CODEWITHSUNDEEP
matlab
jarhead
jarhead

Reputation: 1901

MATLAB - negative values go to NaN in a symmetric function

Can someone please explain why the following symmetric function cannot pass a certain limit of negative values?

D = 0.1; l = 4;
c = @(x,v) (v/D).*exp(-v*x/D)./(1-exp(-v*l/D));
v_vec = -25:0.01:25;

figure(2)
hold on
plot(v_vec,c(l,v_vec),'b')
plot(v_vec,c(0,v_vec),'r')

Notice at the figure where the blue line chops, this is where I get inf/nan values.

enter image description here

Upvotes: 1

Views: 85

Answers (2)

Reiner Franke
Reiner Franke

Reputation: 11

I did encounter the same hurdle using exp() with arguments triggering overflow. Sometimes it is difficult to trace back numeric imprecision or convergence errors. In principle the function definition using exp() only create intermediate issues as your purpose as a transition function. The intention I guess was to provide a continuous function. My solution to this problem is to divide the argument into regions and provide in each region an approximation function. In your case zero for negative x and proportional to x for positive x. In between you can use the orginal function. Care should be taken to match the approximation at the borders of the regions and the number of continuous differentiations which is important for convergence in loops.

Upvotes: 1

Nico B
Nico B

Reputation: 130

It seems that Matlab is trying to compute a result that is too large, outputs +inf, and then operates on that, which yields +/- inf and NaNs. For instance, at v=-25, part of the function computes exp(-(-25)*4/0.1), which is exp(1000), and that outputs +inf. (larger than the largest representable double precision float).

You can potentially solve that problem by rewriting your function to avoid operating of such very large (or very small) numbers, say by reorganising the fraction containing exp() functions.

Upvotes: 2

Related Questions