Creating dynamic number of Symbolic variables

Hi,

(I had some code which was running fine in Matlab 2014a but after upgrading to 2015a today, the same code is not working)

I want to create a symbolic variable t and it's value to contain further symbolic variables called as t1, t2, t3 and so on, wherein the number of these symbolic variable to be determined by a user provided value stored under "NumOfExpVar"

I wrote the below code and it works perfectly in Matlab 2014a and also in 2015a.

NumOfExpVar = 1; % This value will be set by user

t = sym(zeros(1, NumOfExpVar));

for NEVCnter=1:NumOfExpVar

      t(NEVCnter) = sym(sprintf('t%d', NEVCnter));

end

So far so good.

Now, after a long series of intermediate code (which has nothing to do with "t" symbolic variables) I have another code wherein for each of t1, t2 etc I want to create symbolic variables x1_1, x1_2, x1_3...(stored under t1), x2_1, x2_2, x2_3 etc (stored under t2) wherein the number after "_" is determined by "ExpRunCnter". The code I have is

        ExpRunCnter = 2; %This value is set dynamically depending on state of the program

  for NEVCnter = 1:NumOfExpVar

    t(NEVCnter) = sym(zeros(1, ExpRunCnter));

    for ERCCnter = 1:ExpRunCnter

      t(NEVCnter,ERCCnter) = sym(sprintf('x%d_%d', NEVCnter, ERCCnter));

    end

  end

Unfortunately the above does NOT work in 2015a (but did work in 2014a)

Please let know.

Thanks

Hari

PS: the error I get in 2015a is as follows (error pointing to the line "t(NEVCnter) = sym(zeros(1, ExpRunCnter));":

Error using subsasgn In an assignment A(:) = B, the number of elements in A and B must be the same.

Error in sym/privsubsasgn (line 997) L_tilde2 = builtin('subsasgn',L_tilde,struct('type','()','subs',{varargin}),R_tilde);

Error in sym/subsasgn (line 834) C = privsubsasgn(L,R,inds{:});

Answer 1

[Walter Roberson]

For several releases now you have been able to use sym() to generate the names directly.

http://www.mathworks.com/help/symbolic/sym.html

A = sym('a',[m,n]) creates an m-by-n symbolic matrix filled with automatically generated elements. The generated elements do not appear in the MATLAB workspace.

When you use this syntax to create a vector, it generates the elements by using the prefix a and attaching the numbers from 1 to m or n to it. For example, A = sym('a',[1,3]) creates a row vector A = [a1,a2,a3].

When you use this syntax to create a matrix, it generates the elements of the form ai_j, where i = 1:m and j = 1:n. For example, A = sym('a',[2 2]) generates the 2-by-2 symbolic matrix A = [a1_1, a1_2; a2_1, a2_2].

To specify another form for generated names of matrix elements, use combinations of '%d' and the prefix a. For example, A = sym('a_%d',[1 3]) generates a row vector A = [a_1, a_2, a_3], and AB = sym('a%db%d',[2 2]) generates the 2-by-2 symbolic matrix AB = [a1b1, a1b2; a2b1, a2b2].

[Hari]

Hello Walter,

Thanks for the response and letting me know about a simpler approach. It certainly helps me to build the matrix in a straight-forward manner.

Thanks Hari