Error using sym/cat>checkDimensions CAT arguments dimensions not consistent.

I have the following code (there is a space before the minus sign in the first equation):

eqns = [x -y==0;
        x+y==0]

This generates the following series of messages:

Error using sym/cat>checkDimensions CAT arguments dimensions not consistent.

Error in sym/cat>catMany (line 33) [resz, ranges] = checkDimensions(sz,dim);

Error in sym/cat (line 25) ySym = catMany(dim, args);

Error in sym/vertcat (line 19) ySym = cat(1,args{:});

If I delete the space before the minus sign it works fine:

eqns = [x-y==0;
        x+y==0]

Or a space behind the minus sign it also works fine:

eqns = [x - y==0;
        x+y==0]

Why is it not allowed to use a space before the minus sign?

Answer 1

[Bhanu Prakash]

Hi Jos,

As per my understanding, you are trying to initialize a matrix containing linear equations but are facing some errors, when you tried to initialize them differently.

In the first case, where there is a space before the minus sign, both "x" and "-y==0" are identified as separate elements by the compiler. So, the compiler tries to create a row matrix using those two elements. But the second row has only one term i.e., "x+y==0", making the dimensions of the matrix inconsistent. This is leading to the error saying, "CAT arguments dimensions not consistent".

Please have a look at the code below, for your reference:

eqns=[x -y==0]

The output of the above code is mentioned below, which indicates a row matrix:

eqns =

[x, -y == 0]

In all other cases, the compiler considers "x-y==0" as a single element, thereby creating a column matrix having the elements "x-y==0" and "x+y==0".

You can access the documentaion for “Matrices and arrays” here:

Matrices and Arrays - MATLAB & Simulink - MathWorks India

Hope this answer helps you.

Thanks,

Bhanu Prakash.

[Jos van Weert]

Ah, it is totally clear to me. Thank you!