Tag: #ifdef

#undef Directive: C Program

In today’s video tutorial lets see how to use #undef preprocessor directive.

Related Read:
C Program To Find Area of Circle Using Macro Expansion
Conditional Compilation In C: #if #elif #else #endif

What Does #undef Do?

#undef removes or undefines a macro name which is previously created or defined using #define directive.

Syntax

#undef MACRO_NAME

Note: If a Macro template is associated with a Macro expansion, you need not mention the macro expansion to undefine the macro. You simple undefine using this syntax: #undef MACRO_NAME

Video Tutorial: #undef Preprocessor Directive: C Program


[youtube https://www.youtube.com/watch?v=1FwlFR9PQDE]

YouTube Link: https://www.youtube.com/watch?v=1FwlFR9PQDE [Watch the Video In Full Screen.]

Source Code: #undef Directive: C Program

#include<stdio.h>

#define iOS
#undef iOS

int main()
{
    #if defined(iOS)
        printf("I Love Apple Devices\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
Macro iOS has been undefined

In above source code we’re undefining the Macro iOS immediately after defining it. So defined(iOS) returns false or 0. So code inside #else block gets compiled and executed.

#include<stdio.h>

#define iOS

int main()
{
    #if defined(iOS)
        printf("I Love Apple Devices\n");
        printf("Gift me a latest iPhone!\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
I Love Apple Devices
Gift me a latest iPhone!

Here Macro iOS is defined, so defined(iOS) returns true. That’s why block of code inside #if directive gets compiled and executed.

#include<stdio.h>

#define iOS

int main()
{
    #undef iOS
    #if defined(iOS)
        printf("I Love Apple Devices\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
Macro iOS has been undefined

Even though #define iOS is defined at global level you can undefine it inside main() method.

Note: defined() is a preprocessor operator which returns true if the passed macro name is defined orelse returns false or zero if its not defined.

Instead of defined() preprocessor operator you could even use #ifdef or #ifndef to check if the Macro template or Macro name is defined or not defined.

#include<stdio.h>

#define iOS

int main()
{
    int flag = 1;

    do
    {
        #if defined(iOS)
            printf("I Love Apple Devices\n");
            printf("This displays all your iPhone Device Details\n");
        #else
            printf("I'm inside do while block.\n");
        #endif // defined

        printf("Enter your choice? (0/1)\n");
        scanf("%d", &flag);

    }while(flag);


    #undef iOS

    #if defined(iOS)
        printf("I Love Apple Devices\n");
        printf("This displays all your iPhone Device Details\n");
    #else
        printf("Macro iOS has been undefined\n");
    #endif // defined

    return 0;
}

Output:
I Love Apple Devices
This displays all your iPhone Device Details
Enter your choice? (0/1)
1
I Love Apple Devices
This displays all your iPhone Device Details
Enter your choice? (0/1)
1
I Love Apple Devices
This displays all your iPhone Device Details
Enter your choice? (0/1)
0
Macro iOS has been undefined

In above program we are defining iOS macro globally. Inside main method we use do while loop and keep executing #if #else #endif directives. Since iOS is defined, the block of code inside #if gets executed each time. Once user enters 0 as input, the control exits do while loop execution. Next it encounters #undef iOS, so defined(iOS) now returns false or 0, hence code inside #else directive gets executed.

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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 https://www.youtube.com/watch?v=J0nWQRqb3Fc]

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

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

#include<stdio.h>

#define iOS

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #else
        printf("This is code for Android Devices\n");
    #endif // iOS

    return 0;
}

Output:
This is iOS Code

#include<stdio.h>

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #else
        printf("This is code for Android Devices\n");
    #endif // iOS

    return 0;
}

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.

#include<stdio.h>
#define iOS

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #endif // iOS

    printf("This is code for all non iOS Devices\n");

    return 0;
}

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

#include<stdio.h>

int main()
{

    #ifdef iOS
        printf("This is iOS Code\n");
    #endif // iOS

    printf("This is code for all non iOS Devices\n");

    return 0;
}

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.

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