将参数作为参数传递给Fortran-90? [英] Passing parameter as parameter in Fortran-90?

问题描述

假设我有一个子例程:

subroutine foo(x, Nx)
    implicit none
    integer, intent(IN) :: x
    integer, intent(IN) :: Nx

    select case(x)
        case (1)
            write(*,*) "Minimum value"
        case (Nx)
            write(*,*) "Maximum value"
        case default
            write(*,*) "Somewhere in-between"
    end select
end subroutine foo

假设我的驱动程序看起来像这样:

Suppose my driver looks like this:

program main
    implicit none

    interface
        subroutine foo(x,Nx)
            integer, intent(IN)  :: x
            integer, intent(IN)  :: Nx
        end subroutine foo
    end interface

    integer, parameter :: Nx = 100
    integer :: x

    call foo(20, Nx)

end program main

上面的程序将无法编译，因为在子例程中，case (Nx)无效.具体来说，ifort 16会出现以下错误:

The above program will not compile because in the subroutine, case (Nx) is invalid. Specifically, ifort 16 gives the following error:

错误#6601:在CASE语句中，case值必须是一个常量表达式.

error #6601: In a CASE statement, the case-value must be a constant expression.

换句话说，即使通过intent(IN)有效地将Nx声明为子例程常量，它也必须是文字常量或类型为integer的parameter.

In other words, even though Nx is effectively declared as a subroutine constant via intent(IN), it needs to be either a literal constant or parameter of type integer.

是否有任何方法可以使case语句接受Nx作为我们知道的常量参数?有什么方法可以将Nx声明为传入的parameter吗?

Are there any ways to make the case statement accept Nx as the constant parameter we know it to be? Is there some way to declare Nx to be a passed-in parameter?

我意识到在这个简单的简短示例中，if-then-elseif-else-end块就足够了，但是我不知道这个问题的答案. :-)

I realize that in this simple, short example, an if-then-elseif-else-end block would suffice, but then I wouldn't know the answer to this question. :-)

推荐答案

只需使用if语句.子例程参数(您称为参数)当然不是参数(命名为常量). intent(in)不能有效地使其成为 参数，这只是一个承诺，您不会更改它，但是有一些方法可以绕开它. case语句需要一个编译时常量.

Just use an if statement. A subroutine argument (which you call parameter) is certainly not a parameter (named constant). The intent(in) doesn't make it effectively a parameter, it is just a promise you will not change it, but there are ways to circumvent that. The case statement needs a compile-time constant.

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