Introduce optFlag so that one can choose not to build the optimization-related cell of matrices called B. It is too computationally costly and should probably be done in a different way.

function plotf(f,a,b,opt)
    if ~exist('opt')
        opt = '';
    end

    x = linspace(a,b);
    for i = 1:length(x)
        y(i) = f(x(i));
    end
    plot(x,y,opt)
end