save does not recognize text scalar

The following code snippet fails unexpectedly:

clear;
myFilename = 'SavedData';
mustBeTextScalar(myFilename); % Throws exception if not true.
x = 3;
save(myFilename, x);

The variable myFilename passes the mustBeTextScalar as expected. However, the save command fails with the message

Error using save

Argument must be a text scalar.

Error in

saveTest (line 5)

save(myFilename, x);

^^^^^^^^^^^^^^^^^^^

QUESTION: Why does save not see myFilename as a text scalar, even though mustBeTextScalar passes it?

[MATLAB Version: 25.1.0.2943329 (R2025a)]

Answer 1

[Fangjun Jiang]

It is not about the variable name myFilename.

save(myFilename)

or

save(myFilename,'x')

[Richard]

Ah, so it's about the presentation of the name of the variable to be saved. That needs to be in quotes as well. Thank you!

[Walter Roberson]

Right.

save() permits non-constant expressions to be passed as the variable names. You can compute the names of the variables to save.

But more directly: the parsing of

save(myFileName,x)

has to follow the regular rules of MATLAB expressions. MATLAB interprets all function parameters before paying attention to the function being called. So myFileName is first looked up in context and the pointer to the text vector is prepared on the stack.Then, in this hypothetical context, the content of x would be looked up and the pointer to the numeric scalar would be prepared on the stack. Then save() would be called with those two items on the stack. save() would receive first the pointer to the text vector and then the pointer to the numeric scalar. But in doing so it would have lost the name of the numeric scalar and would only have its value.

This is why save() needs text vectors containing the names of variables to be saved, instead of needing the content of those variables.

[Stephen23]

"...But in doing so it would have lost the name of the numeric scalar and would only have its value. This is why save() needs text vectors containing the names of variables to be saved, instead of needing the content of those variables."

Yet TABLE manages to do this, so it turns out that functions do not "need" text inputs like this.

[Walter Roberson]

Well, table() uses inputname under the hood, and invents variable names for cases where expressions were passed. save() does not have the luxary of being able to invent variable names.

x = [1;2;3];
table(x, [4;5;6], +x)

ans = 3×3 table
    x    Var2    Var3
    _    ____    ____

    1     4       1  
    2     5       2  
    3     6       3  

... variable names are used where inputname() is able to figure them out, and otherwise variable names are invented for expressions.