visual C++ string subscript out of range - c++

Question

I am stuck with the error "string subscript out of range".
After testing, I am pretty sure that it's because of this function, which is used for reading values in a file, but have no idea what's wrong:
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
string read(string value) {
ifstream input;
string line="", output="";
size_t pos;
bool a = true;
int i = 0;
input.open("pg_options.txt");
if (!input.is_open()) {
cout << "pg_options.txt missing.";
return "error";
}
while (getline(input, line)) { //get file lines
pos = line.find(value);
if (pos == string::npos) { //if value in line
while (a == true) {
if (line[i] == '=') { //after "="
i++;
break;
}
else {
i++;
}
}
for (i; line[i] != ' '; i++) {
output += line[i]; //put value to output
}
}
}
input.close();
return output;
}
pg_options.txt:
include_special_characters=true
include_upper_case=true
include_lower_case=true
include_numbers=true
digits=10
cout << read("digits") returns the error mentioned above.

Answer 1

Thank you for your comments. I solved this by editing the for loop:
string read(string value) {
ifstream input;
int olength;
string line = "", output = "";
size_t pos;
bool a = true;
int i = 0;
input.open("pg_options.txt");
if (!input.is_open()) {
cout << "pg_options.txt missing.";
return "error";
}
while (getline(input, line)) {
pos = line.find(value);
if (pos != string::npos) {
while (a == true) {
if (line[i] == '=') {
i++;
break;
}
else {
i++;
}
}
olength = line.length() - value.length() - 1;
for (int i2 = 0; i2 < olength; i2++) {
output += line[i];
i++;
}
}
}
input.close();
return output;
}