fortran - 无法在此 Fortran 代码中输入 f(x) 值？

Question

    program prob_1
    implicit real*8(a-h, o-z)
    f(x) = x**2-cos(x)
    df(x) = 2*x+sin(x)
    x0 = 0, x1 = 1
    do i = 1, 3
        if (f((x0+x1)/2) < 0)
            x0 = (x0+x1)/2
        else
            x1 = (x0+x1)/2
    end do
    print *,"x = ", x
end program

我刚刚开始使用 Fortran 90。现在我正在使用 Code::blocks 但我不知道错误到底出现在哪一行。

我猜问题是f((x0+x1)/2) < 0但实际上不知道真正的错误是什么。这里有什么问题吗？

Answer 1

请注意，语句函数(OP 使用的函数定义)已过时。

B.3.4 Statement functions

1. Statement functions are subject to a number of non intuitive restrictions and are a potential source of error because their syntax is easily confused with that of an assignment statement.
2. The internal function is a more generalized form of the statement function and completely supersedes it.

_{source: F2018 Standard}

此外，符号 REAL*8 或任何该形式的内容从未成为任何 Fortran 标准的一部分(请参阅 here ):

我建议将代码重写为:

program prob_1
    implicit none
    double precision :: x1,x0
    integer          :: i
    x0 = 0; x1 = 1
    do i = 1, 3
        if (f((x0+x1)/2.0D0) < 0) then
            x0 = (x0+x1)/2.0D0
        else
            x1 = (x0+x1)/2.0D0
        endif
    end do
    print *,"x = ", (x0+x1)/2.0D0
contains
    function f(x)
      double precision, intent(in) :: x
      double precision             :: f
      f = x**2-cos(x)
    end function f
   function df(x)
      double precision, intent(in) :: x
      double precision             :: df
      df = 2.0D0*x+sin(x)
    end function df
end program