"i cant break the string into its fields and use something like app.(str1).(str2).str(3)..."
You cannot use dynamic fieldnames like that, but you can easily use getfield to do exactly what you want:
>> S.A.B.C = 123;
>> str = 'A.B.C';
>> spl = regexp(str,'\.','split');
>> val = getfield(S,spl{:})
val = 123
And of course setfield does the same for assigning a value. If you have control over the data design, it might be better to store the fieldnames separately anyway (i.e. like spl above).
See also:
