Conditional compilation - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of conditional compilation with defined macro

What is the output of this C program when compiled and run?

C

#include <stdio.h>

#define DEBUG

int main() {
#ifdef DEBUG
    printf("Debug mode\n");
#else
    printf("Release mode\n");
#endif
    return 0;
}

A Release mode

B Debug mode

C Compilation error

D No output

💡 Hint

Look at the #ifdef directive and the defined macro.

Since DEBUG is defined, the #ifdef DEBUG block runs, printing "Debug mode".

❓ Predict Output

intermediate

Output when macro is not defined

What will this program print when compiled and run?

C

#include <stdio.h>

int main() {
#ifndef RELEASE
    printf("Debug build\n");
#else
    printf("Release build\n");
#endif
    return 0;
}

A Compilation error

B Release build

C Debug build

D No output

💡 Hint

Check if RELEASE is defined or not.

RELEASE is not defined, so #ifndef RELEASE is true and prints "Debug build".

❓ Predict Output

advanced

Output with nested conditional compilation

What is the output of this program?

C

#include <stdio.h>

#define FEATURE_X

int main() {
#ifdef FEATURE_X
    #ifdef FEATURE_Y
        printf("Feature X and Y enabled\n");
    #else
        printf("Only Feature X enabled\n");
    #endif
#else
    printf("No features enabled\n");
#endif
    return 0;
}

A Only Feature X enabled

B Feature X and Y enabled

C Compilation error

D No features enabled

💡 Hint

Check which macros are defined.

FEATURE_X is defined but FEATURE_Y is not, so it prints "Only Feature X enabled".

❓ Predict Output

advanced

Output with macro value comparison

What will this program print when compiled and run?

C

#include <stdio.h>

#define VERSION 3

int main() {
#if VERSION >= 5
    printf("Version 5 or higher\n");
#elif VERSION == 3
    printf("Version 3 detected\n");
#else
    printf("Older version\n");
#endif
    return 0;
}

A Compilation error

B Version 5 or higher

C Older version

D Version 3 detected

💡 Hint

Look at the value of VERSION and the #if conditions.

VERSION is 3, so the #elif VERSION == 3 block runs, printing "Version 3 detected".

❓ Predict Output

expert

Output with complex conditional compilation and macro undefinition

What is the output of this program?

C

#include <stdio.h>

#define MODE 1

#if MODE == 1
    #define FEATURE_A
#elif MODE == 2
    #define FEATURE_B
#endif

#undef MODE

int main() {
#ifdef FEATURE_A
    printf("Feature A active\n");
#endif
#ifdef FEATURE_B
    printf("Feature B active\n");
#endif
#ifdef MODE
    printf("Mode defined\n");
#else
    printf("Mode undefined\n");
#endif
    return 0;
}

A

Feature A active
Mode undefined

B

Feature B active
Mode undefined

C

Feature A active
Mode defined

D Compilation error

💡 Hint

Check which macros are defined and which are undefined after #undef.

MODE is initially 1, so FEATURE_A is defined. Then MODE is undefined. So output prints "Feature A active" and "Mode undefined".