Tag: directives

Conditional Compilation In C: #ifndef #else #endif

In this video tutorial lets learn about preprocessor command or directives like #ifndef, #else and #endif. These directives are used for conditional compilation. #ifndef works completely opposite to #ifdef

Related Read:
Conditional Compilation In C: #ifdef #else #endif

How Does #ifndef Work?

The block of code between #ifndef and #endif works only if the macro name is NOT defined. If the macro name is defined, the compiler will skip the entire block of code inside #ifndef from compiling.

When To Use #ifndef

We’ve explained this in detail for video tutorial: Conditional Compilation In C: #ifdef #else #endif It holds good for this video tutorial too.

Video Tutorial: Conditional Compilation In C: #ifndef #else #endif



YouTube Link: https://www.youtube.com/watch?v=Y7UEY7Cxz7Y [Watch the Video In Full Screen.]

Source Code: Conditional Compilation In C: #ifndef #else #endif

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  1. #include<stdio.h>  
  2.   
  3. #define iOS  
  4.   
  5. int main()  
  6. {  
  7.     #ifndef iOS  
  8.         printf("I Love Apple Devices\n");  
  9.     #else  
  10.         printf("Code for Non Apple Devices\n");  
  11.     #endif // iOS  
  12.   
  13.     return 0;  
  14. }  

Output:
Code for Non Apple Devices

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  1. #include<stdio.h>  
  2.   
  3.   
  4. int main()  
  5. {  
  6.     #ifndef iOS  
  7.         printf("I Love Apple Devices\n");  
  8.     #else  
  9.         printf("Code for Non Apple Devices\n");  
  10.     #endif // iOS  
  11.   
  12.     return 0;  
  13. }  

Output:
I Love Apple Devices

In above programs, if iOS macro name is defined, then the set of code present inside #else block gets compiled and executed. If macro name iOS is not defined, then the code inside #ifndef block gets compiled and executed.

Note:

1. Using #else is optional. You can just use #ifndef and #endif.

2. Working of #ifndef is opposite to that of #ifdef.

3. This is similar to if else condition, but here the block of code which do not match the criteria doesn’t even get compiled. It’s treated like regular comments.

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Conditional Compilation In C: #if #elif #else #endif

In this video tutorial lets learn about preprocessor command or directives like #if, #elif, #else and #endif. These directives are used in conditional compilation.

Related Read:
Logical Operators In C
Conditional Compilation In C: #ifdef #else #endif

How Does #if and #elif Work?

The block of code inside #if block works only if the condition or the expression in resolved to non-zero number. If the expression is resolved to zero, then the code inside #if is skipped from compilation.

Same is true for #elif: If the condition or the expression is resolved to 0 then the code inside #elif block is skipped from compilation. If it’s resolved to non-zero number then the block of code inside #elif gets compiled and executed.

#else is optional and when used, it should be present only once between #if and #endif, and it should be present at the end(just before the #endif directive). If none of the conditions in #if and #elif match, then the code inside #else directive block gets executed.

#elif is also optional. But it can be used any number of times inside #if and #endif. But it should always be written after #if and not before it. It should be present before #else and not after #else directive.

Note: We can have nested #if #elif #else #endif inside #if and/or #elif and/or #else block. Nesting can go any number or depth.
But remember to have #endif for every #if.

Video Tutorial: Conditional Compilation In C: #if #elif #else #endif



YouTube Link: https://www.youtube.com/watch?v=0s-y4WADkf0 [Watch the Video In Full Screen.]

When To Use #if, #elif, #else and #endif?

1. When you want certain code to be skipped: #if or #elif or #else part of the code is compiled and executed only when the conditions are met.

2. To write portable code: Again, like that of in #ifdef and #else, we could write portable code. That is, we can write code for specific devices in #if, #elif and #else part and the compiler compiles the code specific to that device. All other code will be treated as comments.

3. Saving memory and file size: We could use it in big projects. We could include device specific files inside #if, #elif and #else blocks depending upon the condition. This way we could avoid including or importing all the files into our source code.

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  1. #include<stdio.h>  
  2.   
  3. int main()  
  4. {  
  5.     #if MARKS  
  6.         printf("GRADE A\n");  
  7.     #endif // MARKS  
  8.   
  9.     printf("Your Result\n");  
  10.   
  11.     return 0;  
  12. }  

Output:
Your Result

Here macro template is not even defined. And for #if to resolve and print the code inside its block, MARKS should be evaluated to non-zero number.

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  1. #include<stdio.h>  
  2.   
  3. #define MARKS  
  4.   
  5. int main()  
  6. {  
  7.     #if MARKS  
  8.         printf("GRADE A\n");  
  9.     #endif // MARKS  
  10.   
  11.     printf("Your Result\n");  
  12.   
  13.     return 0;  
  14. }  

Output:
Your Result

Here even though MARKS is defined, it doesn’t have a non-zero value, so the code inside #if block isn’t compiled.

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  1. #include<stdio.h>  
  2.   
  3. #define MARKS 50  
  4.   
  5. int main()  
  6. {  
  7.     #if MARKS  
  8.         printf("GRADE A\n");  
  9.     #endif // MARKS  
  10.   
  11.     printf("Your Result\n");  
  12.   
  13.     return 0;  
  14. }  

Output:
GRADE A
Your Result

Here MARKS has a macro expansion as 50, which not zero, so code inside #if gets compiled and executed.

Using logical operatation

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  1. #include<stdio.h>  
  2.   
  3. #define MARKS 50  
  4.   
  5. int main()  
  6. {  
  7.     #if(MARKS <= 100 && MARKS >= 80)  
  8.         printf("GRADE A\n");  
  9.     #endif // MARKS  
  10.   
  11.     printf("Your Result\n");  
  12.   
  13.     return 0;  
  14. }  

Output:
GRADE A
Your Result

Check the condition for #if. We can use logical operator and/or arithmetic operators and/or relational operators and/or Conditional Operators in the condition.

Nesting of Directives

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  1. #include<stdio.h>  
  2.   
  3. #define MARKS 50  
  4.   
  5. int main()  
  6. {  
  7.     #if(MARKS <= 100)  
  8.         printf("GRADE A\n");  
  9.         #if(MARKS >= 80)  
  10.             printf("You're excellent!\n");  
  11.         #else  
  12.             printf("You're not that excellent yet!\n");  
  13.         #endif  
  14.     #endif // MARKS  
  15.   
  16.     printf("Your Result\n");  
  17.   
  18.     return 0;  
  19. }  

Output:
GRADE A
You’re not that excellent yet!
Your Result

We can nest any number of #if #endif and/or #if #else #endif and/or #if #elif #endif and/or #if #elif #else #endif inside #if and/or #elif and/or #else.

Source Code: Conditional Compilation In C: #if #elif #else #endif

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  1. #include<stdio.h>  
  2.   
  3. #define MARKS 50  
  4.   
  5. int main()  
  6. {  
  7.     #if(MARKS <= 100 && MARKS >= 80)  
  8.         printf("GRADE A\n");  
  9.     #elif(MARKS <= 79 && MARKS >= 60)  
  10.         printf("GRADE B\n");  
  11.     #elif(MARKS <= 59 && MARKS >= 40)  
  12.         printf("GRADE C\n");  
  13.     #elif(MARKS <= 39 && MARKS >= 30)  
  14.         printf("GRADE D\n");  
  15.     #else  
  16.         printf("Please retake the test!\n");  
  17.     #endif // MARKS  
  18.   
  19.     printf("Your Result\n");  
  20.   
  21.     return 0;  
  22. }  

Output:
GRADE C
Your Result

Only 1 block of code is compiled and executed here, even if multiple conditions are satisfying. Whichever condition returns non-zero number first, the code inside its block gets executed. All other code will be skipped.

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert

Conditional Compilation In C: #ifdef #else #endif

In this video tutorial lets learn about preprocessor command or directives like #ifdef, #else and #endif. These directives are used for conditional compilation.

How Does #ifdef Work?

The block of code between #ifdef and #endif works only if the macro name is defined orelse the compiler will skip the entire block of code from compiling.

When To Use #ifdef

1. When we want the compiler to skip certain part of the source code: For this we could even use multi-line comment, but if we already had multi-line comments in the code, we can’t enclose the source code and multi-line comments with another multi-line comment. Nesting of multi-line comments are not allowed in C programming language. So instead of commenting the code we could make use of #ifdef directive.

2. To write portable code: For example, we could write code both for iOS and non-iOS devices. Using #ifdef we could check the device OS and based on that deliver specific set of codes. For this we can use #ifdef

Video Tutorial: Conditional Compilation In C: #ifdef #else #endif



YouTube Link: https://www.youtube.com/watch?v=J0nWQRqb3Fc [Watch the Video In Full Screen.]

Source Code: Conditional Compilation In C: #ifdef #else #endif

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  1. #include<stdio.h>  
  2.   
  3. #define iOS  
  4.   
  5. int main()  
  6. {  
  7.   
  8.     #ifdef iOS  
  9.         printf("This is iOS Code\n");  
  10.     #else  
  11.         printf("This is code for Android Devices\n");  
  12.     #endif // iOS  
  13.   
  14.     return 0;  
  15. }  

Output:
This is iOS Code

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  1. #include<stdio.h>  
  2.   
  3. int main()  
  4. {  
  5.   
  6.     #ifdef iOS  
  7.         printf("This is iOS Code\n");  
  8.     #else  
  9.         printf("This is code for Android Devices\n");  
  10.     #endif // iOS  
  11.   
  12.     return 0;  
  13. }  

Output:
This is code for Android Devices

In above programs, if iOS macro name is defined, then the set of code present inside #ifdef block gets compiled and executed. If macro name iOS is not defined, then the code inside #else block gets compiled and executed.

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  1. #include<stdio.h>  
  2. #define iOS  
  3.   
  4. int main()  
  5. {  
  6.   
  7.     #ifdef iOS  
  8.         printf("This is iOS Code\n");  
  9.     #endif // iOS  
  10.   
  11.     printf("This is code for all non iOS Devices\n");  
  12.   
  13.     return 0;  
  14. }  

Output:
This is iOS Code
This is code for all non iOS Devices

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  1. #include<stdio.h>  
  2.   
  3. int main()  
  4. {  
  5.   
  6.     #ifdef iOS  
  7.         printf("This is iOS Code\n");  
  8.     #endif // iOS  
  9.   
  10.     printf("This is code for all non iOS Devices\n");  
  11.   
  12.     return 0;  
  13. }  

Output:
This is code for all non iOS Devices

Note: using #else is optional. You can just use #ifdef and #endif.

This is similar to if else condition, but here the block of code which do not match the criteria doesn’t even get compiled. It’s treated like regular comments.

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert

Preprocessors In C Programming Language

In this video tutorial we’ll cover some basics about preprocessors and give you overview of what we’ll be covering in upcoming video tutorials.

Note

1. Preprocessor is a program that processes our source code before it is passed to the compiler. Hence its name pre-processor.

2. Preprocessor Commands are often called as DIRECTIVES.

3. Preprocessor Directives begin with # symbol.

Coming soon ..

1. Macro expansion.
2. File inclusion.
3. Conditional compilation.
4. Miscellaneous directives.
5. We’ll also be covering about predefined macros.

Previously we had covered a little bit about File inclusion, you can take a look at it here include directive in C Program

Video Tutorial: Preprocessors In C Programming Language



YouTube Link: https://www.youtube.com/watch?v=3esrK1iCOc4 [Watch the Video In Full Screen.]

Computer Science Students and Professionals

If you’re a computer science student enrolled in University, your syllabus may not include in-depth learning of preprocessors. But when it comes to competitive exams and real-time application programming you’ll have to use preprocessors a lot. So better learn it now.

In upcoming videos we’ll cover preprocessor concepts in detail with simple example programs to explain the concepts/topics.

So stay tuned, stay subscribed to our YouTube channel and blog.

For list of all c programming interviews / viva question and answers visit: C Programming Interview / Viva Q&A List

For full C programming language free video tutorial list visit:C Programming: Beginner To Advance To Expert