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matlabsymbolic-math
Thomas
Thomas

Reputation: 67

MATLAB subs(): Strange behaviour

syms Gr Ra Ri A B Gr1 Gr2 c d
Gr =  2*c*(Ra^2 - Ri^2) - d*(Ra^4 - Ri^4)/2;
Gr1 = subs(Gr, [c*(Ra^2 - Ri^2), -d*(Ra^4 - Ri^4)/2], [A, B])
Gr2 = subs(Gr, [c*(Ra^2 - Ri^2),  d*(Ra^4 - Ri^4)/2], [A, B])

returns 

Gr1 =

2*A + B


Gr2 =

2*A - (d*(Ra^4 - Ri^4))/2

Is there a way to convince MATLAB to return Gr2 = 2*A - B in the second case without workarounds? I have a much more complex expression to substitute, but I can't work with this subs()-behaviour. Thanks.

Upvotes: 5

Views: 564

Answers (2)

Thomas
Thomas

Reputation: 67

The described behaviour was classified by the Matlab development team as unexpected and will hopefully be fixed in future releases:

http://www.mathworks.com/matlabcentral/answers/213727-subs-unexpected-simplification-behaviour#answer_177179

Upvotes: 0

brainkz
brainkz

Reputation: 1345

I think the problem is in the multiplication/division operations in the second argument of subs since the problem vanishes once I rearrange the code as follows: 

syms Gr Ra Ri A B Gr1 Gr2 c d
Gr =  2*c*(Ra^2 - Ri^2) - d*(Ra^4 - Ri^4)/2;
Gr1 = subs(Gr, [(Ra^2 - Ri^2),(Ra^4 - Ri^4)], [A/c,-2*B/d])
Gr2 = subs(Gr, [(Ra^2 - Ri^2),(Ra^4 - Ri^4)], [A/c,2*B/d])

and the output is:

Gr1 =
2*A + B

Gr2 =  
2*A - B

Upvotes: 1

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