What Is a Structure?

As you've learned, arrays can be used to collect groups of variables of the same type. The question now is how to aggregate pieces of data that are not identically typed.

The answer is that you can group variables of different types with a data type called a structure. In C, a structure collects different data items in such a way that they can be referenced as a single unit.

There are several major differences between an array and a structure. Besides the fact that data items in a structure can have different types, each data item has its own name instead of a subscript value. In fact, data items in a structure are called fields or members of the structure.

The next two subsections teach you how to declare structures and define structure variables.
Declaring Structures

The general form to declare a structure is

struct struct_tag {
     data_type1 variable1;
     data_type2 variable2;
     data_type3 variable3;
     .
     .
     .
     };

Here struct is the keyword used in C to start a structure declaration. struct_tag is the tag name of the structure. variable1, variable2, and variable3 are the members of the structure. Their data types are specified respectively by data_type1, data_type2, and data_type3. As you can see, the declarations of the members have to be enclosed within the opening and closing braces ({ and }) in the structure declaration, and a semicolon (;) has to be included at the end of the declaration.

The following is an example of a structure declaration:

struct automobile {
     int year;
     char model[8];
     int engine_power;
     float weight;
     };

Here struct is used to start a structure declaration. automobile is the tag name of the structure. In this example, there are three types of variables, char, int, and float. The variables have their own names, such as year, model, engine_power, and weight.

Note that a structure tag name, like automobile, is a label to a structure. The compiler uses the tag name to identify the structure labeled by that tag name.
Defining Structure Variables

After declaring a structure, you can define the structure variables. For instance, the following structure variables are defined with the structure data type of automobile from the previous section:

struct automobile sedan, pick_up, sport_utility;

Here three structure variables, sedan, pick_up, and sport_utility, are defined by the structure of automobile. All three structure variables contain the four members of the structure data type of automobile.

Also, you can combine the structure declaration and definition into one statement like this:

struct automobile {
     int year;
     char model[8];
     int engine_power;
     float weight;
     } sedan, pick_up, sport_utility;

Here three structure variables, sedan, pick_up, and sport_utility, are defined with the structure data type of automobile in the single statement.