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Large programs can be organized in separate modules and procedures. Here is an example. We store various mathematical and physical constants in a module named constants_module.

MODULE const_module

REAL, PUBLIC, PARAMETER :: pi = 3.14159265
REAL, PUBLIC, PARAMETER :: e = 2.71828182

END MODULE const_module

Now we write a program that uses the above module:

PROGRAM circ_area
USE const_module


REAL :: rad = 5.0, area

area = pi*rad**2

PRINT *, "Area = ", area
END PROGRAM circ_area

The PRIVATE keyword makes sure everything is private otherwise declared as PUBLIC. Private variables are accessible only to the module, while public variables are accessible to the programs that uses it. There is also PROTECTED keyword, which allows the variable to be accessible but the value cannot be modified. Compile and run our module and program:

gfortran -c 11_const_module.f90
gfortran 11_circ_area.f90 11_const_module.o

Modules need to be compiled before the program that uses it. If both are placed in the same source file, the module should come before the program. Once the module is compiled, it will create .o and .mod files. When we compile the program, we only need to provide the .o file for linking, the compiler will find the .mod files. If the files are on a different directory, specify via -I<dir> option.

gfortran -I./ 11_circ_area.f90

USE keyword

Rename a variable:

use const_module, math_pi => pi

Use only certain variables:

use const_module, only : pi
use const_module, only : pi, e

Last command is same as:

use const_module, only : pi
use const_module, only : e

Use pi and rename e:

use const_module, only : pi, log_base => e