Find Minimum and maximum Value Using Sobroutine

Write a subroutine that attempts to locate the maximum and minimum values of an arbitrary function over a certain range . The function being evaluated should be passed to the subroutine as a calling argument.
The main program should pass to the subroutine the function 

f(x) = x³ – 5x² +5x +2 and search for the minimum and maximum in 200 steps over the range -1 ≤ x ≤ 3 . The subroutine should have the following output arguments minimum value , location of minimum value ,maximum value and location of maximum value.  

Program Maxi_Mini
implicit none
 integer:: i, step
 real:: a, b, x(200), y(200), f, incr
 open(2, file="input.dat")
 open(3, file="output.dat")
 read(2, 7) a
 7 format(/,F10.3)
 read(2,10) b
 10 format(/,F10.3)
 read(2,11) step
 11 format(/,I4)
 incr=(b - a)/Float(step - 1)
 do i= 1, step
 x(i)= a + (i-1)*incr
 y(i)= x(i)**3 - 5*x(i)**2 +5*x(i) +2
 end do
 Call subprog(x,y)
 end program



 subroutine subprog(x,y) ! x and y is x and F(x) value
 implicit none
 integer::i, j, iptr
 real::x(200), y(200), min_loc, max_loc, min_val, max_val, temp
 min_loc=x(1)
 max_loc=x(1)
 min_val=y(1)
 max_val=y(1)
 do i= 2, 200
 if(y(1)>y(i)) then      ! ( finding Minimum )
 call swap(y(1), y(i))
 min_loc=x(i)
 min_val=y(i)
 end if
 end do
 do i= 2, 200
 if(y(1)<y(i)) then       ! ( finding Maximum)
 call swap(y(1), y(i))
max_loc=x(i)
 max_val=y(i)
 end if
 end do
 write(3, 8) "Minimum value =", min_val, ", location =", min_loc
 8 format(A16, f10.3, A13, F10.3)
 write(3, 9) "Maximum value =", max_val, ", location =", max_loc
 9 format(A16, f10.3, A13, F10.3)
 end subroutine



 subroutine swap(a,b)
 implicit none
 real:: a, b, temp
 temp=a
 a=b
 b=temp
 end subroutine