The Concept of Data: Simple Data Types  19  The predeclared functions ord and chr allow the mapping of the  character set onto the ordinal numbers of the character set - and vice  versa;  ord and chr are called  transfer functions.  i.e.,  You can see immediately that ord and chr are inverse functions,  chr(ord(C))  =  C  and  ord (chr  (1))  =  I  Furthermore, the ordering of a given character set is defined by  C1  <  C2  iff  ord(C1)  <  ord(C2)  This definition can be extended to each of the relational operators:  <>, <,  <=, >=,  >. If  R  denotes one of these operators, then  C1  R C2  iff  ord(C1)  R ord(C2)  When the argument ofthe predeclared functions pred and succ is  of type Char, the functions can be defined as:  pred (C)  succ(C)  chr (ord (C) -1)  chr(ord(C)+l)  Note:  The predecessor (successor) of a character is dependent upon the  underlying  character  set.  The  two  properties  hold  only  if the  predecessor or successor exists.  A value of type Real  is an element of the implementation-defined  subset of real numbers.  2.E.  The Type  Real  2.  Upper-case letters' A'  through'  z'  may exist; if so, they are  alphabetically ordered, but not necessarily consecutive  (e.g.,  'A'  <  'B').  3.  Lower-case letters' a'  through' z'  may exist; if so, they are  alphabetically ordered, but not necessarily consecutive  (e.g., '  a'  < '  b').  ord(C)  is the ordinal number of the character C in the  underlying ordered character set.  chr (I)  is the character value with the ordinal number  I. 