How wis works
  word
      |T|H|E|0|.. .          |B|E|L|L|0|T|H|E|0|
       0 1 2 3                          i0      i

two string functions:

#include<string.h>

  program: st2.c

   strlen    string length
   strcmp    string compare

   lexicographical order
      dog
      dag comes before dog in the dictionary order
a bcdefghijklmno

lower case to upper case   'A'-'a' = 'B'-'b'=...= 'Z'-'z'

'A'  'B'

'a'  'b'

background:
   program: st1.c

   "Dog"  The compiler stores the codes and also a 0 at the end
          and it uses the symbol "Dog" to stand for the address of the
          first character in the string, 'D'.

   address arithmetic:  "Dog"+1 is the address of the 'o'.

   printf("%s\n", "Dog");