#include <iostream>
using namespace std;

const int MAX_LETTERS=20;	// Capacity of the array.

void display_reverse(char array[], int num_elems);
// @param: array to reverse display 
// @param: num_elems is the number of elements in array.

void reverse_array(char array[], int num_elems);
// @param: array with num_elems in it.
// @param: num_elems is the number of elements in the array.

void fill_array(char array[], int size, int& num_elems);
// @param array: the array to be filled
// @param size: the capacity of the array parameter
// @param num_elems: initialzed the number of elements in the array. 
// num_elems is updated with the additional elements.

int main()
{
   char array[MAX_LETTERS];
   int num_letters = 0;

   // fill the array with letters, 
   cout << "Enter up to " << MAX_LETTERS << " or enter '.' to end input:\n";
   fill_array(array, MAX_LETTERS, num_letters);

   // Display the reverse, reverse it and display reverse again.
   display_reverse(array, num_letters);
   reverse_array(array, num_letters);
   display_reverse(array, num_letters);

}

void swap_values(char& a, char& b)
// precondition: a and b are characters
// postcondition: a and b are swapped.
{
   char t = a;
   a = b;
   b = t;
}

void display_reverse(char array[], int num_elems)
// precondition: an array of character values;
// postcondition: output of the reverse of the array.
{
   for (int i = num_elems-1; i >= 0; i--)
      cout << array[i];
   cout << endl;
} 

void reverse_array(char array[], int num_elems)
// precondition: array with num_elems elements
// postcondition: array reversed
{
   for (int i = 0; i < (num_elems-1)/2; i++)
       swap_values(array[i], array[num_elems-i-1]);   
}


void fill_array(char array[], int size, int& num_elems)
// precondition: array with num_elems with capacity size. 
// postcondition: array with no more than capacity elems but
// possibly more values added. num_elems is updated.
{
   char next = ' '; 		// Holds next character.

   // Read values until user enters '.' or the array is full.
   while ( next != '.' && num_elems < size )
   {
      cin >> next;
      if (next != '.')
         array[num_elems++] = next;
   }
}