Defining Structures

 Although arrays greatly improved our ability to store data, there is one major drawback to their use ... each element (each box) in an array must be of the same data type.  It is often desirable to group data of different types and work with that grouped data as one entity.  We now have the power to accomplish this grouping with a new data type called a structure.

A structure is a collection of variable types grouped together. You can refer to a structure as a single variable, and to its parts as members of that variable by using the dot (.) operator.  The power of structures lies in the fact that once defined, the structure name becomes a user-defined data type and may be used the same way as other built-in data types, such as int, double, char, and apstring.

Defining a structure

When dealing with the students in a college, many variables of different types are needed.  It may be necessary to keep track of a name, an address (street, city, state, zip code), an age, an ID number, and a grade point average, for example. 

struct STUDENT_TYPE
{
        apstring name, street, city, state, zipcode;
        int age;
        double IDnum;
        double grade;
} ;

Note semicolon!!!

The name of the struct is in capital letters followed by the word TYPE.  While this naming convention is NOT required, it may help you to remember that you have created a new data type.

STUDENT_TYPE is called the structure tag, and is your brand new data type, like int, double, char or apstring.

name, street, city, state, zipcode, age, IDnum, and grade are structure members.

Declaring Variables of Type struct

The most efficient method of dealing with structure variables is to define the structure globally.  This tells "the whole world", namely main and any functions in the program, that a new data type exists.  To declare a structure globally, place it BEFORE int main(void).  The structure variables can then be defined locally in main, for example. 
. . .
struct STUDENT_TYPE
{
       apstring name, street, city, state, zipcode;
       int age;
       double IDnum;
       double grade;
} ;

int main(void)
{
       // declare two variables of the new type
       STUDENT_TYPE student1, student2;

       // array of structures
       apvector <STUDENT_TYPE> roster(10); 
       . . .