定义一个必须声明为常量但在循环中更改的变量 [英] Defining a variable that must be declared constant, but changes in a loop

问题描述

我正在测试函数 selected_real_kind 的值范围(仅用于踢球时为-1:34)，以确定其返回的 kind 参数和实际值使用此种类定义的变量使用的精度位.不过，由于要转换变量 x ， u 和 alpha的函数，我仍然在下面定义变量 RP (例如 real(x，RP)例如)要求 RP (种类类型)必须是常量.

I'm testing out ranges of values (-1:34 just for kicks) for the function selected_real_kind to determine the kind parameter it returns and the actual number of bits of precision used by a variable defined using this kind. I'm stuck with how to define variable RP below, though, since the function to convert the variables x, u and alpha (real(x,RP) e.g.) require that RP (the kind type) to be constant.

如果我将RP定义为参数，即在顶部写 integer，parameter :: RP ，则必须立即对其进行初始化，然后显然可以"不能更改它，因为它是一个 parameter ，所以这行不通.

If I define RP to be a parameter, i.e. at the top write integer, parameter :: RP I have to initialize it immediately, and then I obviously can't change it because, well, it's a parameter, so this won't work.

这就是我所拥有的:

program f1
  implicit none
  integer :: n,t  ! n used in selected_real_kind(n), t is precision (#bits)
  integer :: RP
  real :: x=1.5, u=1.0, alpha=1.0  ! will be reset using _RP below                                                                               

  print '(A2,A4,A4)', "n", "RP", "t"  ! header for values

  do n=-1,34
     RP = selected_real_kind(n)

     ! convert x,u,alpha to type RP.  These functions throw the error!                               
     x = real(x,RP)
     u = real(u,RP)
     alpha = real(alpha,RP)

     ! init precision test variables                                                                                                             
     x=1.5_RP
     u=1.0_RP
     alpha=1.0_RP

     ! reset precision value to zero before each test                                                                                            
     t = 0

     ! precision test                                                                                                                            
     do while(x>alpha)
        u = u/2.0_RP
        x = alpha+u
        t = t+1
     end do

     print '(I2 I4 I4)', n, RP, t
  end do

end program f1

推荐答案

详细说明我的评论，这是一个python示例:

elaborating on my comment, here is a python example:

 fortrancode = """
       implicit none
       integer, parameter :: n=%i
       integer,parameter :: rp=selected_real_kind(n)      
       write(*,*)n,rp
       end
 """
 from subprocess import call
 for n in range(-1,33):
  f=open('test.f','w')
  f.write(fortrancode%(n))  ! <- n here gets string substituted
                            !for the '%i' in fortrancode
  f.close()
   ! optional check  call(['grep','n=','test.f'])
  call(['gfortran','test.f','-o','test.exe'])
  call(['./test.exe'])

python test.py

python test.py

 -1 4
  0 4
  1 4
...
  7 8
...
 18 10
 19 -1
...
 32 -1

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