Fortran 95 中的数值精度: [英] Numerical Precision in Fortran 95:
问题描述
我有以下 Fortran 代码:
奇怪的程序真实(种类=8)::Pi1=3.1415926535897932384626433832795028841971693993751058209;真实(种类=8)::Pi2=3.1415926535897932384626433832795028841971693993751058209_8;打印*, "Pi1=", Pi1;打印*, "Pi2=", Pi2;结束程序奇怪我用gfortran编译，输出是:
Pi1= 3.1415927410125732Pi2= 3.1415926535897931当然第二个是正确的，但应该是这样吗?似乎 Pi1 作为单精度数字输入到内存，然后放入双精度内存插槽.但这对我来说似乎是一个错误.我说的对吗?
我确实懂一点Fortran！@Dougal 的答案是正确的，尽管他引用的片段不是，将字母 d 嵌入到真正的文字常量中不是必需的(从 Fortran 90 开始)，确实许多 Fortran 程序员现在认为这种方法是过时的.该片段在建议使用 3.1415926535d+0 为 pi 初始化 64 位浮点值时也具有误导性，它没有将足够多的数字设置为正确的值.p>
声明:
Real(Kind=8)::Pi1=3.1415926535897932384626433832795028841971693993751058209将 Pi1 定义为类型 8 的实数变量.然而，文字实数值 3.1415926535897932384626433832795028841971693993751058209 是默认类型的实数值，最有可能是 4-byte real 在大多数当前编译器上.这似乎可以解释您的输出，但请检查您的文档.
另一方面，文字实值 Pi2=3.1415926535897932384626433832795028841971693993751058209_8 是，通过种类规范的后缀，声明为 kind=8，这与变量的种类相同它被分配到.
还有三点:
1) 不要误以为 kind=8 与 64 位浮点数 或 double<意思相同/代码>.对于许多编译器来说，它确实如此，而对于某些编译器却没有.种类编号在 Fortran 实现之间不可移植.根据标准，它们是任意正整数.更好的是，使用现代编译器，使用内部模块 iso_fortran_env 中的预定义常量，例如
使用，内在 :: iso_fortran_env...真实(真实64) :: pi = 3.14159265358979323846264338_real64还有其他可移植的方法来使用诸如 selected_real_kind 之类的函数来设置变量种类.
2) 由于 pi 的值在程序执行期间不太可能发生变化，因此您可能需要将其设为参数:
real(real64), 参数:: pi = 3.14159265358979323846264338_real643) 没有必要(或通常)以;"结束 Fortran 语句除非您想在源文件的同一行中包含多个语句.
I have the following Fortran code:
Program Strange
Real(Kind=8)::Pi1=3.1415926535897932384626433832795028841971693993751058209;
Real(Kind=8)::Pi2=3.1415926535897932384626433832795028841971693993751058209_8;
Print*, "Pi1=", Pi1;
Print*, "Pi2=", Pi2;
End Program Strange
I compile with gfortran, and the output is:
Pi1= 3.1415927410125732
Pi2= 3.1415926535897931
Of course the second is correct, but should this be the case? It seems like Pi1 is being input to memory as a single precision number, and then put into a double precision memory slot. But this seems like an error to me. Am I correct?
I do know a bit of Fortran ! @Dougal's answer is correct though the snippet he quotes from is not, embedding the letter d into a real literal constant is not required (since Fortran 90), indeed many Fortran programmers now regard that approach as archaic. The snippet is also misleading in advising the use of 3.1415926535d+0 to initialise a 64-bit floating-point value for pi, it doesn't set enough of the digits to their correct values.
The statement:
Real(Kind=8)::Pi1=3.1415926535897932384626433832795028841971693993751058209
defines Pi1 to be a real variable of kind 8. The literal real value 3.1415926535897932384626433832795028841971693993751058209 is, however, a real value of default kind, most likely to be a 4-byte real on most current compilers. That seems to explain your output but do check your documentation.
On the other hand, the literal real value Pi2=3.1415926535897932384626433832795028841971693993751058209_8 is, by the suffixing of the kind specification, declared to be of kind=8 which is the same as the kind of the variable it is assigned to.
Three more points:
1) Don't fall into the trap of thinking that kind=8 means the same thing as 64-bit floating-point number or double. For many compilers it does, for some it doesn't. Kind numbers are not portable between Fortran implementations. They are, according to the standard, arbitrary positive integers. Better, with a modern compiler, would be to use the predefined constants from the intrinsic module iso_fortran_env, e.g.
use, intrinsic :: iso_fortran_env
...
real(real64) :: pi = 3.14159265358979323846264338_real64
There are other portable approaches to setting variable kinds using functions such as selected_real_kind.
2) Since the value of pi is unlikely to change during the execution of your program you might care to make it a parameter thus:
real(real64), parameter :: pi = 3.14159265358979323846264338_real64
3) It isn't necessary (or usual) to end Fortran statements with a ';' unless you want to have more than one statement on the same line in the source file.
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