Fortran prompting user and reading polynomial input - fortran

Question

I am writing a program to calculate the roots of a polynomial. The user is prompted a polynomial of no greater than fourth order (not a file read). How do I read a polynomial input in Fortran? I thought about using 1D array, each index representing the degree and the number representing the coefficient, but what about the plus and minus signs? I don't want to restrict the user to a polynomial with one mathematical operation.

Answer 1

If you don't care about pretty printing the polynomial without brackets it's not that difficult:
program prompt_read
implicit none
integer, parameter :: wp = kind(1.0d0) ! work precision
!> Polynomial coefficients (maximum degree = 4), only real values allowed
real(wp) :: a(0:4)
!> Polynomial degree
integer :: n
! initialize coefficients to zero
a = 0
write(*,'(A)',advance='no') "Enter polynomial degree (1-4): "
read(*,*) n
! handle erroneous input
if (n > 4 .or. n < 1) then
write(*,'(A)') 'Input error. Try again.'
stop
end if
write(*,'(A)',advance='no') "Enter polynomial coefficients (ascending powers of x): "
read(*,*) a(0:n)
write(*,'(A)') "The polynomial received was " // polystr(a(0:n))
! ... continue with calculation of roots
contains
pure function polystr(coeffs)
real(wp), intent(in) :: coeffs(0:)
character(len=:), allocatable :: polystr
character(32) :: s
integer :: i
write(s,'("(",F0.3,")")') coeffs(0)
polystr = trim(s)
do i = 1, ubound(coeffs,1)
write(s,'("(",F0.3,")")') coeffs(i)
if (i == 1) then
s = trim(s) // '*x'
else if (i > 1) then
s = trim(s) // '*x^' // achar(iachar('0') + i)
end if
polystr = polystr // ' + ' // trim(s)
end do
end function
end program
Example output:
$ gfortran -Wall -o prompt_read prompt_read.f90
$ ./prompt_read
Enter polynomial degree (1-4): 3
Enter polynomial coefficients (ascending powers): 1,-2,3.,-4.2
The polynomial entered is (1.000) + (-2.000)*x + (3.000)*x^2 + (-4.200)*x^3
A few notes/ideas:
use a custom lower bound of 0 to make the array index match the power of the monomial
you can read both space- or comma-delimited input
the polystr function can only handle polynomials up to degree 9
have a look at polyroots-fortran, a collection of existing polynomial root solvers in Fortran
put the body of the program in an "infinite" do-end do loop with a suitable exit condition (for example upon pressing the key q); add a brief help message to explain usage of the program