[求助] 调试时变量不能赋值

program solver
    implicit none
    real,external::getroot,polyeq
    real::tol=0.0001
    real::coeff(4)=(/-1.,1.,0.,-1./)!多项式系数向量
    real::root
    root=getroot(polyeq,coeff,4,0.,1.)!求根函数
    write(*,*)root
    end program
    real function getroot(polyeq,coeff,n,a,b)
    implicit none
    real,external::polyeq
    integer,intent(in)::n
    real,intent(in)::coeff(n)
    real::a,b
    integer::i
    real::root
    do
        if(abs((b-a)/2) .lt. 0.0001)exit
        root=(b+a)/2
        if(abs(polyeq(coeff,n,root)) .lt. 0.0001)exit
        if(polyeq(coeff,n,a)*polyeq(coeff,n,root) .lt. 0.0)then
            b=root
        else
            a=root
        end if
    end do
    getroot=root
!    return getroot
    end function getroot
    real function polyeq(coeff,n,x)!多项式
    implicit none
    integer,intent(in)::n
    integer::i
    real,intent(in)::coeff(n)
    real,intent(in)::x
    real::vec(n)
    real::res=0.0
    vec=[((x**(i-1)),i=1,n)]
    res=res+dot_product(coeff,vec)
    polyeq=res
!    return polyeq
    end function polyeq

program solver
  implicit none
  real,external::getroot,polyeq
  real::tol=0.0001
  real::coeff(4)=(/-1.,1.,0.,-1./)!多项式系数向量
  real::root,a=0.0,b=1.0
  root=getroot(polyeq,coeff,4,a,b)!求根函数
  write(*,*)root
end program