lower string characters and add a _ in front of converted capital letter - c++

Question

I have one more question, I want to add a _ in front of every Capital letter which will be converted to lowercase, plus the first letter cannot be capital!! I cant figure out how to do it... :{ example:
input: loLollL, output: lo_loll_l
and I want it to go backwards too: input: lo_loll_l output: loLollL
code is here:
#include <iostream>
#include <algorithm>
using namespace std;
int main ()
{
const int max = 100;
string slovo;
int pocet_r;
cout << "Zadaj pocet uloh:" << endl;
cin >> pocet_r;
if(pocet_r >= 1 && pocet_r <=100)
{
// funkcia na zabezpecenie minimalneho poctu chars
for (int i = 0; i <pocet_r; i++)
{
cout << "Uloha " << i+1 << ":" << endl;
cin >> slovo;
if(slovo.size() > max)
{
cout << "slovo musi mat minimalne 1 a maximalne 100 znakov" << endl;
}
while( slovo.size() > max)
{
cin >> slovo;
}
for (int i=0; i <= slovo.size(); i++)
{
int s = slovo[i];
while (s > 'A' && s <= 'Z')
{
if(s<='Z' && s>='A'){
return s-('Z'-'_z');
}else{
cout << "chyba";
}
}
}
cout << slovo[i] << endl;
}
}else{
cout << "Minimalne 1 a maximalne 100 uloh" << endl;
}
system("pause");
}
EDIT>
for (int i=0; i <= slovo.size(); i++)
{
while (slovo[i] >= 'A' && slovo[i] <= 'Z')
{
string s = transform(slovo[i]);
cout << s << endl;
s = untransform(s);
cout << s << endl;
}
}

Answer 1

This should work:
#include <string>
#include <cctype>
#include <iostream>
using namespace std;
string
transform(const string& s)
{
const size_t n = s.size();
string t;
for (size_t i = 0; i < n; ++i)
{
const char c = s[i];
if (isupper(c))
{
t.push_back('_');
}
t.push_back(tolower(c));
}
return t;
}
string
untransform(const string& s)
{
string t;
const size_t n = s.size();
size_t i = 0;
while (i < n)
{
char c = s[i++];
if (c != '_')
{
t.push_back(c);
continue;
}
c = s[i++];
t.push_back(toupper(c));
}
return t;
}
int
main()
{
string s = transform("loLollL");
cout << s << endl;
s = untransform(s);
cout << s << endl;
}