What do we mean by a "Struct"?

A Struct (or a strucure) is a way of storing many different variables of different types under the same name. In other words it is a created data type that groups several data types together. For some it would be easier to understand it as a “record.” This allows a program to become modular, which is easier to modify because its design makes things more compact. It is very useful in the use of databases.

It is very similar to an array. It includes all of the properties an array has with the exception that a structure does not require all elements to be of the same data type, which can be understood from its basic definition.

Structs and classes are often compared. They are diversified particularly because of their use of heap or stack. This is the way in which they use memory. Heap is used by classes and it uses up memory. Stack is what structs use and is much more memory efficient. It can be seen as something similar to recycling memory.

Text Box: Data Type
Used

The following is and example of a database that can be used

to illustrate structured data objects:

Name

Text Box: Name

char

Text Box: char

S.S.N.

Text Box: S.S.N.

char

Date of Birth

Text Box: Date of Birth

char

Address

Text Box: Address

char

Text Box: char

Occupation

Text Box: Occupation

char

Age

int

NAME	SSN	DOB	ADDRESS	OCCUPATION	AGE
*Angel Medrano*	*987654321*	*08/29/81*	*123 Somewhere*	*Student*	23

In the previous example, you can see the purpose of a structured data object. In C, you can create a data type to represent the previous variables, however, it must be declared. How? It is explained in the next section.

How do we declare a struct in C?

This can be illustrated through a list of steps. Considering the database example above, the steps included are to:

1 Determine what fields we wish to have in our record.

2 Determine the data type for each variable that will be used in each of the fields.

3 Determine the amount of bytes to be assigned to each of the fields.

4 Declare the record in C according to the fields and variables selected in the previous steps.

The following segments of a C program illustrate these steps in executable code:

struct Customer Info

{ char Name [31];

char SSN [10];

char DOB [8];

char Address [41];

int Age [2]; };

int main()

{ struct Custmer Info active =

{"Favre, Brett",12345,"CIS",3.27,67};

struct student alumni =

{"White, Reggie", 23456, "Pain", 1.34, 89};

printf("Name: %13s %14s\n",active.student_name,

alumni.student_name);

printf("ID: %13d %14d\n",active.student_id,

alumni.student_id);

printf("Major: %13s %14s\n",active.major,

alumni.major);

printf("gpa: %13.3f %14.3f\n",active.gpa,

alumni.gpa);

printf("Hours: %13d %14d\n",active.total_hrs,

alumni.total_hrs);

return 0;

}

Why are Name and Address given an odd number of bytes? Is this the desire of the programmer or is there a reason for this choice?

The truth is that without the extra byte given to “Name” and “Address” we would need to keep track of the string size. However, by previously adding a byte to allow a null character, keeping track of a string size is not necessary.

How do we declare a struct in other languages?

The way a struct is declared in C can be compared to the way it is declared in other languages since there are some similarities. Take for instance, the following structures in their respective languages:

In COBOL

01 Customer Info

02 Name PIC X(40).

02 SSN PIC 9(9).

02 DOB PIC 9(6).

02 Address PIC X(35).

02 Occupation PIC X(30).

02 Age PIC 9(3).

In PASCAL

Type Customer Info = record

Name : array [1..40] char

SSN : interger;

DOB : integer;

Adress :array[1..35] of char;

Occupation : array [1..30];

Age : integer;

end;

In SQL

create table student

( student_name char(30) not null,

student_id integer not null,

major char(10) default ‘CIS’,

gpa decimal(4,2),

total_hours smallint,

primary key (student_id),

check (gpa > 0) )

In JAVA

{
   int i = 3;

   public static void main (String [] args)
   {
      CODemo2 obj1 = new CODemo2 ();

      System.out.println ("obj1.i = " + obj1.i);
      obj1.printHello ();

      CODemo2 obj2 = new CODemo2 ();

      obj1.i = 5;

      System.out.println ("obj1.i = " + obj1.i);
      obj1.printHello ();

      System.out.println ("obj2.i = " + obj2.i);
      obj2.printHello ();
   }

   void printHello ()
   {
      System.out.println ("Hello! i = " + i + "\n");
   }
}

Learning the basics of structured data objects is important in any language. As seen in the previous examples of code, how you declare them depends on the syntax (or rules) of the language.

Tutorial Questions:

1. Why is there an extra byte allocated for Name and Address if they should consist of 30 and 40 correspondingly?

The reason for this, is because we are storing these fields as the data type character array, which means that we need to reserve one extra byte for the null character. This is necessary, otherwise we would have to keep track of the length of each of the strings Name and Address.