elm.

computeNeighbors : String -> Maybe (List Int)
computeNeighbors phoneNumber =
case String.toInt phoneNumber of
Just integerPhoneNumber ->
Just [integerPhoneNumber + 1, integerPhoneNumber - 1]
Nothing ->
Nothing


Python one-liner to the rescue again. (Horribly formatted this time tho)

f = lambda x: print(*[x[:len(x)-1]+str(int(x[len(x)-1])+1), x[:len(x)-1]+str(int(x[len(x)-1])-1)])

>>> f('555-555-5555')
555-555-5556 555-555-5554


any numbers ±1

🤔
Most people interpreted that as only a single number could vary at a time, others took the example too literally as the full number +- 1...

So I thought...

What if any numberS (plural) means any number of digits can vary...?

const neighbourSingle = (num) => [...new Set([ clamp(num-1), num, clamp(num+1) ])];
const clamp = (num) => Math.min(Math.max(num, 0), 9);
/*
neighbourSingle(5) === [4, 5, 6]
neighbourSingle(0) === [0, 1]
*/

const neighbours = (phoneString) => {
return phoneString.split("").reduce((a,b) => a.flatMap(c=> neighbourSingle(parseInt(b)).map(d=> c+d)) ,[""])
}
/*
neighbours("55")
(9) ["44", "45", "46", "54", "55", "56", "64", "65", "66"]

neighbours("555")
(27) ["444", "445", "446", "454", "455", "456", "464", "465", "466", "544", "545", "546", "554", "555", "556", "564", "565", "566", "644", "645", "646", "654", "655", "656", "664", "665", "666"]

neighbours("5555555555")
(59049) ["4444444444", "4444444445", "4444444446", "4444444454", "4444444455", "4444444456", "4444444464", "4444444465", "4444444466", "4444444544", "4444444545", "4444444546", "4444444554", "4444444555", "4444444556", "4444444564", "4444444565", "4444444566", "4444444644", "4444444645", "4444444646", "4444444654", "4444444655", "4444444656", "4444444664", "4444444665", "4444444666", "4444445444", "4444445445", "4444445446", "4444445454", "4444445455", "4444445456", "4444445464", "4444445465", "4444445466", "4444445544", "4444445545", "4444445546", "4444445554", "4444445555", "4444445556", "4444445564", "4444445565", "4444445566", "4444445644", "4444445645", "4444445646", "4444445654", "4444445655", "4444445656", "4444445664", "4444445665", "4444445666", "4444446444", "4444446445", "4444446446", "4444446454", "4444446455", "4444446456", "4444446464", "4444446465", "4444446466", "4444446544", "4444446545", "4444446546", "4444446554", "4444446555", "4444446556", "4444446564", "4444446565", "4444446566", "4444446644", "4444446645", "4444446646", "4444446654", "4444446655", "4444446656", "4444446664", "4444446665", "4444446666", "4444454444", "4444454445", "4444454446", "4444454454", "4444454455", "4444454456", "4444454464", "4444454465", "4444454466", "4444454544", "4444454545", "4444454546", "4444454554", "4444454555", "4444454556", "4444454564", "4444454565", "4444454566", "4444454644", …]
*/


This was a little harder, at least to me it was.

phone_number.go

package phonenumbers

// PhoneNeighbors returns all the phone numbers that are adjacent to the given phone number
func PhoneNeighbors(phone string) []string {
numbers := []string{}

if phone == "" {
return numbers
}

runes := []rune(phone)

for i, r := range runes {
// Copy the phone number so we can replace the current digit with its neighbors during the loop
instance := append(runes[:0:0], runes...)

// Get the neighbors of this digit
n := neighbors(r)

// Loop over the neighbors
for _, num := range n {
// Replace the digit at this index with the neighbor and append the result to the slice
instance[i] = num
numbers = append(numbers, string(instance))
}
}

return numbers
}

func neighbors(number rune) []rune {
switch number {
case '0':
return []rune{'8'}
case '1':
return []rune{'2', '4'}
case '2':
return []rune{'1', '3', '5'}
case '3':
return []rune{'2', '6'}
case '4':
return []rune{'1', '5', '7'}
case '5':
return []rune{'2', '4', '6', '8'}
case '6':
return []rune{'3', '5', '9'}
case '7':
return []rune{'4', '8'}
case '8':
return []rune{'0', '5', '7', '9'}
case '9':
return []rune{'6', '8'}
default:
return []rune{}
}
}



phone_number_test.go

package phonenumbers

import "testing"

var testCases = []struct {
description string
input       string
expected    []string
}{
{
"empty string",
"",
[]string{},
},
{
"single digit",
"5",
[]string{"2", "4", "6", "8"},
},
{
"many digits",
"4531",
[]string{
"1531",
"5531",
"7531",
"4231",
"4431",
"4631",
"4831",
"4521",
"4561",
"4532",
"4534",
},
},
}

func equal(result []string, expected []string) bool {
if len(result) != len(expected) {
return false
}

for i := range result {
if result[i] != expected[i] {
return false
}
}

return true
}

func TestPhoneNeighbors(t *testing.T) {
for _, test := range testCases {
if result := PhoneNeighbors(test.input); !equal(result, test.expected) {
t.Fatalf("FAIL: %s - PhoneNeighbors(%s): %v - expected: %v", test.description, test.input, result, test.expected)
}
t.Logf("PASS: %s", test.description)
}
}



python

def number(n):
if len(n.replace('-','')) >= 1 and len(n.replace('-','')) <= 10:
print(*[n[:len(n)-1]+str(int(n[-1])+1), n[:len(n)-1]+str(int(n[-1])-1)])
else:
print('Error')


So... something like this?

const neighbours = str => [ (parseInt(str) + 1).toString(), parseInt(str) - 1).toString() ]


const neighborNumbers = number => [${Number(number)-1}, ${Number(number)+1}];

neighborsNumbers("5555555555") // ["5555555554", "5555555556"]  