freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-python/69738771fb5a7b8b24cca29e.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
69738771fb5a7b8b24cca29e	Challenge 172: Letters-Numbers	29	challenge-172

--description--

Given a string containing only letters and numbers, return a new string where a hyphen (-) is inserted every time the string switches from a letter to a number, or a number to a letter.

--hints--

separate_letters_and_numbers("ABC123") should return "ABC-123".

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(separate_letters_and_numbers("ABC123"), "ABC-123")`)
}})

separate_letters_and_numbers("Route66") should return "Route-66".

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(separate_letters_and_numbers("Route66"), "Route-66")`)
}})

separate_letters_and_numbers("H3LL0W0RLD") should return "H-3-LL-0-W-0-RLD".

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(separate_letters_and_numbers("H3LL0W0RLD"), "H-3-LL-0-W-0-RLD")`)
}})

separate_letters_and_numbers("a1b2c3d4") should return "a-1-b-2-c-3-d-4".

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(separate_letters_and_numbers("a1b2c3d4"), "a-1-b-2-c-3-d-4")`)
}})

--seed--

--seed-contents--

def separate_letters_and_numbers(s):

    return s

--solutions--

def separate_letters_and_numbers(s):
    if not s:
        return s

    result = s[0]

    for i in range(1, len(s)):
        prev = s[i - 1]
        curr = s[i]

        prev_is_letter = prev.isalpha()
        curr_is_letter = curr.isalpha()

        if prev_is_letter != curr_is_letter:
            result += "-"

        result += curr

    return result