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fulldiff.m

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fulldiff.m

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17 Mar 2005 (Updated )

Symbolically computes full derivative wrt time using chain rule and time variables.

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Description

Symbolically computes full time derivative of given function and time dependant variables. It's an expansion of the partial derivative functionality of DIFF. Chain rule application automatically creates necessary higher order derivatives. Only base dependent variable need to be input, eg {x y}, dx and dy, etc. are assumed time dependant and d2x and d2y are created if required. This is motivated by the combination of partial derivative and full derivative required to compute Lagrange?s equations of motion.

Acknowledgements

This file inspired Dynamics Simulator For Kinematic Chains and Ddt.

Required Products Symbolic Math Toolbox
MATLAB release MATLAB 8.0 (R2012b)
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Comments and Ratings (9)
01 May 2014 Henrique Leandro Silveira

Tim Jorris, I found a bug? See my code and results:

clear all;
syms e1 e2 e10 e20
x=[e1;e10;e2;e20];
y=fulldiff(x,{e1,e10,e2,e20})
z=fulldiff(x,{e1,e10,e2,e20},2)

y =

de1
0
de2
0


z =

d2e1
0
d2e2
0

15 Nov 2012 Sinan  
08 Apr 2012 Amr

I am having a problem running this code on Matlab 2010a.
I even tried the example in the mfile
clc
clear all
close all
syms x y dx d2y

f = x*y*(dx^2)*(d2y)
fulldiff(f) % (assumes x only) produces
% 3
% y dx d2y + 2 x y dx d2y d2x
% whereas
fulldiff(f,{x, y}) % produces
% 3 2 2
% y dx d2y + 2 x y dx d2y d2x + x dx d2y dy + x y dx d3y
But I keep getting the following error:
??? Error using ==> strfind
Inputs must be character arrays.

Error in ==> fulldiff>findhigher at 153
cid=strfind(allvar,',');

Error in ==> fulldiff at 120
dvars=findhigher(fun,vars,dvars);

Please advise

22 Aug 2011 Bassam Jalgha

Really useful! I just used your code to develop a dynamics simulator for kinematic chains.

20 Jun 2008 Scott Boynton

Very nicely done and effective. I'm also working with the Lagrange equations and it saved me a lot of time. Thank you.

01 Feb 2007 Peter Pott

Hi Tim,

this piece of code seems to be the answer to all my (matlab) problems! But it produces an error message:
??? Undefined function or variable 'true'.
Error in ==> C:\path\fulldiff.m
On line 87 ==> while true
Is it a matlab-release-numer incompatibility or my fault?

Best regards

Peter
(pppott_at_web.de)

29 Nov 2006 Francesco Igino Cosco

thank to your code I was able to do my work without translate my code in Mathematica.

20 Nov 2006 Daichi Nozaki  
03 Feb 2006 Bruno Guerreiro

This function is extremly useful!

However I think I found a bug. Try this sequence and observe that the two last lines should give the same result:

syms m g R a real; y = 1/m*R*a + g; fulldiff(y,{R,a},2)
syms m g R a real; y = 1/m*R*a + g; fulldiff(y,{R,a},3)
syms m g R dR d2R a da d2a real; d2y = 2/m*da*dR+1/m*a*d2R+1/m*R*d2a; fulldiff(d2y,{R,a})

According to my calculus, only the last result is correct. I conclude that there's a bug with the multiple derivatives.

Matlab should think of including this kind of calculus into the symbolic toolbox, as well as "undefined" matrix and vector symbolic calculus.

Updates
28 Mar 2005

Improved to properly captures higher (>2) order derivatives

30 Jun 2014

Fixes the bug pointed out by Henrique.

26 Sep 2014

Bug fixed. Code now looks for an exact match for variable names. Thank you Henrique.

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