MATLAB: Does the SUBS command in Symbolic Math Toolbox 5.1 (R2008b) return an error when used to substitute variable ‘D’

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When I execute the following:

subs('A+D','D',3)

I receive the error, ERROR: ??? Error using ==> mupadfeval at 28 Error: Illegal operand [subs]; during evaluation of 'mlsubs' Error in ==> sym.subs at 219 G = sym(mupadfeval('mlsubs', charcmd(G), [x '=' charcmd(y)])); Error in ==> sym.subs at 237 G = subs(F,Y,X,0); Error in ==> subs at 60 r = subs(sym(f),varargin{:});

This only seems to occur when the second argument of the SUBS function is the string representation of variable 'D'. This command worked perfectly in the previous version of the Symbolic Math Toolbox (R2008a)

Best Answer

The symbol 'D' is a reserved keyword in the MuPAD engine and is used to specify the differentiation operator. This and other such keywords cannot be used in string representations of expressions in the Symbolic Math Toolbox 5.1 (R2008b). More information about this can be found at the following link:

https://www.mathworks.com/help/symbolic/reserved-variable-and-function-names.html

You can work around this by using a symbolic variable instead. For example,

syms A D
subs(A + D, D, 3)

Related Solutions

MATLAB: Is SIMPLIFY not able to simplify a straightforward expression in Symbolic Math Toolbox 5.3 (R2009b)

This is the expected behavior of the SIMPLIFY function in the Symbolic Math Toolbox 5.3 (R2009b). By default, the function tries to make as few assumptions about the symbolic variables as possible. Considering the above example, the expression 'log(exp(x))' cannot always be simplified to the expression 'x'. In particular, consider if x = 2*pi*i (where i is the imaginary unit). Then log(exp(x)) = 0, which is clearly not equal to x.

In order to do simplifications with looser assumptions about the symbolic variables, call the MuPAD SIMPLIFY function in the MuPAD notebook interface or in MATLAB through the FEVAL or EVALIN commands. Using the MuPAD command directly allows the inclusion of the 'IgnoreAnalyticConstraints' option which controls the level of mathematical rigor that the solver uses on the analytical constraints on the solution.

For example:

syms x
feval(symengine,'simplify',log(exp(x)),'IgnoreAnalyticConstraints')

For more information on the MuPAD SIMPLIFY command, the 'IgnoreAnalyticConstraints' option, and other options which can be used with the MuPAD SIMPLIFY command you may refer to:

http://www.mathworks.com/help/releases/R2009b/toolbox/mupad/stdlib/simplify.html

http://www.mathworks.com/help/releases/R2009b/toolbox/mupad/stdlib/newSimplify.html

MATLAB: How to programmatically replace a referenced model with a Subsystem in Simulink

Assuming that you have a 'parent.slx' model which references a model 'child.slx' (whose block name is 'Model' in the parent Simulink model), use either of the following options:

>> open_system('parent');

>> add_block('built-in/Subsystem',['parent/mySub']);

>> load_system('child');

>> Simulink.BlockDiagram.copyContentsToSubsystem('child',['parent/mySub']);

>> pos = get_param('parent/Model','position');

>> delete_block('parent/Model');

>> set_param('parent/mySub','position',pos);

>> open_system('parent');

>> load_system('child');

>> convert2subsys('child');

>> add_block('child/Subsystem','parent/mySub');

>> pos = get_param('parent/Model','position');

>> delete_block('parent/Model');

>> set_param('parent/mySub','position',pos);

>> close_system('child',0);

where 'convert2subsys' is defined at the following link:

https://uk.mathworks.com/help/simulink/slref/simulink.blockdiagram.createsubsystem.html#br5mgzk-6

Please refer to the attachment for further details.

Best Answer

Related Solutions

MATLAB: Is SIMPLIFY not able to simplify a straightforward expression in Symbolic Math Toolbox 5.3 (R2009b)

MATLAB: How to programmatically replace a referenced model with a Subsystem in Simulink

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