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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 −

- Literal constants
- Named constants

A literal constant have a value, but no name.

For example, following are the literal constants −

Type | Example |
---|---|

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. |

Character constants |
"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

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