HELP eval - Undefined function 'eval' for input arguments of type 'cell'. - find structure (from workspace) with part of name and accessing it

Hello, I am currently writing a code to analyse blood pressure. The blood pressure has been extracted using another language, the code renamed my channels a bit weirdly. I just need to modify my matlab code to make it flexible to all those weird names .. and I am a bit stuck.

My matlab code is looped on my participants.

For each participant, I load a .mat file (easy).

As I said the channel are weirdly renamed :

name of channel1:     XYPlot *1_01_1*__Ch1 
name of channel2:     XYPlot *1_01_1*__Ch2 
name of channel3:     XYPlot *1_01_1*__Ch3

The bold part is always different between subjects.

**MY GOAL

To define:

time_Spike= channel2.xvalues;
SBP_peaks_values = channel2.yvalues;

**MY SCRIPT

namesWorkspace = who;
outStr = regexpi(namesWorkspace, 'Ch2');
ind = ~cellfun('isempty',outStr);
var = namesWorkspace(ind);
str={var};
out=str{1};   
channel2=eval(out);
time_Spike= channel2.xvalues;
SBP_peaks_values = channel2.yvalues;

**OUTCOME

But I have a problem with eval

"Undefined function 'eval' for input arguments of type 'cell'."

I am sure there is another easier way to do what I want to do,

Your help and feedback would be very appreciated :)

thanks

Sophie

[Image Analyst]

Sophie, you forgot to attach the .mat file so I can't do anything (yet).

"out" appears to be the contents of a cell. We don't know what data type it is - it could be anything. But there is no reason to send in into eval(). Why did you do that???

Answer 1

[Jan]

The topic of the dynamic creation of variables is discussed very frequently. The answer is always the same: Don't do this. If causes more problems than it solves. See this exhaustive explanation: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval .

The actual problem here seems to be:

var = namesWorkspace(ind);
str={var};
out=str{1};
channel2 = eval(out);

It would work with:

variable = namesWorkspace{ind};    % "var" is the varianze command
channel2 = eval(variable);

But this is still a shot in your knee. Much better catch the output of load:

Data     = load(FileName);
Fields   = fieldnames(Data);
ch2Str   = regexpi(namesWorkspace, 'Ch2');
ind      = ~cellfun('isempty',outStr);
channel2 = Data.(Fields{ind});

Now your workspace is not polluted by variables from the MAT file and no eval confuses the JIT acceleration. You know while reading the source code where the variables are coming from and this supports an efficient debugging.

In short: Don't EVAL!

[Sophie]

Hi, thanks for your explanation, your trick with eval is working well. thanks.

I tried your last suggestion but "Index exceeds matrix dimensions."

Answer 2

[Steven Lord]

DON'T use eval. load your data into a struct array and use fieldnames and dynamic field names to access the data in the struct array.

[Sophie]

thanks