#if-#else in C

In C Language, the #if and #else directives are preprocessor directives used for conditional compilation.

They allow us to include or exclude sections of code based on the evaluation of constant expressions at compile-time.

Syntax:

#if constant_expression
    // Code to be compiled if constant_expression is true (non-zero)
#else
    // Code to be compiled if constant_expression is false (zero)
#endif

"constant_expression" is a constant expression that is evaluated at compile-time.
If the expression evaluates to non-zero (true), the code between "#if" and "#else" is included in the compilation.
Otherwise, the code between "#else" and "#endif" is included.

Conditional Compilation with "#if" and "#else":

Example:

#include <stdio.h>

#define DEBUG 1

int main() {
#if DEBUG
    printf("Debugging information.\n");
#else
    printf("Release mode.\n");
#endif

    return 0;
}

Output:

Debugging information.

the macro "DEBUG" is defined with a value of 1.
Therefore, the condition "#if DEBUG" evaluates to "true", and the debugging information message will be included in the compiled program.
If "DEBUG" were defined as 0 or not defined at all, the "#else" block would be compiled, and the "Release mode." message would be included.

Conditional Compilation with Numeric Expressions:

Example:

#include <stdio.h>

#define MAX_SIZE 100
#define ARRAY_SIZE 90

int main() {
#if MAX_SIZE > ARRAY_SIZE
    printf("Max size is greater than array size.\n");
#else
    printf("Array size is greater than or equal to max size.\n");
#endif

    return 0;
}

Output:

Max size is greater than array size.

the condition "#if MAX_SIZE > ARRAY_SIZE" is evaluated at compile-time.
If "MAX_SIZE" is greater than "ARRAY_SIZE", the first message will be printed; otherwise, the second message will be printed.

Using #else for Fallback:

Example:

#include <stdio.h>

#define USE_FEATURE_X

int main() {
#if defined(USE_FEATURE_X)
    printf("Feature X is enabled.\n");
#else
    printf("Feature X is not enabled.\n");
#endif

    return 0;
}

Output:

Feature X is enabled.

if "USE_FEATURE_X" is defined, the code inside the "#if" block will be compiled, and "Feature X is enabled." will be printed.
Otherwise, the code inside the "#else" block will be compiled, and "Feature X is not enabled." will be printed.

Note:

"#if" and "#else" can be used for conditional compilation in C programming, allowing us to control which sections of code are included in the compiled program based on the evaluation of constant expressions.

<-- #ifndef

#undef -->

#if-#else in C

In C Language, the #if and #else directives are preprocessor directives used for conditional compilation.

They allow us to include or exclude sections of code based on the evaluation of constant expressions at compile-time.

Syntax:

#if constant_expression
    // Code to be compiled if constant_expression is true (non-zero)
#else
    // Code to be compiled if constant_expression is false (zero)
#endif

"constant_expression" is a constant expression that is evaluated at compile-time.
If the expression evaluates to non-zero (true), the code between "#if" and "#else" is included in the compilation.
Otherwise, the code between "#else" and "#endif" is included.

Conditional Compilation with "#if" and "#else":

Example:

#include <stdio.h>

#define DEBUG 1

int main() {
#if DEBUG
    printf("Debugging information.\n");
#else
    printf("Release mode.\n");
#endif

    return 0;
}

Output:

Debugging information.

the macro "DEBUG" is defined with a value of 1.
Therefore, the condition "#if DEBUG" evaluates to "true", and the debugging information message will be included in the compiled program.
If "DEBUG" were defined as 0 or not defined at all, the "#else" block would be compiled, and the "Release mode." message would be included.

Conditional Compilation with Numeric Expressions:

Example:

#include <stdio.h>

#define MAX_SIZE 100
#define ARRAY_SIZE 90

int main() {
#if MAX_SIZE > ARRAY_SIZE
    printf("Max size is greater than array size.\n");
#else
    printf("Array size is greater than or equal to max size.\n");
#endif

    return 0;
}

Output:

Max size is greater than array size.

the condition "#if MAX_SIZE > ARRAY_SIZE" is evaluated at compile-time.
If "MAX_SIZE" is greater than "ARRAY_SIZE", the first message will be printed; otherwise, the second message will be printed.

Using #else for Fallback:

Example:

#include <stdio.h>

#define USE_FEATURE_X

int main() {
#if defined(USE_FEATURE_X)
    printf("Feature X is enabled.\n");
#else
    printf("Feature X is not enabled.\n");
#endif

    return 0;
}

Output:

Feature X is enabled.

if "USE_FEATURE_X" is defined, the code inside the "#if" block will be compiled, and "Feature X is enabled." will be printed.
Otherwise, the code inside the "#else" block will be compiled, and "Feature X is not enabled." will be printed.

Note:

"#if" and "#else" can be used for conditional compilation in C programming, allowing us to control which sections of code are included in the compiled program based on the evaluation of constant expressions.

<-- #ifndef

#undef -->