#if, #elif and #else Directives in C

What are #if, #elif, and #else in C

The #if, #elif, and #else directives in C are part of the preprocessor. They help you control which parts of your code get compiled based on certain conditions. These directives are not regular C code – they are processed before compilation begins.

Purpose of #if, #elif, and #else:

• When you want to compile different code for different platforms or operating systems.
• You are testing different features or debug options.
• You want to include or exclude certain code blocks without deleting them.

#if Directive in C

The #if directive checks if a given constant expression evaluates to true (non-zero). If so, the following code is included at compile-time.

Syntax:

#if constant_expression
    // Code to compile if condition is true
#endif

Example:

#include <stdio.h>
#define QUIZ_LEVEL 1
 
int main() {
#if QUIZ_LEVEL == 1
    printf("Beginner level quiz selected.\n");
#endif
    return 0;
}

This C program uses a macro QUIZ_LEVEL set to 1. Inside the main function, a #if directive checks if QUIZ_LEVEL equals 1. Since it does, it prints “Beginner level quiz selected”. This shows how macros can control which code runs based on conditions at compile time.

#elif Directive in C

The #elif directive stands for “else if”. It provides another condition if the #if fails.

Syntax:

#if condition1
    // Code for condition1
#elif condition2
    // Code for condition2
#endif

Example:

#include <stdio.h>
#define QUIZ_LEVEL 2
 
int main() {
#if QUIZ_LEVEL == 1
    printf("Beginner level selected.\n");
#elif QUIZ_LEVEL == 2
    printf("Intermediate level selected.\n");
#endif
    return 0;
}

This C program defines a macro QUIZ_LEVEL as 2. In the main function, it uses #if and #elif to check the value of QUIZ_LEVEL. Since the value is 2, it prints “Intermediate level selected.” This shows how conditional compilation can select different code blocks based on macro values.

#else Directive in C

The #else directive runs when none of the previous #if or #elif conditions were true.

Syntax:

#if condition
    // Code for condition
#else
    // Fallback code
#endif

Example:

#include <stdio.h>
#define QUIZ_LEVEL 3
 
int main() {
#if QUIZ_LEVEL == 1
    printf("Beginner quiz mode\n");
#else
    printf("Advanced quiz mode\n");
#endif
    return 0;
}

This C program sets the macro QUIZ_LEVEL to 3. It uses #if to check if the level is 1; since it’s not, the #else block runs, printing “Advanced quiz mode”. This shows how #if and #else can control code flow based on macro values.

Combining #if, #elif, and #else

These directives can be combined to handle complex conditional compilation scenarios.

#include <stdio.h>
#define QUIZ_LEVEL 2
 
int main() {
#if QUIZ_LEVEL == 1
    printf("Quiz Mode: Beginner\n");
#elif QUIZ_LEVEL == 2
    printf("Quiz Mode: Intermediate\n");
#else
    printf("Quiz Mode: Advanced\n");
#endif
    return 0;
}

Output:

Quiz Mode: Intermediate

This C program defines QUIZ_LEVEL as 2. It uses #if, #elif, and #else to choose which message to print. Since QUIZ_LEVEL is 2, it prints “Quiz Mode: Intermediate”. This shows how multiple conditions can control which code runs during compilation.

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