Fortran 简明教程

Fortran - Constants

常量是指程序在其执行期间不能更改的固定值。这些固定值也称为 literals 。

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

常量可以是任意基本数据类型，例如整型常量、浮点常量、字符常量、复常量或字符串常量。只有两个逻辑常量： .true. 和 .false.

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.

常量与常规变量一样，不同之处在于它们的的值在定义后不能修改。

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

Named Constants and Literals

有两种类型的常量：

There are two types of constants −

  1. Literal constants

  2. Named constants

字面常量有一个值，但没有名称。

A literal constant have a value, but no name.

例如，以下是字面常量 -

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 '' '

已命名常量既有值又有名称。

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

已命名常量应在程序或过程的开头声明，如同变量类型声明，指示其名称和类型。已命名常量用参数属性声明。例如，

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

Example

以下程序计算重力作用下垂直运动的位移。

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

编译并执行上述代码后，将产生以下结果 −

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

Time = 5.00000000
Displacement = 127.374992