Converting a string to roman numerals in C++ [duplicate] - c++

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How to convert integer value to Roman numeral string?
first time asking a question here. I have a project on the horizon I've been working a bit on, and can't seem to find anything like it asked here or elsewhere. The goal is to accept an integer with no upper constraint, and convert that into a roman numeral. Since integers do in fact have an upper boundary, I have to convert it into a string before parsing it, using apostrophes to denote each set of three characters placed into a sub-string. I'm having trouble conceptualizing the loops that a)assign roman numerals to what they see based on their location b)count each set of three by displaying apostrophes.
So far I have:
for(int i=0;i<(int)input.length()/3;i++){
temp=input.substr(i,3);
for(int j = 0; j < (int)temp.length(); j++){
if(j == 0){
if(temp[j] == '9') cout<<"CM";
else if(temp[j] >= '8') cout<<"DCCC";
else if(temp[j] >= '7') cout<<"DCC";
else if(temp[j] >= '6') cout<<"DC";
else if(temp[j] >= '5') cout<<"D";
else if(temp[j] == '4') cout<<"CD";
else if(temp[j] == '3') cout<<"CCC";
else if(temp[j] == '2') cout<<"CC";
else if(temp[j] == '1') cout<<"C";
}
else if(j == 1){
if(temp[j] == '9') cout<<"XC";
else if(temp[j] >= '8') cout<<"LXXX";
else if(temp[j] >= '7') cout<<"LXX";
else if(temp[j] >= '6') cout<<"LX";
else if(temp[j] >= '5') cout<<"L";
else if(temp[j] == '4') cout<<"XL";
else if(temp[j] == '3') cout<<"XXX";
else if(temp[j] == '2') cout<<"XX";
else if(temp[j] == '1') cout<<"X";
}
else if(j ==2){
if(temp[j] == '9') cout<<"IX";
else if(temp[j] == '8') cout<<"VIII";
else if(temp[j] == '7') cout<<"VII";
else if(temp[j] == '6') cout<<"VI";
else if(temp[j] >= '5') cout<<"V";
else if(temp[j] == '4') cout<<"IV";
else if(temp[j] == '3') cout<<"III";
else if(temp[j] == '2') cout<<"II";
else if(temp[j] == '1') cout<<"I";
}
}
}
The numerals display well enough on their own, but I'm having trouble figuring out how to tell the loop to start on the right, and work it's way left by threes, maintaining the actual place of the number in the input (e.g. 1234 should display 1 as I, not C. I also need to figure out the loop to write in the apostrophes.

Answer 1

Simplest way i can think of to convert to roman numerals, is to check starting from the largest possible digit/combo and work down. Include combos, and check them from largest to smallest, so that say "XC" always gets checked before "L" and you don't have to worry about "LXXXX" and "LXL" and such.
// This code requires C++11 support. Namely, initializer lists and type inference.
// If your compiler sucks, there's equivalents for the important stuff. What really
// really matters is the order of the value=>digits mappings, and the iteration over
// them in the for loop.
#include <vector>
#include <string>
#include <utility>
std::string romanNumeralFor(int n, int markCount = 0) {
typedef std::pair<int, std::string> valueMapping;
static std::vector<valueMapping> importantNumbers = {
{1000, "M"}, {900, "CM"}, {500, "D"}, {400, "CD"},
{100, "C"}, { 90, "XC"}, { 50, "L"}, { 40, "XL"},
{10, "X"}, { 9, "IX"}, { 5, "V"}, { 4, "IV"},
{1, "I"},
};
std::string result;
bool needMark = false;
std::string marks(markCount, '\'');
for (auto mapping : importantNumbers) {
int value = mapping.first;
std::string &digits = mapping.second;
while (n >= value) {
result += digits;
n -= value;
needMark = true;
}
if ((value == 1000 || value == 100 || value == 10 || value == 1) && needMark) {
result += marks;
needMark = false;
}
}
return result;
}
As for converting a string to a number:
// in C++11
int n = std::stoi(str);
// in C++03
std::istringstream iss(str);
int n;
iss >> n;
So, split your string up into three-digit chunks (starting from the end!), and pass them in with appropriate mark counts.

Answer 2

Not the best solution but it works.
#include <iostream>
#include <map>
#include <algorithm>
#include <string>
void replaceAll(std::string& str, const std::string& from, const std::string& to) {
if(from.empty())
return;
size_t start_pos = 0;
while((start_pos = str.find(from, start_pos)) != std::string::npos) {
str.replace(start_pos, from.length(), to);
start_pos += to.length();
}
}
int romanNumeralToInt(std::string s) {
std::map<char,int> romanNum = {
{'I',1}, {'V',5}, {'X',10}, {'L',50}, {'C',100},
{'D',500}, {'M',1000}
};
//{"IIIII",5}, {"VV",10}, {"XXXXX",50}, {"LL",100}, {"CCCCC",500}, {"DD",1000}
int g = 0;
std::sort(s.begin(),s.end());
if(s.find("IIIII") != std::string::npos)
replaceAll(s,"IIIII","V");
if(s.find("VV") != std::string::npos)
replaceAll(s,"VV","X");
if(s.find("XXXXX") != std::string::npos)
replaceAll(s,"XXXXX","L");
if(s.find("LL") != std::string::npos)
replaceAll(s,"LL","C");
for(auto& i : s) {
if(romanNum[i] != 0)
g += romanNum[i];
}
return g;
}
int main() {
std::string st = "XXXXXIIIIIVVVXLLD";
int a = romanNumeralToInt(st);
std::cout << a;
}
Prints out 680.