Programming in C

5.0 Expressions 443

1. As the value after a case in a switchstatement


2. For specifying the size of an array that is initialized or globally declared


3. For assigning a value to an enumeration identifier


4. For specifying the bit field size in a structure definition


5. For assigning initial values to static variables


6. For assigning initial values to global variables


7. As the expression following the #ifin a #ifpreprocessor statement
   In the first four cases, the constant expression must consist of integer constants, character
   constants, enumeration constants, and sizeofexpressions.The only operators that can be
   used are the arithmetic operators, the bitwise operators, the relational operators, the con-
   ditional expression operator, and the type cast operator.The sizeofoperator cannot be
   used on an expression with a variable-length array because the result is evaluated at run-
   time and is, therefore, not a constant expression.
   In the fifth and sixth cases, in addition to the rules cited earlier, the address operator
   can be implicitly or explicitly used. However, it can only be applied to global or static
   variables or functions. So, for example, the expression
   &x + 10
   is a valid constant expression, provided that xis a global or static variable. Furthermore,
   the expression
   &a[10] - 5
   is a valid constant expression if ais a global or static array. Finally, because &a[0]is
   equivalent to the expression a,
   a + sizeof (char) * 100
   is also a valid constant expression.
   For the last situation that requires a constant expression (after the #if), the rules are
   the same as for the first four cases, except the sizeofoperator, enumeration constants,
   and the type cast operator cannot be used. However, the special definedoperator is per-
   mitted (see Section 9.2.3).

5.3 Arithmetic Operators

Given that

a, b are expressions of any basic data type except void;

i, j are expressions of any integer data type;

then the expression

-a negates the value of a;

+a gives the value of a;

a + b adds awith b;

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