MATLAB: Vector function is only returning a scalar output

Question

I have the function:

function [ theta4 ] = ThetaFour( L, theta2 )
%given equations, finding polynomials for the quad formula
k1 = (L(1)/L(2));
k2 = (L(1)/L(4));
k3 = ((L(1)^2) + (L(2)^2) - (L(3)^2) + (L(4))^2) / (2*L(2)*L(4));
a = cosd(theta2) - k1 -k2*cosd(theta2) + k3;
b = -2*sind(theta2);
c = k1 - (k2 + 1) * cosd(theta2) + k3;
%finding final angle using the quadratic function function created for
%projectile motion script
theta4 = 2*atand(Quadratic(a,b,c,-1))
end

• L is a vector [0.1313, 0.0475, 0.0880, 0.0960]
• theta 2 is a vector [0:90]

During this process: a, b, and c all become vectors of length 90

My Quadratic function is:

function Roots = Quadratic( a, b, c, plusOrMinus )
%Uses the quadratic formula to solve for roots
if ((b .^ 2)-(4 .* a .* c) >= 0)
    if (plusOrMinus == 1)
        Roots = (-b + sqrt( (b .^2 - 4.* a.* c) ) ) / (2 .* a);
    else
        Roots = (-b - sqrt( (b .^2 - 4.* a.* c) ) ) / (2 .* a);
    end
else
    error('Coefficiants yield a complex root. b^2 - 4ac must be 0 or greater.')
end

a, b, c should be going in as vectors, while plusOrMinus is -1, returning a vector

Changing a, b, or c to a scalar produces a vector, but all three as vectors produces a scalar.

For example, Quadratic(-.5, b, c, -1) returns a full vector as does Quadratic(a, 10, c, -1) while Quadratic(a, b, c, -1) is just returning one value.

I would like to know how to get Quadratic(a, b, c, -1) to return a vector. Thank you in advance.

Answer 1

function Roots = Quadratic( a, b, c, plusOrMinus )
    Roots = (-b + plusOrMinus.*sqrt( (b .^2 - 4.* a.* c) ) ) ./ (2 .* a);
     Roots(imag(Roots)~=0)=nan;