how to evaluate a string named matrix

I have a name changing matrix

x = sprintf('a_%s_y(1:2)', '2g');

y = sprintf('b_%s_y(1:2)', '2g');

They generate x='a_2g_y(1:2)' and 'b_2g_y(1:2)'.

First how can I detect a_2g_y(1:2) exist or not to avoid undefined variable error?

Second, how can I plot(a_2g_y(1:2), b_2g_y(1:2)) if the matrix exist

[Adam Danz]

"First how can I detect a_2g_y(1:2) exist or not to avoid undefined variable error"

It sounds like you might be creating dynamic variable names which causes more problems than it solves.

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

If so, instead, store your values in a cell array or structure.

Answer 1

[Walter Roberson]

xvar = sprintf('a_%s_y', '2g');
xcol = 1:2;
yvar = sprintf('b_%s_y', '2g');
ycol = 1:2;

if exist(xvar, 'var') && exist(yvar, 'var')
    xval = eval(xvar); yval = eval(yvar);
    if numel(xvar) >= max(xcol) & numel(yvar) >= max(ycol) 
      plot(xval(xcol), yval(ycol));
    else
        fprintf('variables both exist but not large enough\n')
    end
else
    fprintf('variables do not both exist\n')
end

This code is not recommended.

Do not even give the user an opportunity to specify a variable that does not exist: create a listbox containing only valid variable names, and use it to index into a cell array of the variable values. Or if the variables are being loaded from a .mat file,

variable_struct = load('appropriate .mat file name');
variable_names = fieldnames(variable_struct);

Now give them a listbox from the variable names, and use the selected variable name with dynamic field names:

variable1_index = get(handles.FirstVariable, 'value');
variable2_index = get(handles.SecondVariable, 'value');
first_variable = variable_struct.(variable_names{variable1_index});
second_variable = variable_struct.(variable_names{variable2_index});

[vincent lin]

Thank you so much. It works!