C - ifdef
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C - ifdef - #ifdef - Definition and Usage
- If the value of expression is true, then the code that immediately follows the command will be compiled.
- “#ifdef” directive checks whether the particular macro is defined or not.
- #ifdef directive checks whether the identifier is currently defined.
- Identifiers can be defined by a #define directive or on the command line.
- If such identifiers have not been subsequently undefined, they are considered currently defined.
- If #ifdef directive is defined, “If” clause statements are included in source file. Otherwise, “else” clause statements are included in source file for compilation and execution..
- #endif always matches the nearest #ifdef (or #ifndef, or #if).
- Also, we cannot start a conditional group in one file and end it in another.
- The number of necessary #endif directives changes according to whether the elseif or #else directive is used.
Sample - C Code
C Code - Explanation
- In C programming #include<stdio.h> specifies a statement which tells the compiler to insert the contents of stdio at that particular place. A header file is a C file, that typically ends in ".h".
- In this example, #define WIKITECHY 16 defines the constant name (WIKITECHY) with its value (16).
- In this example, #ifdef WIKITECHY specifies this directive checks whether the identifier(WIKITECHY) is currently defined, there by executing the statement “Hello! welcome to wikitechy”.
- The command #elseif simply truncates the if statement and prints the else statement www.wikitechy.com.
- In this example, #endif specifies the end of the program.
Sample Output - Programming Examples
- Here in this output the “Hello! welcome to wikitechy” is printed in the console window since the ifdef directive defines that the identifier(WIKITECHY) is currently defined.
- Here in this output , the ifdef directive doesn’t define the identifier(WIKITECHY) so the else statement prints the statement “www.wikitechy.com”.