MATLAB: Difference between [ handles.comp1 ]=axes and [ comp1=axes ]

axishandles

What is the difference between defining an axis using {handles.comp1=axes} and {comp1= axes}

I'm guessing in the first case the axes has its handle stored in the handles structure and in the second it is not stored into it?

Yes, that is the only difference. And it would be perfectly valid to use

comp1 = axes;
handles.comp1 = comp1;

Your code logic looks good. The issue, I think, is that you are indexing incorrectly into contents to assign a value to popupmenu1. Your code is

contents(get(hObject,'Value'))

but the code Guide provides in the callback hint is

contents{get(hObject,'Value')}

Note the use of curly braces {}

Use set command

Example

t=0:0.1:10;
y=sin(t);
plot(t,y);
set(gca,'xlim',[0 5])

With the new version of Matlab

h=gca
h.XLim=[0 5]