C++ Encoding Error - c++

Question

The object of this code is to take the command line argument ./Program -encode|decode 0-9 and apply it to a text string written on the next line. IF encode, then shift each character of the text however many places specified in the alphabet. Decode does the opposite. I've gotten it good enough that the compiler finds nothing wrong, but when running the program nothing returns after entering the text string.
#include <iostream>
#include <string>
#include <cctype>
#include <cstdlib>
#include <cstring>
using namespace std;
string encode(string& to_encode, int to_shift)
{for(int i = 0; I < to_encode.length(); i++)
{if(isalpha(to_encode[i]))
{char sum = to_encode[i] + to_shift;
if(isupper(to_encode[i]) != 0 && isupper(sum) == 0)
{to_encode[i] = sum - 'Z' + 'A';
}
if(isupper(to_encode[i]) != 0 && isupper(sum) != 0)
{to_encode[i] = sum;
}
if(islower(to_encode[i]) != 0 && islower(sum) == 0)
{to_encode[i] = sum - 'z' + 'a';
}
if(islower(to_encode[i]) != 0 && islower(sum) != 0)
{to_encode[i] = sum;
}
}
}
return to_encode;
}
string decode(string& to_decode, int to_shift)
{for(int i = 0; I < to_decode.length(); i++)
{if(isalpha(to_decode[i]))
{char difference = to_decode[i] + to_shift;
if(isupper(to_decode[i]) != 0 && isupper(difference) == 0)
{to_encode[i] = difference + 'Z' - 'A';
}
if(isupper(to_decode[i]) != 0 && isupper(difference) != 0)
{to_encode[i] = difference;
}
if(islower(to_decode[i]) != 0 && islower(difference) == 0)
{to_encode[i] = difference + 'z' - 'a';
}
if(islower(to_decode[i]) != 0 && islower(difference) != 0)
{to_encode[i] = difference;
}
}
}
return to_decode;
}
int main(int argc, char* argv[])
{string cryption;
in shift;
if(argc != 3)
{cerr<<"Usage: ./Prog1d -encode|decode 0-9"<<endl;
return -1;
}
if(!(strcmp(argv[1],"-encode") == 0 || strcmp(argv[1],"-decode") == 0)
{cerr<<"Usage: ./Prog1d -encode|decode 0-9"<<endl;
return -1;
}
shift = atoi(argv[2]);
if(shift > 0 && shift <= 9)
{while(getline(cin,cryption))
{if(argv[1] == "-encode")
{encode(cryption, shift);
}
else
{decode(cryption, shift);
}
}
}
else
{cerr<<"Usage: ./Prog1d -encode|decode 0-9"<<endl;
return -1;
}
}