HP C A.06.05 Reference Manual

Preprocessing Directives

Macro Replacement (#deﬁne, #undef)

Chapter 7192

In this example, the array x is dimensioned using the macro ARRAY_SIZE rather than the

constant 1000. Note that expressions that may use the array can also use the macro instead of

the actual constant:

for (i=0; i<ARRAY_SIZE; ++i) f+=x[i];

Changing the dimension of x means only changing the macro for ARRAY_SIZE; the dimension

will change and so will all the expressions that make use of the dimension.

Some other common macros used by C programmers include:

#define FALSE 0

#define TRUE 1

The following macro is more complex. It has two parameters and will produce an in-line

expression which is equal to the maximum of its two parameters:

#define MAX(x,y) ((x) > (y) ? (x) : (y))

Parentheses surrounding each argument and the resulting expression insure that the

precedences of the arguments and the result will not improperly interact with any other

operators that might be used with the MAX macro.

Using a macro deﬁnition for MAX has some advantages over a function deﬁnition. First, it

executes faster because the macro generates in-line code, avoiding the overhead of a function

call. Second, the MAX macro accepts any argument types. A functional implementation of MAX

would be restricted to the types deﬁned for the function. Note further that because each

argument to the MAX macro appears in the token string more than once, check to be sure that

the actual arguments to the MAX macro do not have any "side effects." The following example

MAX(a++, b);

might not work as expected because the argument a is incremented two times when a is the

maximum.

The following statement

i = MAX(a, b+2);

is expanded to:

i = ((a) > (b+2) ? (a) : (b+2));

Examples

#define isodd(n) ( ((n % 2) == 1) ? (TRUE) : (FALSE))

/* This macro tests a number and returns TRUE if the number is odd. It will

/* return FALSE otherwise..........................*/