@jinxonik / UniLecs #263

int_to_roman.cpp

#include <string>

// Convert positive integer 1..3999 to roman number string
// Empty string is returned for out-of-range values
std::string int_to_roman(int n)
{
  // Range check
  if (n < 1 || n > 3999) { return ""; }

  // Roman digits and result string
  static const char* roman_digits = "MDCLXVI";  // 1000,500,100...
  std::string result;

  // Conversion loop: level - value order to check, idx - index of current digit in roman_digits string (array)
  for (int level = 1000, idx = 0; n > 0; level /= 10, idx += 2) {
    if (n >= level * 4) {
      if (n < level * 5 || n >= level * 9) {  // IV, XL, CD or IX, XC, CM
        result.push_back(roman_digits[idx]);  // I,X,C...
        n += level;
      }

      if (n >= level * 9) {  // IX, XC, CM
        result.push_back(roman_digits[idx-2]);  // (I,X,C) + X,C,M (previous level digit)
        n -= level * 10;
      } else {  // IV, XL, CD or V, L, D
        result.push_back(roman_digits[idx-1]);  // [if (n < level * 5): I,X,C] + V,L,D
        n -= level * 5;
      } // if (n >= level * 9) / else
    } // if (n >= level * 4)

    // Rest (from 0 to 3) digits: I,X,C,M
    for (; n >= level; n -= level) {
      result.push_back(roman_digits[idx]);  // [if (n >= level * 5): V,L,D] + I,X,C,M
    }
  } // for

  return result;
} // int_to_roman

#include <iostream>

int main()
{
  int list[] = { 1, 4, 58, 256, 700, 1009, 1982, 2021, 3674, 1111, 2222, 3333, 444, 555, 666, 777, 888, 999, -100, 0, 1000000 };
  for (int n : list) {
    std::cout << n << " = \"" << int_to_roman(n) << '"' << std::endl;
  }
}

int_to_roman_min.cpp

// Old minimalistic version

#include <string>

std::string int_to_roman(int n)
{
  if (n < 1 || n > 3999) { return ""; }
  static const char* roman_dig = "MDCLXVI";
  std::string result;
  for (int exp = 1000, idx = 0; exp > 0; exp /= 10, idx += 2) {
    int dig = n / exp;
    if (dig >= 4) {
      if (dig == 4 || dig == 9) {  // IV, IX
        result.push_back(roman_dig[idx]);  // I
      }
      result.push_back(roman_dig[idx - 1 - (dig == 9)]);  // (I)V, V[...] or (I)X
      n -= exp * (dig == 4 || dig == 9 ? dig : 5);
    }
    for (; n >= exp; n -= exp) {
      result.push_back(roman_dig[idx]);  // [V]I...
    }
  }
  return result;
}

#include <iostream>

int main()
{
  int list[] = { 1, 4, 58, 256, 700, 1009, 1982, 2021, 3674, 1111, 2222, 3333, 444, 555, 666, 777, 888, 999, -100, 0, 1000000 };
  for (int n : list) {
    std::cout << n << " = \"" << int_to_roman(n) << '"' << std::endl;
  }
}