Fortran - Constants

The constants refer to the fixed values that the program cannot alter during its execution. These fixed values are also called pterals.

Constants can be of any of the basic data types pke an integer constant, a floating constant, a character constant, a complex constant, or a string pteral. There are only two logical constants : .true. and .false.

The constants are treated just pke regular variables, except that their values cannot be modified after their definition.

Named Constants and Literals

There are two types of constants −

Literal constants

Named constants

A pteral constant have a value, but no name.

For example, following are the pteral constants −

A named constant has a value as well as a name.

Named constants should be declared at the beginning of a program or procedure, just pke a variable type declaration, indicating its name and type. Named constants are declared with the parameter attribute. For example,

real, parameter :: pi = 3.1415927

Example

The following program calculates the displacement due to vertical motion under gravity.

program gravitationalDisp

! this program calculates vertical motion under gravity 
imppcit none  

   ! gravitational acceleration
   real, parameter :: g = 9.81   
   
   ! variable declaration
   real :: s ! displacement   
   real :: t ! time  
   real :: u ! initial speed  
   
   ! assigning values 
   t = 5.0   
   u = 50  
   
   ! displacement   
   s = u * t - g * (t**2) / 2  
   
   ! output 
   print *, "Time = ", t
   print *,  Displacement =  ,s  
   
end program gravitationalDisp

Time = 5.00000000    
Displacement = 127.374992