C User defined data type

User defined data type:

1. typedef
2. enum

1.typedef

• In C language a user can define an identifier that represents an existing data type.
• The user defined datatype identifier can later be used to declare variables.
• Syntax is
          typedef datatype identifier;
      here datatype represents the existing primary or derived data type. 
•  Example:
      typedef int marks;
      Now marks can be used  to declare integer variables as
      marks mark1,mark2; 

2.enum 

• enum is short form of enumeration 
• enumeration consists of a set of named integer constants. ie enumeration is a list of constant integer values.
• Syntax for enum declaration is
          Enum identifier {value1,value2,.....value n}
• The identifier is a user defined enumerated datatype which can be used to declare variables that have one of the values enclosed with in the braces.
• after the declaration,we can define variables with this new type as
           enum identifier v1,v2,v3....vn
• The enumerated variables v1,v2,v3...vn can have only one of the values value1,value2....value n.
• Example
  declaration
  enum day {Monday,Tuesday,Wednesday,Thursday,Friday,Saturday};
  definition
  enum day day1,day2,day3;
  
  day1=Monday;
  day2=Friday;
  
  day3=Sunday;//Error because sunday is not in the enum definition
• The first name in an enum has value 0, the next has value 1 and so on,unless explicit values are specified. 
• In the above example Monday is assigned to 0,Tuesday is assigned to 1 and so on and Saturday is assigned to 5 internally.
• we can also explicitly assign a value to the set
    enum day {Monday,Tuesday=0,Wednesday,Thursday,Friday,Saturday};
  The value 0 is associated with Monday by default.Then Tuesday is explicitly set to 0.so Wednesday is set to 1 and so on.

Points to consider:

1. To explicitly assign an integer value to a variable of an enumerated data type,use a type cast
      enum day today;
       today=(enum day)Monday;
   now today has value 0
2.  In declaration, enum identifier {value1,value2,.....value n};value1,value2...valuen must all be distinct from each other and different from ordinary variable names,but the values need not be distinct.
       enum student1 {Anand=0,Ashok,Alex} Boys;
       enum student2 {skippy=0,slowvy,Ashok} Girls;
   The above code will create an compiler error because 'Ashok' is available in both the list.
3. Enum is a set of named integer constant.so while printing the enum members the integer values only printed, Sometimes we need to print the actual name of the enum value for troubleshooting.But we cant directly print the enum members using %s.we can use switch statement or if-else to print the enum values.But if the enum consist of more than 100 constants then the program becomes very large. A simple strategy is to use array of strings(char *)
   Example program to print the enum data

#include<stdio.h>
enum new_enum{sunday,monday,tuesday,wednesday,thursday,friday,saturday};
const char *enum_names[]={"sunday","monday","tuesday","wednesday","thursday","friday","saturday"};
int main()
{
    enum new_enum e1,e2;
    e1=sunday;
    e2=monday;
    printf("day1=%s\n",enum_names[e1]);
    printf("day2=%s\n",enum_names[e2]);
    return 0;
}
Output
day1=sunday
day2=monday