8.1. Basic Data Structures

• 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 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 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 typedef to define a new variable type, then we do not need to use the 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 );
  

Homework 12 - Numerical Proof of Euler’s Formula provides an opportunity to build a program using a simple data structure.

8.2. 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);
}