chlor - if, in your OpeningFcn you define local variables as you have done as
function next_previous_in_plotty_OpeningFcn(hObject, eventdata, handles, varargin)
handles.output = hObject;
guidata(hObject, handles);
filelist = {'1.xls', '2.xls', '3.xls', '4.xls'}
i = 1;
k = length(filelist);
then filelist, i, and k will only be visible within this function and be available for the duration of this function call (since not persistent and this function is only ever called once). If you want this data to be available to other callbacks, then you need to save it to the handles structure as
function next_previous_in_plotty_OpeningFcn(hObject, eventdata, handles, varargin)
handles.output = hObject;
handles.filelist = {'1.xls', '2.xls', '3.xls', '4.xls'}
handles.i = 1;
handles.k = length(file list);
guidata(hObject, handles);
Order is important in the above - we create new fields in the handles struct and then save it using guidata. (Note that you may want to consider renaming the i and k variables to something that gives an idea as to what they represent.)
And you should be able to declare your function wherever you want in the m file. Typically, I do this before I use (or call) the function.
