How to fix exception error with 3 inputs into arithmetic equation in Erlang?

Question

I am trying to figure out how to fix this error. I should be able to input 3 numbers and it will solve for the X value of the quadratic equation.

-module(main).

-export([main/3]).

main(A, B, C) ->
  [(-B + math:sqrt(B * B - 4 * A * C)) / (2 * A), (-B - math:sqrt(B * B - 4 * A * C)) / (2 * A)].

Here is the result that I got after running the code.

** exception error: an error occurred when evaluating an arithmetic expression in function math:sqrt/1 called as math:sqrt(-4) in call from main:main/3 (/Users/ianrogers/IdeaProjects/CS381 Projects/src/main.erl, line 14)

Answer 1

You must test the number before evaluating the square root. As you do it when calculating by yourself.

main(A, B, C) ->
  D = (B * B - 4 * A * C) + 0.0, % add 0.0 to cevert to float in any case
  case D of
    0.0 -> {ok, -B / (2 * A)};
    D when D > 0.0 -> {ok, [(-B + math:sqrt(D)) / (2 * A), (-B - math:sqrt(D)) / (2 * A)]};
    _ -> {error, no_real_solution}
  end.

Answer 2

If you input negative numbers into math:sqrt/1 you will get an error. For example

2> math:sqrt(-1).
** exception error: an error occurred when evaluating an arithmetic expression
     in function  math:sqrt/1
        called as math:sqrt(-1)

You function does work for some input. It does not work in your posted example because of "called as math:sqrt(-4)"