• A structure in C is a collection of items of different types. You can think of a structure as a "record" is in Pascal or a class in Java without methods.

• Structures, or structs, are very useful in creating data structures larger and more complex than the ones we have discussed so far.

• Simply you can group various built-in data types into a structure.

• Object conepts was derived from Structure concept. You can achieve few object oriented goals using C structure but it is very complex.

Following is the example how to define a structure.

struct student {
    char firstName[20];
    char lastName[20];
    char SSN[9];
    float gpa;
  };

Now you have a new datatype called student and you can use this datatype define your variables of student type:

struct student student_a, student_b;
or an array of students as
struct student students[50];

Another way to declare the same thing is:

struct {
    char firstName[20];
    char lastName[20];
    char SSN[10];
    float gpa;
  } student_a, student_b;

All the variables inside an structure will be accessed using these values as student_a.firstName will give value of firstName variable. Similarly we can aqccess other variables.

Structure Example:

Try out following example to understand the concept:

#include <stdio.h>
struct student {
    char firstName[20];
    char lastName[20];
    char SSN[10];
    float gpa;
};
main()
{
  struct student student_a;
  strcpy(student_a.firstName, "Hasan");
  strcpy(student_a.lastName, "Kole");
  strcpy(student_a.SSN, "2333234" );
  student_a.gpa = 2015.20;
 printf( "First Name: %s\n", student_a.firstName );
 printf( "Last Name: %s\n", student_a.lastName );
 printf( "SNN : %s\n", student_a.SSN );
 printf( "GPA : %f\n", student_a.gpa );
}

This will produce following results:

First Name: Hasan
Last Name: Kole
SSN : 2333234
GPA : 2015.20

Pointers to Structs:

Sometimes it is useful to assign pointers to structures (this will be evident in the next section with self-referential structures). Declaring pointers to structures is basically the same as declaring a normal pointer:

struct student *student_a;

To dereference, you can use the infix operator: ->.

printf("%s\n", student_a->SSN);

typedef Keyword

There is an easier way to define structs or you could "alias" types you create. For example:

typedef struct{
    char firstName[20];
    char lastName[20];
    char SSN[10];
    float gpa;
  }student;

Now you can use student directly to define variables of student type without using struct keyword. Following is the example:

student student_a;

You can use typedef for non-structs:

typedef long int *pint32;
pint32 x, y, z;

x, y and z are all pointers to long ints

Unions Datatype

Unions are declared in the same fashion as structs, but have a fundamental difference. Only one item within the union can be used at any time, because the memory allocated for each item inside the union is in a shared memory location.

Here is how we define a Union

union Shape {
      int circle;
      int triangle;
      int ovel;
};

We use union in such case where only one condition will be applied and only one variable will be used.

Conclusion:

• You can create arrays of structs.
• Structs can be copied or assigned.
• The & operator may be used with structs to show addresses.
• Structs can be passed into functions. Structs can also be returned from functions.
• Structs cannot be compared!
• Structures can store non-homogenous data types into a single collection, much like an array does for common data (except it isn't accessed in the same manner).
• Pointers to structs have a special infix operator: -> for dereferencing the pointer.
• typedef can help you clear your code up and can help save some keystrokes.