Поиск буквы бинарным поиском в тексте

#include <iostream>
#include <string>
 
int main() {
  std::string s = "abcdefghijklmnop";
  int first = 0, average = 0, last = s.length() - 1;
  char ch;
  std::cout << "String: " << s << std::endl;
  std::cout << "Please enter char: ";
  std::cin >> ch;
  while(first < last) {
    average = first + (last - first) / 2;
    if(ch <= s[average])
      last = average;
    else
      first++;
  }
  
  if(s[last] == ch)
    std::cout << "Index: " << last << std::endl;
  else
    std::cout << "Not Found!" << std::endl;
  
  return 0;
}

#include <iostream>
#include <fstream>
#include <string>
 
int binarySearch(std::string, char);
 
int main() {
  std::ifstream fin("file.txt");
  std::string str, tmp;
  
  while(!fin.eof()) {
    std::getline(fin, tmp);
    str.append(tmp);
  }
  fin.close();
  
  char ch;
  std::cout << "String: " << str << std::endl;
  std::cout << "Enter char: ";
  std::cin >> ch;
  
  int index = binarySearch(str, ch);
  if(ch == str[index])
    std::cout << "Index: " << index << std::endl;
  else
    std::cout << "Not found" << std::endl;
  return 0;
}
 
int binarySearch(std::string str, char ch) {
  int first = 0, average = 0, last = str.length() - 1;
  while(first < last) {
    average = first + (last - first) / 2;
    if(ch <= str[average])
      last = average;
    else
      first++;
  }
  return last;
}