fortran保留的有效位数问题？

jacobi

测试程序如下:

PROGRAM PQ
IMPLICIT NONE
INTEGER::H
INTEGER, PARAMETER :: P2=SELECTED_REAL_KIND(15)
REAL(P2)::A
REAL(KIND=8)::B
REAL(KIND=4)::C
A=SQRT(2.0)
B=SQRT(2.0)
C=SQRT(2.0)
WRITE(*,'(F50.40)')A
WRITE(*,'(F50.40)')B
WRITE(*,'(F50.40)')C
WRITE(*,'(F20.16)')C
END PROGRAM PQ

GWinston

2.0是单精度常量，SQRT(2.0)的返回值也是单精度的，修改如下再试试：
A=SQRT(2.D0)
B=SQRT(2.D0)
C=SQRT(2.D0)

hillyuan

INTEGER, PARAMETER :: P1=SELECTED_REAL_KIND( 8 )
INTEGER, PARAMETER :: P2=SELECTED_REAL_KIND(15)
WRITE(*,'(I3,F50.40)') P1,1.0/3.0_P1
WRITE(*,'(I3,F50.40)') P2,1.0/3.0_P2

P1, P2 is machine depedent constants. Running above program in your machine, if P1 and P2 share the same value, the result is of course the same.

jacobi

GWinston wrote:
2.0是单精度常量，SQRT(2.0)的返回值也是单精度的，修改如下再试试：
A=SQRT(2.D0)
B=SQRT(2.D0)
C=SQRT(2.D0)

如果要提高精度,那么 2.D0 又该怎么写呢?
谢谢.

jacobi

hillyuan wrote:
INTEGER, PARAMETER :: P1=SELECTED_REAL_KIND( 8 )
INTEGER, PARAMETER :: P2=SELECTED_REAL_KIND(15)
WRITE(*,'(I3,F50.40)') P1,1.0/3.0_P1
WRITE(*,'(I3,F50.40)') P2,1.0/3.0_P2

P1, P2 is machine depedent constants. Running above program in your machine, if P1 and P2 share the same value, the result is of course the same.

偶试了,P1=P2,
那么,偶的机器只能有REAL(8)的精度了?
其他软件却可以,如MATHEMATIC等.
谢谢指点.