Enum (Enumerated Type)

An enumeration is a user-defined data type that consists of integral constants. To define an enumeration, keyword enum is used.

enum flag { const1, const2, ..., constN };

Here, name of the enumeration is flag.

And, const1, const2,...., constN are values of type flag.

By default, const1 is 0, const2  is 1 and so on. You can change default values of enum elements during declaration (if necessary).

// Changing default values of enum
enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3,
};

Enumerated Type Declaration

When you create an enumerated type, only blueprint for the variable is created. Here's how you can create variables of enum type.

enum boolean { false, true };
enum boolean check;

Example: Enumeration Type

#include <stdio.h>

 enum week 
{ 
 sunday, monday, tuesday, wednesday, thursday, friday, saturday 
};

 int main()  
{ 
  enum week today; 
  today = wednesday;
  printf("Day %d",today+1); 
  return 0; 
}

Output
Day 4

Why enums are used in C programming?

Enum variable takes only one value out of many possible values. Example to demonstrate it,

#include <stdio.h>

enum suit {
    club = 0,
    diamonds = 10,
    hearts = 20,
    spades = 3
} card;

int main() 
{
        card = club;
 printf("Size of enum variable = %d bytes", sizeof(card)); 
 return 0;
}

Output

Size of enum variable = 4 bytes

It's because the size of an integer is 4 bytes.
This makes enum a good choice to work with flags.

How to use enums for flags?

Let us take an example,

enum designFlags {
 ITALICS = 1,
 BOLD = 2,
 UNDERLINE = 4
} button;

Suppose you are designing a button for Windows application. You can set flags ITALICS, BOLD and UNDERLINE to work with text.
There is a reason why all the integral constants are power of 2 in above pseudocode.

// In binary

ITALICS = 00000001
BOLD = 00000010
UNDERLINE = 00000100 

Since, the integral constants are power of 2, you can combine two or more flags at once without overlapping using bitwise OR | operator. This allows you to choose two or more flags at once. For example,

#include <stdio.h>

enum designFlags {
 BOLD = 1,
 ITALICS = 2,
 UNDERLINE = 4
};

int main() {
 int myDesign = BOLD | UNDERLINE; 

        //    00000001
        //  | 00000100
        //  ___________
        //    00000101

 printf("%d", myDesign);

 return 0;
}

Output

5 

ungetc

  // the following c program displays Whatever is typed by the user second time   
  #include <stdio.h>

        int main()

        {

            int n;

            scanf("%d", &n);

            ungetc(n, stdin);

            scanf("%d", &n);

            printf("%d\n", n);

            return 0;

        }

storage management in c

// 1.   The following program displays welcome

       #include <stdio.h>
        void main()
        {
            char *p = calloc(100, 1);
            p = "welcome";
            printf("%s\n", p);
        }


// 2.  the following memory allocation program displays 0
        #include <stdio.h>

        struct p

        {

            struct p *next;

            int x;

        };

        int main()

        {

            struct p *p1 = calloc(1, sizeof(struct p));

            p1->x = 1;

            p1->next = calloc(1, sizeof(struct p));

            printf("%d\n", p1->next->x);

            return 0;

        }