"I'm trying to dynamically create variable names, for array and struc variables"
But your example shows you changing a structure fieldname, not a variable name. So your description does not match your example. It is not clear what you mean by "structure variables" as all structures are arrays, just like every other fundamental data type in MATLAB:
In any case, your EVAL-approach is slow, very ill-advised, counter-productive, and best avoided: https://en.wikipedia.org/wiki/Anti-pattern
The neat, simple, and very efficient MATLAB approach for the example you show is by using a dynamic fieldname:
For example:
F = sprintf('sampleNo%d',n);
varToPlot = a(i,j).(F)
Numbered fieldnames indicates poor data design which would be more efficiently accessed using indexing.
