def roman_numeral(num):
    """Convert an integer into a string representing its writing in roman
    numerals.
    """
    val = [1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1]
    sym = ["M", "CM", "D", "CD", "C", "XC",
           "L", "XL", "X", "IX", "V", "IV", "I"]
    roman_num = ""
    i = 0
    while num > 0:
        for _ in range(num // val[i]):
            roman_num += sym[i]
            num -= val[i]
        i += 1
    return roman_num