6.17.cpp

#include <string>
#include <iostream>

bool hasCapital(const std::string &s) {  // Only ASCII
  for (auto &c : s)  // `c` would be `const char &` since `s` is `const`
    if (c >= 'A' && c <= 'Z')
      return true;
  return false;
}

void toLowerStr(std::string &s) {  // Only ASCII
  for (auto &c : s)
    if (c >= 'A' && c <= 'Z')
      c -= 'A' - 'a';
}

// The parameters have different type, because in `hasCapital`, we don't need
// to change the string, but in `toLowerStr`, we need to change the string.

int main() {
  std::string str("Hello World!");
  std::cout << hasCapital("Abc") << " " << hasCapital("abc") << " "
            << hasCapital(str) << std::endl;
  std::cout << str << std::endl;
  toLowerStr(str);
  std::cout << str << std::endl;

  return 0;
}