Macros allow code to be substituted before compilation by replacing text. During preprocessing: 1) Macros find and replace text without evaluating code. 2) Macro arguments are substituted but not evaluated. 3) Unintended side effects can occur from operations inside macros.

1. CSE240
Macros and Preprocessing

2. What is preprocessing?
 It is the state of the program that occurs BEFORE any
code is compiled
 At this point, no values are initialized and nothing is
evaluated
 Code swapping and replacing can occur here, but
no computations

3. What is a Macro?
 A macro, as defined in C and C++, substitutes a definition (a.k.a.
code) for a name in a program
 Basically, this says that a Macro acts like a find an replace function given a
name and it replaces it with some text (called a definition)
 Macros allow arguments which create “in-line” functions or
procedures
 Since macros only substitute code, nothing is actually processed or
evaluated
 Ex: #define PI 3.14
 PI is not a variable with a value assigned to be 3.14
 Whenever the compiler sees PI it replaces the text with 3.14

4. Macro Preprocessing: Before and After
Before Preprocessing
#include <stdio.h>
#define GREETING "Hello, my name is Sam.n"
#define QUESTION "What is the product of 4 and 7?n"
#define COMPUTATION 4*7
#define FAREWELL "The answer is %d, Goodbye!n"
int main()
{
int product = 0;
printf(GREETING);
printf(QUESTION);
product = COMPUTATION;
printf(FAREWELL, product);
return 0;
}
After Preprocessing but Before Compilation
#include <stdio.h>
#define GREETING "Hello, my name is Sam.n"
#define QUESTION "What is the product of 4 and 7?n"
#define COMPUTATION 4*7
#define FAREWELL "The answer is %d, Goodbye!n"
int main()
{
int product = 0;
printf("Hello, my name is Sam.n");
printf("What is the product of 4 and 7?n");
product = 4*7;
printf("The answer is %d, Goodbye!n", product);
return 0;
}

5. Macro In-Line Functions
 Uses notation similar to a function, but does not do any
computation or evaluation; it only finds and replaces text
 Example: #define RECTANGLE_AREA(length, width)
length*width
 If RECTANGLE_AREA(3,10) were used in a function, the text
“length” with would be replaced with “3”, which would be
followed by “*”, and then the text “width” would be replaced
by “10”
 int x = RECTANGLE_AREA(3,10); becomes int x = 3*10; after
preprocessing

6. Macro Side Effects
 Since macros substitute code, using the pre-increment ++x and
post increment x++ has unintended results. The same is true for
the pre and post decrements.
Ex: #define function(x, y) ( x < y) ? x : y
int x = 1, int y = 5;
If you call function(++x, y), it is replaced with (++x < y ) ? ++x : y.
When compiled, it does (++x < y) ? ++x : y.
x is now 2 x is now 3
At the end, x will be 3, y will be 5, and the result of the computation is 3 because it
calculates ++x twice and then returns it.

7. Question 1
#include <stdio.h>
#define INCREMENTER(a,b) (a > b) ? b : a
int main()
{
int a = 2;
int b = 3;
int result;
result = INCREMENTER(a++, b);
printf("a is %d, b is %d, and result is %dn", a, b,
result);
result = INCREMENTER(++a, b);
printf("a is %d, b is %d, and result is %dn", a, b,
result);
return 0;
}
 What is the output of the program?
A.) a is 4, b is 3, and result is 3
a is 5, b is 3, and result is 3
B.) a is 3, b is 3, and result is 3
a is 4, b is 3, and result is 3
C.) a is 2, b is 3, and result is 2
a is 2, b is 3, and result is 2
D.) a is 4, b is 3, and result is 4
a is 5, b is 3, and result is 3

8. Question 2
#include <stdio.h>
#define PI 3.14159265
#define DENSITY(mass,volume) mass/volume // Q2
#define CIRCUMFERENCE(r) 2*r*PI
#define INITIALIZE_FLOAT(name, value) float name =
value
int main()
{
INITIALIZE_FLOAT(density, 0.0);
INITIALIZE_FLOAT(circumference, 0.0);
INITIALIZE_FLOAT(radius, 5.0); // Q3
density = DENSITY(10.0, 2.0); // Q4
circumference = CIRCUMFERENCE(radius); // Q5
return 0;
}
 How does the following macro work during
preprocessing?
#define DENSITY(mass,volume) mass/volume
A.) Two local variables are created then divided
B.) Two passed in parameters are divided and the
result is returned
C.) The words mass and volume are substituted by
corresponding values in the parameter and the divide
symbol is placed between them.
D.) The compiler evaluates both parameters and
performs a calculation

9. Question 3
#include <stdio.h>
#define PI 3.14159265
#define DENSITY(mass,volume) mass/volume // Q2
#define CIRCUMFERENCE(r) 2*r*PI
#define INITIALIZE_FLOAT(name, value) float name = value
int main()
{
INITIALIZE_FLOAT(density, 0.0);
INITIALIZE_FLOAT(circumference, 0.0);
INITIALIZE_FLOAT(radius, 5.0); // Q3
density = DENSITY(10.0, 2.0); // Q4
circumference = CIRCUMFERENCE(radius); // Q5
return 0;
}
 At line Q3 and after preprocessing but before
compilation, what would this line look like?
A.) INITIALIZE_FLOAT(radius, 5.0);
B.) float radius = 5.0;
C.) radius = 5.0;
D.) INITIALIZE_FLOAT(radius, 5.0) float name =
value;

10. Question 4
#include <stdio.h>
#define PI 3.14159265
#define DENSITY(mass,volume) mass/volume // Q2
#define CIRCUMFERENCE(r) 2*r*PI
#define INITIALIZE_FLOAT(name, value) float name = value
int main()
{
INITIALIZE_FLOAT(density, 0.0);
INITIALIZE_FLOAT(circumference, 0.0);
INITIALIZE_FLOAT(radius, 5.0); // Q3
density = DENSITY(10.0, 2.0); // Q4
circumference = CIRCUMFERENCE(radius); // Q5
return 0;
}
 At line Q4 and after preprocessing but
before compilation, what would this line
look like?
A.) density = 10.0/2.0;
B.) density = 5.0;
C.) density = DENSITY(10.0, 2.0) mass/volume;
D.) density = DENSITY(10.0, 2.0);

11. Question 5
#include <stdio.h>
#define PI 3.14159265
#define DENSITY(mass,volume) mass/volume // Q2
#define CIRCUMFERENCE(r) 2*r*PI
#define INITIALIZE_FLOAT(name, value) float name = value
int main()
{
INITIALIZE_FLOAT(density, 0.0);
INITIALIZE_FLOAT(circumference, 0.0);
INITIALIZE_FLOAT(radius, 5.0); // Q3
density = DENSITY(10.0, 2.0); // Q4
circumference = CIRCUMFERENCE(radius); // Q5
return 0;
}
 At line Q5 and after preprocessing but
before compilation, what would this line
look like?
A.) circumference = CIRCUMFERENCE(radius);
B.) circumference = CIRCUMFERENCE(5.0);
C.) circumference = 31.416;
D.) circumference = 2*radius*3.14159265;