Simple Quadratic Equation Solver

Question

Still pretty new to coding. I posted that Twin Primes Finder the other day and you guys were a great help so thanks. Here's my quadratic equation solver:

import math

#variables for initial equation
a = int(raw_input("Enter a:"))
b = int(raw_input("Enter b:"))
c = int(raw_input("Enter c:"))

#variables for quad. equation
negative_b = -b
b_squared = b**2
four_a_c = 4*a*c
determinant = b_squared - four_a_c

#prints initial equation
equation = [str(a)+"x^2", str(b)+"x", str(c)]

print "Solve: " + " + ".join(equation)


if determinant < 0:
    print "No real solutions."
elif a == 0:
    y_int = float(-c)/b
    print "x =", y_int
    print "But this is a straight line."
else:
    root_determinant = math.sqrt(determinant)
    two_a = 2*a
    numerator_one = negative_b - root_determinant
    answer_one = numerator_one/two_a
    if determinant == 0:
      print "x =", answer_one
    else:
      numerator_two = negative_b + root_determinant
      answer_two = numerator_two/two_a
      print "x =", answer_one, "or", answer_two

Answer 1

I wold separate the concerns somewhat and put them into functions.

One major concern is actually solving the quadratic equation. Another is the fancy input/output around it (if you want to be pedantic, that is actually two concerns).

The solving function, which I would call solve_quadratic returns a list of the solutions. This list can be empty (if there are no resolutions) or have multiple entries. We can use this fact to just append a solution if there are multiple.

I inlined some of the computations, if they were only used once.

import math

def solve_quadratic(a, b, c):
    """
    Given three real coefficients,
    returns the (real) roots of the second degree polynomial
    """
    negative_b = -b
    determinant = b**2 - 4*a*c

    if determinant < 0:
        return []
    elif a == 0:
        return [float(-c)/b]
    else:
        root_determinant = math.sqrt(determinant)
        two_a = 2*a
        answers = [(negative_b - root_determinant)/two_a]
        if determinant:
            answers.append((negative_b + root_determinant)/two_a)
        return answers

Now, the input part and the printing which serves as interpretation. For the initial printing I would use str.format, instead of first building a list and then joining it. It is slightly more readable IMO.

Finally, I would use if __name__ == "__main__": to be able to import these functions in another script without calling the input function.

def fancy_quadratic_solver():
    a = int(raw_input("Enter a:"))
    b = int(raw_input("Enter b:"))
    c = int(raw_input("Enter c:"))
    print "Solve: {}x^2 + {}x + {}".format(a, b, c)

    x = solve_quadratic(a, b, c)
    if not x:
        print "No real solutions."
    else:
        print "x =", " or ".join(map(str, x))
        if a == 0:
            print "But this is a straight line."

if __name__ == "__main__":
    fancy_quadratic_solver()