fortran编写用牛顿迭代法求方程ax3+bx2+cx+d=0在x0附近的一个根的函数，其中a,b,c,d,x0的值均在主程序输入

PROGRAM MAIN_ROOT
IMPLICIT NONE
REAL::ROOT,A,B,C,D,X0,X
write( *,* ) "PLEASE INPUT A,B,C,D,X0"
READ (*,*) A,B,C,D,X0
X=ROOT(A,B,C,D,X0)
write(*,*) X
return
END PROGRAM MAIN_ROOT

FUNCTION ROOT(A,B,C,D,X0) RESULT(ROOT_RESULT)
IMPLICIT NONE
REAL,INTENT(IN)::A,B,C,D,X0
REAL::ROOT_RESULT
reAL::X1,F,F1
X1 = X0
F=A*X1**3+B*X1**2+C*X1+D
write(*,*) F
DO WHILE(ABS(F).GT.1E-6)
F=A*X1**3+B*X1**2+C*X1+D
c |
F1=3*A*X1*X1+2*B*X1+C
X1 = X1-F/F1
write(*,*) X1
ROOT_RESULT = X1
END DO
END FUNCTION ROOT

因为INTENT(IN)就表明这个值是传入的，不可修改。
而你的函数中X0要修改。所以就出错了。

你可以把X0设为INTENT(INOUT)。或把X0赋值给另一个变量。

有问题可以再讨论。

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