Non-constant expression or empty matrix. This expression must be constant because its value determines the size or class of some expression.

Hello,

I'm trying to use the MATLAB coder and getting the above error when loading a mat file.

My code:

data = struct('Value',zeros(500,1));  %% No change with or without this line
data = load(Folder+"\Value.mat");   %% Error on this line
All_data(:,1) = data.Value;

Thank you

[Matt J]

What is the result if yolu just evaluate the argument to load()?

Folder+"\Value.mat"

[Tal Levy]

load(Folder+"\Value.mat")

Returns a vector (named "Value").

But the coder can't handle load without placing it in a variable

Answer 1

[Abhijeet]

Hi,

The error message you received suggests that the expression you are using to load data is not constant or is an empty matrix, which makes it impossible to determine the size or class of the expression.

Here are some steps you can take to resolve this error:

1. Check that the path to the folder containing the file you want to load is correct. Make sure that the folder path is enclosed in quotes if it contains spaces.
2. Check that the file you are trying to load exists in the specified folder.
3. Make sure that the file you are trying to load is in the correct format for the function you are using to load it. For this case, if you are using the load() function, the file must be a .mat file.
4. Check that the variable "Folder" has been defined and is pointing to the correct folder.
5. Make sure that the expression you are using to load data is constant and not an empty matrix.

Hope these may resolve your errors.

[cui,xingxing]

@Abhijeet,@Ryan Livingston

Hi, I'm getting a similar error message but I've tried many things and still can't solve it, any better suggestions? Thank you. my problem is following link.

https://ww2.mathworks.cn/matlabcentral/answers/1948868-non-constant-expression-or-empty-matrix-this-expression-must-be-constant-because-its-value-determin

[Ryan Livingston]

I added a comment there. Your example triggers this same message for a different reason. Walter's answer summarizes the limitation and I added a debugging/sanity checking step you can use when generating struct field names.

Answer 2

[Ryan Livingston]

For code generation, the file name input to load must be a constant:

https://www.mathworks.com/help/matlab/ref/load.html#refsect-extended-capabilities

So here Coder is trying to tell you that Folder+"\Value.mat"is not constant and that it can't deal with that. This error is confusing, so I've made an internal note for our team to improve the message in the future.