Solve cubic equation in MATLAB

I need to solve a cubic equation but I don't know how to do it. Here is my coding:

    B=80.73233784;

    xc=152.01;

    R=82.05746;

    for i=1:0.1:200

        x(i)=-50+i;

        y(i)=(10^(7.23086-(1175.581/(x(i)+271.079))))*0.001315789473684;

        a(i)=y(i)/(R*x(i));

        b(i)=(4.9343*((xc/x(i))^1.5)*B)-B-((y(i)/(R*x(i)))*(B^2));

        c(i)=4.9343*((xc/x(i))^1.5)*(B^2);

        m=[a -1 b -c];

        r=roots(m);

    end

This is my equation to be solved for "v": ((a(i)*(v^3))-(v^2)+(b(i)*v)-c(i))=0;

Here "a", "b" and "c" changes with respect to "x" and "y". I want to eliminate the negative and complex roots but I am unable to do so. Moreover program shows error that roots can't be found. I am new to MATLAB and don't know much about its functions. Kindly help me. Thank you.

Answer 1

[Rick Rosson]

   >> doc roots
   >> doc isreal

[PRIYANK AGARWAL]

Thanks for replying. Sorry I don't know about these functions. Please tell me how to declare them and what are they used for.

[Rick Rosson]

Those are commands that you can type at the MATLAB command line. Please try it.