C Macro & Preprocessor

Question 1

#include <stdio.h>
#define PRINT(i, limit) do 
                        { 
                            if (i++ < limit) 
                            { 
                                printf("GeeksQuizn"); 
                                continue; 
                            } 
                        }while(1)

int main()
{
    PRINT(0, 3);
    return 0;
}

How many times GeeksQuiz is printed in the above program?

• A. 1
• B. 3
• C. 4
• D. Compile-time error

Answer: (D)

C Macro & Preprocessor
Discuss it

The PRINT macro gets expanded at the pre-processor time i.e. before the compilation time. After the macro expansion, the if expression becomes: if (0++ < 3). Since 0 is a constant figure and represents only r-value, applying increment operator gives compile-time error: lvalue required. lvalue means a memory location with some address.

Question 2

C

#include <stdio.h>
#if X == 3
    #define Y 3
#else
    #define Y 5
#endif

int main()
{
    printf("%d", Y);
    return 0;
}

What is the output of the above program?

• A. 3
  
• B. 5
  
• C. 3 or 5 depending on value of X
  
• D. Compile time error
  

Answer: (B)

C Macro & Preprocessor 50 C Language MCQs with Answers
Discuss it

In the first look, the output seems to be compile-time error because macro X has not been defined. In C, if a macro is not defined, the pre-processor assigns 0 to it by default. Hence, the control goes to the conditional else part and 5 is printed. See the next question for better understanding.

Question 3

What is the output of following program?

#include <stdio.h>
#define macro(n, a, i, m) m##a##i##n
#define MAIN macro(n, a, i, m)

int MAIN()
{
    printf("GeeksQuiz");
    return 0;
}

• A. Compiler Error
• B. GeeksQuiz
• C. MAIN
• D. main

Answer: (B)

C Macro & Preprocessor
Discuss it

The program has a preprocessor that replaces "MAIN" with "macro(n, a, i, m)". The line "macro(n, a, i, m)" is again replaced by main. The key thing to note is token pasting operator ## which concatenates parameters to macro.

Question 4

#include <stdio.h>
#define X 3
#if !X
    printf("Geeks");
#else
    printf("Quiz");
 
#endif
int main()
{
        return 0;
}

• A. Geeks
• B. Quiz
• C. Compiler Error
• D. Runtime Error

Answer: (C)

C Macro & Preprocessor
Discuss it

A program is converted to executable using following steps 1) Preprocessing 2) C code to object code conversion 3) Linking The first step processes macros. So the code is converted to following after the preprocessing step.

printf("Quiz");
int main()
{
        return 0;
}

The above code produces error because printf() is called outside main. The following program works fine and prints "Quiz"

#include 
#define X 3

int main()
{
#if !X
    printf("Geeks");
#else
    printf("Quiz");

#endif
    return 0;
}

Question 5

#include <stdio.h>
#define ISEQUAL(X, Y) X == Y
int main()
{
    #if ISEQUAL(X, 0)
        printf("Geeks");
    #else
        printf("Quiz");
    #endif
    return 0;
}

Output of the above program?

• A. Geeks
• B. Quiz
• C. Any of Geeks or Quiz
• D. Compile time error

Answer: (A)

C Macro & Preprocessor
Discuss it

The conditional macro #if ISEQUAL(X, 0) is expanded to #if X == 0. After the pre-processing is over, all the undefined macros are initialized with default value 0. Since macro X has not been defined, it is initialized with 0. So, Geeks is printed.

Question 6

#include <stdio.h>
#define square(x) x*x
int main()
{
  int x;
  x = 36/square(6);
  printf("%d", x);
  return 0;
}

• A. 1
• B. 36
• C. 0
• D. Compiler Error

Answer: (B)

C Macro & Preprocessor
Discuss it

Preprocessor replaces square(6) by 6*6 and the expression becomes x = 36/6*6 and value of x is calculated as 36. Note that the macro will also fail for expressions "x = square(6-2)" If we want correct behavior from macro square(x), we should declare the macro as

#define square(x) ((x)*(x))  

Question 7

Output?

# include <stdio.h>
# define scanf  "%s Geeks Quiz "
int main()
{
   printf(scanf, scanf);
   return 0;
}

• A. Compiler Error
• B. %s Geeks Quiz
• C. Geeks Quiz
• D. %s Geeks Quiz Geeks Quiz

Answer: (D)

C Macro & Preprocessor
Discuss it

After pre-processing phase of compilation, printf statement will become. printf("%s Geeks Quiz ", "%s Geeks Quiz "); Now you can easily guess why output is "%s Geeks Quiz Geeks Quiz".

Question 8

#include <stdio.h>
#define a 10
int main()
{
  printf("%d ",a);

  #define a 50

  printf("%d ",a);
  return 0;
}

• A. Compiler Error
• B. 10 50
• C. 50 50
• D. 10 10

Answer: (B)

C Macro & Preprocessor
Discuss it

Preprocessor doesn't give any error if we redefine a preprocessor directive. It may give warning though. Preprocessor takes the most recent value before use of and put it in place of a.

Question 9

Output?

#include<stdio.h> 
#define f(g,g2) g##g2 
int main() 
{ 
   int var12 = 100; 
   printf("%d", f(var,12)); 
   return 0; 
}

• A. 100
• B. Compiler Error
• C. 0
• D. 1

Answer: (A)

C Macro & Preprocessor
Discuss it

The operator ## is called “Token-Pasting” or “Merge” Operator. It merges two tokens into one token. So, after preprocessing, the main function becomes as follows, and prints 100.

int main() 
{ 
   int var12 = 100; 
   printf("%d", var12); 
   return 0; 
}

Question 10

Which file is generated after pre-processing of a C program?

• A. .p
  
• B. .i
  
• C. .o
  
• D. .m
  

Answer: (B)

C Macro & Preprocessor 50 C Language MCQs with Answers
Discuss it

After the pre-processing of a C program, a .i file is generated which is passed to the compiler for compilation.