Fortran - Derived Data Types

Fortran allows you to define derived data types. A derived data type is also called a structure, and it can consist of data objects of different types.
Derived data types are used to represent a record. E.g. you want to keep track of your books in a library, you might want to track the following attributes about each book:

• Title
• Author
• Subject
• Book ID

Defining a Derived data type

To define a derived data type, the type and end type statements are used. . The type statement defines a new data type, with more than one member for your program. The format of the type statement is this:

type type_name      
   declarations
end type 

Here is the way you would declare the Book structure:

type Books
   character(len=50) :: title
   character(len=50) :: author
   character(len=150) :: subject
   integer :: book_id
end type Books

Accessing Structure Members

An object of a derived data type is called a structure
A structure of type Books can be created in a type declaration statement like:

type(Books) :: book1 

The components of the structure can be accessed using the component selector character (%):

book1%title = "C Programming"
book1%author = "Nuha Ali"
book1%subject = "C Programming Tutorial"
book1%book_id = 6495407

Note that there are no spaces before and after the % symbol.
Example
The following program illustrates the above concepts:

program deriveDataType

   !type declaration
   type Books
      character(len=50) :: title
      character(len=50) :: author
      character(len=150) :: subject
      integer :: book_id
   end type Books
   
   !declaring type variables
   type(Books) :: book1 
   type(Books) :: book2 
   
   !accessing the components of the structure
   
   book1%title = "C Programming"
   book1%author = "Nuha Ali"
   book1%subject = "C Programming Tutorial"
   book1%book_id = 6495407 
   
   book2%title = "Telecom Billing"
   book2%author = "Zara Ali"
   book2%subject = "Telecom Billing Tutorial"
   book2%book_id = 6495700
  
   !display book info
   
   Print *, book1%title 
   Print *, book1%author 
   Print *, book1%subject 
   Print *, book1%book_id  
   
   Print *, book2%title 
   Print *, book2%author 
   Print *, book2%subject 
   Print *, book2%book_id  

end program deriveDataType

When the above code is compiled and executed, it produces the following result:

 C Programming                                     
 Nuha Ali                                          
 C Programming Tutorial            
     6495407
 Telecom Billing                                   
 Zara Ali                                          
 Telecom Billing Tutorial            
     6495700

Array of Structures

You can also create arrays of a derived type:

type(Books), dimension(2) :: list

Individual elements of the array could be accessed as:

list(1)%title = "C Programming"
list(1)%author = "Nuha Ali"
list(1)%subject = "C Programming Tutorial"
list(1)%book_id = 6495407

The following program illustrates the concept:

program deriveDataType

   !type declaration
   type Books
      character(len=50) :: title
      character(len=50) :: author
      character(len=150) :: subject
      integer :: book_id
   end type Books
   
   !declaring array of books
   type(Books), dimension(2) :: list 
    
   !accessing the components of the structure
   
   list(1)%title = "C Programming"
   list(1)%author = "Nuha Ali"
   list(1)%subject = "C Programming Tutorial"
   list(1)%book_id = 6495407 
   
   list(2)%title = "Telecom Billing"
   list(2)%author = "Zara Ali"
   list(2)%subject = "Telecom Billing Tutorial"
   list(2)%book_id = 6495700
  
   !display book info
   
   Print *, list(1)%title 
   Print *, list(1)%author 
   Print *, list(1)%subject 
   Print *, list(1)%book_id  
   
   Print *, list(1)%title 
   Print *, list(2)%author 
   Print *, list(2)%subject 
   Print *, list(2)%book_id  

end program deriveDataType

When the above code is compiled and executed, it produces the following result:

C Programming                                     
Nuha Ali                                          
C Programming Tutorial               
   6495407
C Programming                                     
Zara Ali                                          
Telecom Billing Tutorial                                      
   6495700