Programming in C

310 Chapter 13 The Preprocessor
assigns the value of v 2 to y.What do you think would happen in the case of the statement
y = SQUARE (v + 1);
This statement does not assign the value of (v + 1) 2 to y as you would expect. Because
the preprocessor performs a literal text substitution of the argument into the macro definition,
the preceding expression would actually be evaluated as
y = v + 1 * v + 1;
which would obviously not produce the expected results.To handle this situation properly,
parentheses are needed in the definition of the SQUARE macro:
#define SQUARE(x) ( (x) * (x) )
Even though the preceding definition might look strange, remember that it is the entire
expression as given to the SQUARE macro that is literally substituted wherever x appears
in the definition.With your new macro definition for SQUARE, the statement
y = SQUARE (v + 1);
is then correctly evaluated as
y = ( (v + 1) * (v + 1) );
The conditional expression operator can be particularly handy when defining macros.
The following defines a macro called MAX that gives the maximum of two values:
#define MAX(a,b) ( ((a) > (b)) ? (a) : (b) )
This macro enables you to subsequently write statements such as
limit = MAX (x + y, minValue);
which would assign to limit the maximum of x + y and minValue.Parentheses were
placed around the entire MAX definition to ensure that an expression such as
MAX (x, y) * 100
gets evaluated properly; and parentheses were individually placed around each argument
to ensure that expressions such as
MAX (x & y, z)
get correctly evaluated.The bitwise AND operator has lower precedence than the > operator
used in the macro.Without the parentheses in the macro definition, the > operator
would be evaluated before the bitwise AND,producing the incorrect result.
The following macro tests if a character is a lowercase letter:
#define IS_LOWER_CASE(x) ( ((x) >= 'a') && ((x)