C Macro & Preprocessor

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

int main()
{
    PRINT(0, 3);
    return 0;
}
How many times GeeksQuiz is printed in the above program?
Cross
1
Cross
3
Cross
4
Tick
Compile-time error


Question 1-Explanation: 
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?
 

Cross

3
 

Tick

5
 

Cross

3 or 5 depending on value of X
 

Cross

Compile time error
 



Question 2-Explanation: 

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? 

C

#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;
}

Cross

Compiler Error

Tick

GeeksQuiz

Cross

MAIN

Cross

main



Question 3-Explanation: 

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;
}
Cross
Geeks
Cross
Quiz
Tick
Compiler Error
Cross
Runtime Error


Question 4-Explanation: 
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?
Tick
Geeks
Cross
Quiz
Cross
Any of Geeks or Quiz
Cross
Compile time error


Question 5-Explanation: 
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;
}
Cross
1
Tick
36
Cross
0
Cross
Compiler Error


Question 6-Explanation: 
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;
}
Cross
Compiler Error
Cross
%s Geeks Quiz
Cross
Geeks Quiz
Tick
%s Geeks Quiz Geeks Quiz


Question 7-Explanation: 
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;
}
Cross
Compiler Error
Tick
10 50
Cross
50 50
Cross
10 10


Question 8-Explanation: 
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; 
}
Tick
100
Cross
Compiler Error
Cross
0
Cross
1


Question 9-Explanation: 
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?
 

Cross

.p
 

Tick

.i
 

Cross

.o
 

Cross

.m
 



Question 10-Explanation: 

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

There are 21 questions to complete.

  • Last Updated : 28 Sep, 2023

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