defined

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

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.

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

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In this video tutorial lets see how to use preprocessor operator “defined” in C programming language.

How Does ‘defined’ preprocessor operator work?

defined preprocessor operator can only be used with #if and #elif directives.

If the Macro name passed to define is actually defined, then it returns true or else it’ll return false.

Syntac of “defined”

#if( defined(macro_name1) )
  // Some Set of Code
#elif( defined(macro_name2) )
  // Some More Set of Code
#endif

#if defined(macro_name1) 
  // Some Set of Code
#elif defined(macro_name2) 
  // Some More Set of Code
#endif

Video Tutorial: C Preprocessor Operator: defined

[youtube https://www.youtube.com/watch?v=IDiV22rMPBk]

YouTube Link: https://www.youtube.com/watch?v=IDiV22rMPBk [Watch the Video In Full Screen.]

Source Code: C Preprocessor Operator: defined

#include<stdio.h>

#define PI 3.14159265358979323846

#define AREA

void area_circle(float r);
void circumference_circle(float r);

int main()
{
    float r;

    printf("Enter radius of Circle\n");
    scanf("%f", &r);

    #if defined(AREA)
        area_circle(r);
    #elif defined(CIRCUM)
        circumference_circle(r);
    #else
        printf("Code To Be Implemented\n");

    #endif // defined

    return 0;
}

void area_circle(float r)
{
    printf("Area of Circle is %f\n", (PI * r * r) );
}

void circumference_circle(float r)
{
    printf("Circumference of Circle is %f\n", (2 * PI * r) );
}

Output:
Enter radius of Circle
5
Area of Circle is 78.539816

In above source code AREA macro template or macro name is defined. So defined(AREA) returns true in #if condition. Hence the code inside #if block gets compiled and executed.

#include<stdio.h>

#define PI 3.14159265358979323846

#define CIRCUM

void area_circle(float r);
void circumference_circle(float r);

int main()
{
    float r;

    printf("Enter radius of Circle\n");
    scanf("%f", &r);

    #if defined(AREA)
        area_circle(r);
    #elif defined(CIRCUM)
        circumference_circle(r);
    #else
        printf("Code To Be Implemented\n");

    #endif // defined

    return 0;
}

void area_circle(float r)
{
    printf("Area of Circle is %f\n", (PI * r * r) );
}

void circumference_circle(float r)
{
    printf("Circumference of Circle is %f\n", (2 * PI * r) );
}

Output:
Enter radius of Circle
5
Circumference of Circle is 31.415927

Here macro name AREA is not defined, hence code inside #if gets skipped. Whereas macro CIRCUM is defined, so the code inside #elif gets compiled and executed.

If neither AREA and nor CIRCUM is defined, then code inside #else block gets compiled and executed.

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