.. _struct:

Basic Data Structures
=====================

.. index:: struct

- 
   A *structure*, like an array, allows us to aggregate multiple
   pieces of data into a single named variable.

- 
   With structures, however, each member data element may have a
   distinct data type.

-  The :keyword:`struct` keyword is used to define the data structure.

 
    ::

        struct employee {
            char *name;    /* only alloc the pointer here. */
            int  ssn;
            int  salary;
            enum job_grade position;  /* any known data type may be used */
            int  hire_date;
        };


- 
   The :keyword:`struct` construct only defines the structure. It is still
   necessary to declare a variable in order to use the structure.

    ::

        struct employee intern;


- 
   If we use :keyword:`typedef` to define a new variable type, then we do not
   need to use the keyword :keyword:`struct` when we make a reference to the
   structure in our program.

    ::

        typedef struct employee EMPLOYEE;
        
        -or-
        
        typedef struct employee {
            char *name;
            int  ssn;
            ....
        } EMPLOYEE;
        
        EMPLOYEE intern;


- 
   Use a dot (``.``) to access member data for local variables. Use
   (``->``) to access member data when the variable is a pointer.

       ::

           EMPLOYEE boss, *temp;
           int input_data;
           char input_string[50];
           
           temp = &boss;
           
           temp->ssn = input_data;
           boss.name = (char *)calloc( strlen(input_string)+1, sizeof(char) );
           strcpy( temp->name, input_string );

:ref:`hw12` provides an opportunity to build a program using a simple data
structure.

Allocating memory for structures
================================

It is common to dynamically allocate the memory for a structure,
especially if we do not know in advance how much data will be
stored.

::

    struct employee *temp_employee;
    
    if((temp_employee=(struct employee *)malloc(sizeof(struct employee))) == NULL) {
        fprintf( stderr, "Memory allocation error\n" );
        exit(1);
    }