Could I trust Matlab?

Hello, I recently write a code and debug it.

I have a key sentence,

    if true
      % code
    end while 1

      ...
                           if f(x_new)-f(x_old)<0
                               break;
                           elseif
                              L=1.1*L;
                            ...
                           end
But it seems that the returned L is always wrong. And it always happened when f(x_new)-f(x_old)=1e-20. So I doubt maybe it should that f(x_new)<f(x_old), but the returned values is a little wrong to lead f(x_new)-f(x_old)=1e-20.

Could this thing happen in matlab.

Thank you in advance. I have already spent a lot time on it.

[Jos (10584)]

Based on this snippet, I have no idea what f, L, x_new and x_old are doing inside the loop. Are they being updated on the fly? And how do you know the returned L is wrong?

The elseif statement also seems out of place ...

[Vivian]

Yes,they are computer on the fly. I do not show the f since it is complex and takes time. It just input x and then output is the value of f with the variable x. I know L is wrong , actually is large, since I calculate using a simple data set.

Answer 1

[Jos (10584)]

Perhaps this is what you really want

if abs(f(x_new)-f(x_old)) < eps
…
end

[Vivian]

Hi, thank you, I do not think this is a good idea. I do not try to tell f(x_new)==f(x_old)

Answer 2

[Andreas Goser]

Looks like yet another flavor of the numerical question with the highest volume: http://www.mathworks.com/matlabcentral/answers/102419-how-do-i-determine-if-the-error-in-my-answer-is-the-result-of-round-off-error-or-a-bug

[Vivian]

Thank you.