C Preprocessor Questions and Answers

Q: What will be the output of the following C code? #include <stdio.h> #define max 20 main() { #ifndef max #define min 10 #else #define min 30 #endif printf("%d",min); }

Correct answer: 30max is defined, so #ifndef max is false and the #else branch is taken, defining min as 30. The program prints 30.

Q: What will be the output of the following C code? #include <stdio.h> #define hello 10 main() { #ifdef hello #undef hello #define hello 100 #else #define hello 200 #endif printf("%d",hello); }

Correct answer: 100hello is defined, so the #ifdef branch runs: the old definition is removed with #undef and a new one, 100, put in its place. Using #undef before redefining is exactly the right way to do this, since it avoids the redefinition warning. The output is 100.

Q: What will be the output of the following C code? #include <stdio.h> void f() { #define sf 100 printf("%d",sf); } int main() { #define sf 99; f(); printf("%d",sf); }

Correct answer: errorThe macro body is 99; - the semicolon is part of the text. Substituting it gives printf("%d",99;);, a syntax error. (The redefinition of sf without an intervening #undef is a second fault.) A #define for a constant should never end in a semicolon.

Q: What will be the output of the following C code? #include <stdio.h> #define INDIA 1 #define US 2 #define CC US main() { #if CC==INDIA printf("Rupee"); #elif CC==US printf("Dollar"); #else printf("Euro"); #endif }

Correct answer: DollarCC expands to US, which expands to 2 - expansions are rescanned. The first test 2 == 1 fails, the #elif test 2 == 2 succeeds, so printf("Dollar") is the only line that reaches the compiler.

Q: Which of the following is not a preprocessor directive?

Correct answer: #ifelseThere is no #ifelse directive. The conditional set is #if, #ifdef, #ifndef, #elif, #else and #endif - the “else if” form is spelt #elif. #error and #pragma are both real directives.

Q: Which of the following are C preprocessors?

Correct answer: all of the mentioned#ifdef, #define and #endif are all preprocessor directives - the first and third belonging to conditional compilation and the second to macro definition. Every directive begins with # and is processed before the compiler proper sees the file.

61.

What will be the output of the following C code?

#include <stdio.h>
#define hello 
main()
{
    #ifdef hello
    #define hi 4
    #else
    #define hi 5
    #endif
    printf("%d",hi);
 
}

A.4
B.5
C.45
D.error

Answer: A

The code shown above illustrates #define, #ifdef, #else, #endif. Since the identifier hello is defined (condition of #if is true), another identifier names hi is defined and it’s value is 4. If hello was not defined, then the value of hi would be 5.

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62.

The purpose of the preprocessor directive #error is that ____________

A.It rectifies any error present in the code
B.It rectifies only the first error which occurs in the code
C.It causes the preprocessor to report a fatal error
D.It causes the preprocessor to ignore an error

Answer: C

Correct answer: It causes the preprocessor to report a fatal error

#error message makes the preprocessor stop with a diagnostic. It is used to fail loudly when a configuration makes no sense - for example #if !defined(__STDC__) followed by #error This code requires a standard C compiler - rather than letting the build produce something broken.

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63.

What will be the output of the following C code?

#include <stdio.h>
#define max 20
main()
{
    #ifndef max
    #define min 10
    #else
    #define min 30
    #endif
    printf("%d",min);
}

A.10
B.20
C.30
D.error

Answer: C

Correct answer: 30

max is defined, so #ifndef max is false and the #else branch is taken, defining min as 30. The program prints 30.

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64.

What will be the output of the following C code?

#include <stdio.h>
#define hello 10
main()
{
    #ifdef hello
    #undef hello
    #define hello 100
    #else
    #define hello 200
    #endif
    printf("%d",hello);
}

A.Error
B.10
C.100
D.200

Answer: C

Correct answer: 100

hello is defined, so the #ifdef branch runs: the old definition is removed with #undef and a new one, 100, put in its place. Using #undef before redefining is exactly the right way to do this, since it avoids the redefinition warning. The output is 100.

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65.

What will be the output of the following C code?

#include <stdio.h>
#define sf 10
main()
{
    if(sf==100)
        printf("good");
    else
    {
        printf("bad");
        sf=100;
    }
    printf("%d",sf);
}

A.100
B.bad
C.10
D.error

Answer: D

The above code will result in an error because a macro cannot be defined using a simple assignment operator. In the code shown above, the value of sf is changed to 100 using the assignment operator. This causes an error.

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66.

What will be the output of the following C code?

#include <stdio.h>
void f()
{
    #define sf 100
    printf("%d",sf);
}
int main()
{
    #define sf 99;
    f();
    printf("%d",sf);
}

A.error
B.100
C.99
D.10099

Answer: A

Correct answer: error

The macro body is 99; - the semicolon is part of the text. Substituting it gives printf("%d",99;);, a syntax error. (The redefinition of sf without an intervening #undef is a second fault.) A #define for a constant should never end in a semicolon.

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67.

What will be the output of the following C code?

#include <stdio.h>
#define INDIA 1
#define US 2
#define CC US
main()
{
    #if CC==INDIA
    printf("Rupee");
    #elif CC==US
    printf("Dollar");
    #else
    printf("Euro");
    #endif 
}

A.Euro
B.Rupee
C.Dollar
D.Error

Answer: C

Correct answer: Dollar

CC expands to US, which expands to 2 - expansions are rescanned. The first test 2 == 1 fails, the #elif test 2 == 2 succeeds, so printf("Dollar") is the only line that reaches the compiler.

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68.

What will be the output of the following C code?

#define sqr(x) x*x
main()
{
    int a1;
    a1=25/sqr(5);
    printf("%d",a1);
}

A.25
B.1
C.5
D.error

Answer: A

The output of the code shown above is 25. This is because the arithmetic statement takes the form of: 25/5*5 (which is equal to 1). If the macro had been defined as #define sqr(x) (x*x), the output would have been 1.

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69.

Which of the following is not a preprocessor directive?

A.#error
B.#pragma
C.#if
D.#ifelse

Answer: D

Correct answer: #ifelse

There is no #ifelse directive. The conditional set is #if, #ifdef, #ifndef, #elif, #else and #endif - the “else if” form is spelt #elif. #error and #pragma are both real directives.

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70.

Which of the following are C preprocessors?

A.#ifdef
B.#define
C.#endif
D.all of the mentioned

Answer: D

Correct answer: all of the mentioned

#ifdef, #define and #endif are all preprocessor directives - the first and third belonging to conditional compilation and the second to macro definition. Every directive begins with # and is processed before the compiler proper sees the file.

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