Fortran 95中的数值精度： [英] Numerical Precision in Fortran 95:
问题描述
程序奇怪
Real（Kind = 8）:: Pi1 = 3.1415926535897932384626433832795028841971693993751058209;
Real（Kind = 8）:: Pi2 = 3.1415926535897932384626433832795028841971693993751058209_8;
打印*，Pi1 =，Pi1;
打印*，Pi2 =，Pi2;
结束程序奇怪
我用gfortran进行编译，输出结果如下：
Pi1 = 3.1415927410125732
Pi2 = 3.1415926535897931
当然第二个是正确的，但情况应该如何？看起来Pi1正在作为一个单精度数输入到存储器中，然后放入一个双精度内存插槽。但这对我来说似乎是一个错误。我是否正确？
我知道一点Fortran！ @ Dougal的回答是正确的，尽管他引用的片段不是，将字母 d 嵌入真正的文字常量中并不是必需的（因为Fortran 90），现在很多Fortran程序员认为这种做法过时了。该片段在建议使用 3.1415926535d + 0 初始化pi的64位浮点值时也存在误导，它没有将足够的数字设置为他们的正确价值。
声明：
$ b $ pre $ code真实（种类= 8）:: Pi1 = 3.1415926535897932384626433832795028841971693993751058209
定义 Pi1 是一种真正的变量8.然而，字面实际值 3.1415926535897932384626433832795028841971693993751058209 是一种实际的默认值，最可能是大多数情况下为4字节的实数目前的编译器。这似乎解释你的输出，但检查你的文档。另一方面，字面上的真实值 Pi2 = 3.1415926535897932384626433832795028841971693993751058209_8
还有三点：$ b
$ b 1）不要陷入认为 kind = 8 意味着相同的陷阱例如 64位浮点数或 double 。对于许多编译器来说，它并不是。有关数字在Fortran实现之间不可移植。根据标准，它们是任意的正整数。更好的是，使用现代编译器，将使用来自内置模块 iso_fortran_env 的预定义常量，例如
use，intrinsic :: iso_fortran_env
...
real（real64）:: pi = 3.14159265358979323846264338_real64
还有其他一些使用 selected_real_kind 等函数设置变量种类的便携方法。
<2>由于
pi 的值在程序执行过程中不太可能发生变化，因此您可能需要将其设置为一个参数： real（real64），parameter :: pi = 3.14159265358979323846264338_real64
<3>除非要在源文件的同一行上有多个语句，否则不需要（或平常）用';'结束Fortran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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