Can't input value in f(x) in this Fortran code? - fortran

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
I'm just starting to use Fortran 90.
Now I'm using Code::blocks but I don't know exactly which line the error exists on.
I guess the problem is f((x0+x1)/2) < 0 but don't know actually what is the real error.
what's problem is here?

Answer 1

Be advised that statement functions, the function definitions the OP uses, are obsolescent.
B.3.4 Statement functions
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.
The internal function is a more generalized form of the statement function and completely supersedes it.
source: F2018 Standard
Also the notation REAL*8 or anything of that form has never been part of any Fortran standard (see here):
I would suggest to rewrite the code as:
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

Answer 2

If you change your program as follows then it will compile:
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) then
x0 = (x0+x1)/2
else
x1 = (x0+x1)/2
endif
end do
print *,"x = ", x
end program
As mentionned in the comments, you have to add the semicolon ; to separate statements in one line and you have to add the then as well as endif to your if condition.
Hope it helps.