Using a map, write a main() function to
   loop and each time in the loop, ask the
   user to enter an integer n.

 Store in the map the integer n and its
   corresponding sqrt(n).

 We let the user know when a NEW element
   is added to the map --or-- when we're
   just reading from the map.

              map< int, double >

       ==>user enters 100
       ==>adds 100-->10.0 to the map
       ==>user enters 64
       ==>adds 64-->8.0 to the map
       ==>user enters 100
       ==>looks up 100 and finds it
          in the map (reads 10.0)


 Sample output and input:
   Enter an int and I'll tell you the sqrt:
   100
   Oooo! That's new. I'll add it to the map.
   sqrt of 100 is 10

   Enter an int and I'll tell you the sqrt:
   64
   Oooo! That's new. I'll add it to the map.
   sqrt of 64 is 8

   Enter an int and I'll tell you the sqrt:
   100
   Already know that
   sqrt of 100 is 10



 #include <cmath>  <== math library from C


 String and Character manipulation

   #include <cctype>

   char c = ' ';
   char d;

   if ( isspace( c ) ) {
     // ...
   } else if ( isalpha( c ) ) { // a-zA-Z
     // valid character ...
   }

   // isdigit( c )     // 0-9
   // ispunct( c )     // .,;: etc.
   // isupper( c )     // A-Z
   // islower( c )     // a-z

   // d = tolower( c )   // A-Z ==> a-z
   // d = toupper( c )   // a-z ==> A-Z

   string w;