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

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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
  2 comentarios
Matt J
Matt J el 26 de Mzo. de 2023
Editada: Matt J el 26 de Mzo. de 2023
What is the result if yolu just evaluate the argument to load()?
Folder+"\Value.mat"
Tal Levy
Tal Levy el 27 de Mzo. de 2023
load(Folder+"\Value.mat")
Returns a vector (named "Value").
But the coder can't handle load without placing it in a variable

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Respuestas (2)

Abhijeet
Abhijeet el 5 de Abr. de 2023
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.
  2 comentarios
Ryan Livingston
Ryan Livingston el 18 de Abr. de 2023
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.

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Ryan Livingston
Ryan Livingston el 5 de Abr. de 2023
Editada: Ryan Livingston el 5 de Abr. de 2023
For code generation, the file name input to load must be a constant:
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.

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