STRUCTURE AND UNIONS

STRUCTURE  AND  UNIONS


Structure is the collection of variables of different types under a single name for better handling. For example: You want to store the information about person about his/her name, citizenship number and salary. You can create these information separately but, better approach will be collection of these information under single name because all these information are related to person.

Structure Definition in C

Keyword struct is used for creating a structure.
Syntax of structure
struct structure_name
{
    data_type member1;
    data_type member2;
    .
    .
    data_type memeber;
};
We can create the structure for a person as mentioned above as:
struct person
{
    char name[50];
    int cit_no;
    float salary;
};
This declaration above creates the derived data type struct person.
Structure variable declaration
When a structure is defined, it creates a user-defined type but, no storage is allocated. For the above structure of person, variable can be declared as:
struct person
{
    char name[50];
    int cit_no;
    float salary;
};

Inside main function:

struct person p1, p2, p[20];
Another way of creating sturcture variable is:
struct person
{
    char name[50];
    int cit_no;
    float salary;
}p1 ,p2 ,p[20];
In both cases, 2 variables p1, p2 and array p having 20 elements of type struct person are created.

Accessing members of a structure

There are two types of operators used for accessing members of a structure.

Member operator(.)
Structure pointer operator(->)

Any member of a structure can be accessed as: structure_variable_name.member_name
Suppose, we want to access salary for variable p2. Then, it can be accessed as:
p2.salary

Example of structure


Write a C program to add two distances entered by user. Measurement of distance should be in inch and feet.(Note: 12 inches = 1 foot)

#include <stdio.h>
struct Distance{
    int feet;
    float inch;
}d1,d2,sum;
int main(){
    printf("1st distance\n");
    printf("Enter feet: ");
    scanf("%d",&d1.feet);  /* input of feet for structure variable d1 */
    printf("Enter inch: ");
    scanf("%f",&d1.inch);  /* input of inch for structure variable d1 */
    printf("2nd distance\n");
    printf("Enter feet: ");
    scanf("%d",&d2.feet);  /* input of feet for structure variable d2 */
    printf("Enter inch: ");
    scanf("%f",&d2.inch);  /* input of inch for structure variable d2 */
    sum.feet=d1.feet+d2.feet;
    sum.inch=d1.inch+d2.inch;
    if (sum.inch>12){  //If inch is greater than 12, changing it to feet.
        ++sum.feet;
        sum.inch=sum.inch-12;
    }
    printf("Sum of distances=%d\'-%.1f\"",sum.feet,sum.inch);
/* printing sum of distance d1 and d2 */
    return 0;
}

Output
1st distance
Enter feet: 12
Enter inch: 7.9
2nd distance
Enter feet: 2
Enter inch: 9.8
Sum of distances= 15'-5.7"

Keyword typedef while using structure

Programmer generally use typedef while using structure in C language. For example:
typedef struct complex{
  int imag;
  float real;
}comp;

Inside main:
comp c1,c2;
Here, typedef keyword is used in creating a type comp(which is of type as struct complex). Then, two structure variables c1 and c2 are created by this comp type.
Structures within structures
Structures can be nested within other structures in C programming.
struct complex
{
 int imag_value;
 float real_value;
};
struct number{
   struct complex c1;
   int real;
}n1,n2;
Suppose you want to access imag_value for n2 structure variable then, structure member n1.c1.imag_value is used.

Pointers can be accessed along with structures. A pointer variable of structure can be created as below:
struct name {
    member1;
    member2;
    .
    .
};
-------- Inside function -------
struct name *ptr;
Here, the pointer variable of type struct name is created.
Structure's member through pointer can be used in two ways:
Referencing pointer to another address to access memory
Using dynamic memory allocation
Consider an example to access structure's member through pointer.
#include <stdio.h>
struct name{
   int a;
   float b;
};
int main(){
    struct name *ptr,p;
    ptr=&p;            /* Referencing pointer to memory address of p */
    printf("Enter integer: ");
    scanf("%d",&(*ptr).a);
    printf("Enter number: ");
    scanf("%f",&(*ptr).b);
    printf("Displaying: ");
    printf("%d%f",(*ptr).a,(*ptr).b);
    return 0;
}
In this example, the pointer variable of type struct name is referenced to the address of p. Then, only the structure member through pointer can can accessed.

Structure pointer member can also be accessed using -> operator.
(*ptr).a is same as ptr->a
(*ptr).b is same as ptr->b

Accessing structure member through pointer using dynamic memory allocation
To access structure member using pointers, memory can be allocated dynamically using malloc() function defined under "stdlib.h" header file.
Syntax to use malloc()
ptr=(cast-type*)malloc(byte-size)
Example to use structure's member through pointer using malloc() function.
#include <stdio.h>
#include<stdlib.h>
struct name {
   int a;
   float b;
   char c[30];
};
int main(){
   struct name *ptr;
   int i,n;
   printf("Enter n: ");
   scanf("%d",&n);
   ptr=(struct name*)malloc(n*sizeof(struct name));
/* Above statement allocates the memory for n structures with pointer ptr pointing to base address */
   for(i=0;i<n;++i){
       printf("Enter string, integer and floating number  respectively:\n");
       scanf("%s%d%f",&(ptr+i)->c,&(ptr+i)->a,&(ptr+i)->b);
   }
   printf("Displaying Infromation:\n");
   for(i=0;i<n;++i)
       printf("%s\t%d\t%.2f\n",(ptr+i)->c,(ptr+i)->a,(ptr+i)->b);
   return 0;
}

Output
Enter n: 2
Enter string, integer and floating number  respectively:
Programming
2
3.2
Enter string, integer and floating number  respectively:
Structure
6
2.3
Displaying Information
Programming      2      3.20
Structure      6      2.30