NaN for zero divide by zero.

Will illustrate with an example, I.e:

a = (0:0.1:1); b = (1-a)*2./(1-a);

Matlab will output the end value in b as NaN, when in reality b should converge to 2 for a = 1.

How can I avoid this?

Answer 1

[Matt Fig]

There are two things to remember when looking at this problem.

1. A function having a limit as the independent variable approaches a point does not imply that the function value at that point is defined. You plugged in for the function value at that point. The fact that you get NAN says nothing about the limit of the function as the independent variable approaches that point.

2. NAN is the correct value when evaluating the function at that point. This is a standard double precision definition.

[Stian Molvik]

So the best way to go about this would be to manually edit the last value in b?

Thank you.

[Walter Roberson]

You will probably want to check out isnan()

[Matt Fig]

Yes, if you know the limit, you can do

b(isnan(b)) = 2;

[Stian Molvik]

Ah. Ok. Hoped there would be an easier way, but still.

Thank you very much everyone for your help! :)

Answer 2

[Walter Roberson]

In order to avoid this, you will need to use the symbolic toolbox.

MATLAB is giving the correct numeric answer according to IEEE 754 floating point definitions. MATLAB works with actual values computed on real machines, not with limits or ideal values. See for example this portion of the FAQ

Answer 3

[Sean de Wolski]

0/0 is the definition of a nan

doc nan

Answer 4

[Stian Molvik]

Thanks for the reply Walter.

I haven't had too much experience with Matlab earlier. How would I go about using the symbolic toolbox?

[Walter Roberson]

Try this:

sym a

b = (1-a)*2/(1-a);

and see what b contains afterwards.