for loop

[Tomheza]

for(char& c: input)

How does this work?

[keskiverto]

http://en.cppreference.com/w/cpp/language/range-for

[Tomheza]

So it would be equal to

 for(int i = 0; i <= input.length; i++)

[Arslan7041]

"For every char in 'input'...."

[keskiverto]

So it would be equal to
for(int i = 0; i <= input.length; i++)

You seem to have an indexing error in that.

But close,

for ( char & c : input ) {
  // code
}

// is similar to
for ( size_t pos = 0; pos < count_of_elements_in_input; ++pos ) {
  char & c = input[pos];
  // code
}


// there is also
std::for_each( std::begin(input), std::end(input), []( char & c ) {
    // code
  });
// the std::for_each predates C++11, unlike the std::begin, std::end, and lambda syntax