Use subexpr function in matlab

It is easy via MATLAB, see for example https://au.mathworks.com/help/matlab/import_export/analyze-big-data-in-matlab-using-tall-arrays.html

Dear Muhammad Ali ,

I am not talking about big data. I'm talking about such a parameter obtained by Matlab symbolic equations. You can see in the attached figure that a variable called A23 is equivalent to the text file. You can see how huge the parameter is by looking at the points I pointed out in the figure.

Hi,

in MAPLE, it is possible to transform an analytical expression in FORTRAN using the following lines:

with(CodeGeneration):

Fortran(expression,optimize);

the "optimize" argument make common sub-expressions appear.

In MATLAB, you can use the subexpr command as follows:

example:

syms a b c d x

solutions=solve(a*x^3+b*x^2+c*x+d==0,x,'MaxDegree',3);

[r,sigma]=subexpr(solutions)

Hi Richard Epenoy ,

Thanks. That was a good answer to my problem. It reduced 1/3 of my result. In the subexpr command, we can only use one sub-expression. Is there a way to use more than one to shorten the expression even more?

Hi, I think subexpr reduces automatically the complexity of the expression. An alternative consits in using the "simplify" function. Best.

Thanks Richard Epenoy . Simplify doesn't help in this case. It can't handle the computation.

Hi, MAPLE is more powerful than MATLAB for these manipulations. Did you try MAPLE?

Yes, I also used Maple before. The result was huge again. But I couldn't compare it to the one I obtained by Matlab because I don't know how to import the result to Matlab.

Is there any way to send the obtained Maple result to Matlab?

Use the "vpa" function to compute the numerical value of the symbolic expression https://in.mathworks.com/help/symbolic/vpa.html

Thanks Ranjan Parekh

But there is no specific numeric value to use vpa command. Most of the result are symbolic values.

When values are assigned to variables q1 and q2 the symbolic expression will evaluate to a numerical value. Otherwise there is nothing to calculate, as per your question you have asked.

Yes Ranjan Parekh

But I think my problem is related to the phase before assigning values to the variables. As you can see in the figure I uploaded, the symbolic expression is huge. This is before assigning values to the q1,...q4 variables.

So you want to shorten the expression. Usually this is done by "simplify" and sometimes "eval". The page below might be of further help.

https://in.mathworks.com/help/symbolic/choose-function-to-rearrange-expression.html

Thanks Ranjan Parekh

Simplify wasn't useful. After 16 hours of running, it gave an error of memory.

Use subexpr function in matlab

Read this example:

https://www.mathworks.com/help/symbolic/subexpr.html

Best regards