实变量精度 [英] precision of real variable

问题描述

我在 FORTRAN 77 中有以下代码:

REAL*8 :: dmdm=1.-1.E-12写(6，*)'dm:'，dm

我得到:dm: 1

这样好吗?我想获得 dm=0.999999999999

解决方案

如注释中所述，您需要指定常量的精度.此外，real*8 已过时.(它总是扩展吗?)这是一种现代的编写方式，使用 ISO Fortran 环境获取 64 位实数类型，并在声明和常量中使用该类型.

使用 ISO_FORTRAN_ENV真实的(real64)::dmdm = 1.0_real64 - 1.0E-12_real64

有关更多信息，请参阅 `real*8` 是什么意思?p>

I have the following code in FORTRAN 77:

REAL*8 :: dm

dm=1.-1.E-12

write(6,*) 'dm: ', dm

I get: dm: 1

Is this OK? I would like to get dm=0.999999999999

解决方案

As stated in a comment, you need to specify the precision of the constants. Also, real*8 is obsolete. (Was it always an extension?) Here is a modern way to write this, using the ISO Fortran Environment to obtain a 64-bit real type and using that type both in the declaration and in the constants.

use ISO_FORTRAN_ENV

real (real64) :: dm  
dm = 1.0_real64 - 1.0E-12_real64

For more info, see What does `real*8` mean?

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