The constants refer to the fixed values that the program cannot alter during its execution. These fixed values are also called literals.
Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, a complex constant, or a string literal. There are only two logical constants : .true. and .false.
The constants are treated just like regular variables, except that their values cannot be modified after their definition.
There are two types of constants −
A literal constant have a value, but no name.
For example, following are the literal constants −
|Integer constants||0 1 -1 300 123456789|
|Real constants||0.0 1.0 -1.0 123.456 7.1E+10 -52.715E-30|
|Complex constants||(0.0, 0.0) (-123.456E+30, 987.654E-29)|
|Logical constants||.true. .false.|
"PQR" "a" "123'abc$%#@!"
" a quote "" "
'PQR' 'a' '123"abc$%#@!'
' an apostrophe '' '
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 like a variable type declaration, indicating its name and type. Named constants are declared with the parameter attribute. For example,
real, parameter :: pi = 3.1415927
The following program calculates the displacement due to vertical motion under gravity.
program gravitationalDisp ! this program calculates vertical motion under gravity implicit 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
When the above code is compiled and executed, it produces the following result −
Time = 5.00000000 Displacement = 127.374992