Replace a multiple of two symbolic numbers

Hi,

I'm attempting to replace (to linearise my equation) the multiple of two very small values dx and dy.

I attempted to use subs but it doesn't seem to work.

Code: PO=subs(PO,[(dx*dy),(dy*dx)],[0,0]);

PO is my symbolic vector. There is no error message but it just doesn't pick up the combination. Any Ideas?

Thanks!

Matlab Version: 2012a

Answer 1

[Star Strider]

I honestly don't understand why your subs call doesn't work. I can't make it work either, in spite of a number of different approaches that have worked for me previously. I consider this a bug, so you may want to report it to TMW as such.

All is not lost, however. Here's a kludgy but effective work-around. Note that it still only recognizes (dx*dy) and not such as (dx*dy)^2:

syms dx dy x y
PO = expand( (x + dx) * (y + dy) + (dx * dy) )  % Prototype expression
[POc, T] = coeffs(PO, [dx dy])
Tfind = find(T == 'dx*dy')
POc(Tfind) = 0
PO = POc * T.'

This could probably be made more efficient. I wrote it as I did to make the logic behind it transparent.

[Moritz]

Thanks! I adapted it to make it work with vectors and now it works like a charm.

Thank you very much!

P.S.: I'm gonna report it as a bug and see what Mathworks has to say about it.

[Star Strider]

Great! I'm glad it worked for you. It is always my pleasure to help.

Please post what TMW replies to your bug report here in a new ‘Answers’ question (use ‘announcement’ as the first tag and then the same tags and products you used here), and include a link to this thread so you don't have to completely re-describe the problem. This seems to me to be a significant issue — and one I hadn't previously discovered — so it merits special attention.

Answer 2

[Walter Roberson]

In the Symbolic Toolbox, the order of operands is treated algebraically, so any subexpression might get rewritten to an equivalent subexpression. For example, x*2*x would get rewritten to 2*x^2. When you replace dx*dy with dy*dx the order might get rewritten arbitrarily.

I do not know the order used by the Symbolic Toolbox. In another package I am familiar with, the order used is the order the operands were first encountered in (or so it used to be; the package added new controls around this in later releases.)