Bisection in Octave, correct version.

function nst = bisekt (a, b, eps, f)
    if((f(a) .* f(b)) >= 0)
        disp("Invalid arguments! f(a) xor f(b) must be negative.");
    endif

    c = (a.+b)/2;
    fc = f(c);
    if (abs(fc) < eps)
        nst = c;
    else if (fc > 0)
            nst = bisekt(a, c, eps, f);
        else
        nst = bisekt(c, b, eps, f);
        endif
    endif
endfunction