Exercise 14.04

For each of the following macros, give an example that illustrates a problem with the macro and show how to fix it.

(a) #define AVG(x,y) (x-y)/2
(b) #define AREA(x,y) (x)*(y)

Solution

(a)

AVG(1+1, 2+2) will incorrectly follow standard order of operations: (1+1-2+2)/2, evaluating to 2 rather than predicted -2. Placing parentheses around the parameters would fix this problem:

#define AVG(x,y) (((x)-(y))/2)

(b)

25 / AREA(5,5) will incorrectly evaluate to 25 rather than predicted 1, following standard order of operations. Placing parentheses around the entire definition should fix this problem.

#define AREA(x,y) ((x)*(y))