eval for string to variable

I have a GUI with a string as variable and a returned value for it.

var="simulation type"

returned GUI value:

DataEdit.Value = 1x1 cell array {'RUN 1'}

eval([var '=' char(DataEdit.Value) ';']) gives Error using eval, Argument must be a text scalar.

Any suggestions?

[Stephen23]

"Any suggestions?"

The fact that EVAL lets you obfuscate such buggy code should be a big hint that this should be avoided.

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

[Walter Roberson]

And when the XML file has in it

app = ''

then should your code delete your GUI, writing over the app variable that is holding the framework of your entire GUI ?

Answer 1

[Voss]

Since var is a string (as opposed to character vector), when you concatenate it with character vectors, those character vectors are converted to strings as well, and the result is a string array:

var="simulation type";
DataEdit.Value = {'RUN 1'};
[var '=' char(DataEdit.Value) ';']

ans = 1×4 string array
    "simulation type"    "="    "RUN 1"    ";"

However, if you convert var into a character vector, then the concatenation works:

[char(var) '=' char(DataEdit.Value) ';']

ans = 'simulation type=RUN 1;'

You could also use the + operator for string concatenation, to concatenate everything as a string:

var + '=' + char(DataEdit.Value) + ';'

ans = "simulation type=RUN 1;"

But in either case, you cannot eval the result because simulation type is not a valid variable name:

try
    eval([char(var) '=' char(DataEdit.Value) ';'])
catch ME
    disp(ME.message)
end

Undefined function 'simulation' for input arguments of type 'char'.

try
    eval(var + '=' + char(DataEdit.Value) + ';')
catch ME
    disp(ME.message)
end

Undefined function 'simulation' for input arguments of type 'char'.

So what is it you want to do with this?

[Jonathon Cheah]

Thanks for the quick solution. This GUI generates a XML file to have an entry -- Simulation_Type = "RUN 1", using your code, it works with "RUN 1" in quotations, with or without char():

eval(var + '= "' + DataEdit.Value + '";')

ME.message complains Invalid expression for with or witout char(), but Matlab does not crash as before.

Ugly, but I got the right answer in the XML file generated.

[Stephen23]

@Jonathon Cheah: why on earth do you need to EVAL some text to write some other text to an XML file?

https://xyproblem.info/

https://en.wikipedia.org/wiki/XY_problem

[Jonathon Cheah]

Hi Stephen,

I have a vey large multi layer deep xml config file for a simulator. I did a readstruct (*.xml) to start with to search the multi thread, multi layer decendents, calibration data by the GUI. The updated struct generates an identical xml file with desired values...simulator is finicky about the config.

Guess eval() is a nice hack function to have, to do this....quickly.

-jc

[Walter Roberson]

@Jonathon Cheah

What XML writing routine are you using that cares about the name of the variable holding the data to be written??

Have you considered using dynamic field names of a struct, and using writestruct to write the output XML file ? Perhaps having used readstruct to read the original in?