JC,
If you have an internal null data set at program initialisation, then you need to input all the data values before you can do a meaningful analysis, but if you launch your program with a data set that is internally complete, then you can (subject to what you do with the data values) update them one at a time.
In a Clearwin+ / Windows context, this is the difference between (say) ‘File | New’ and ‘File | New from template’.
In the former, you might present the user with a set of input boxes asking for ‘Number of elements’, ‘Number of nodes’, ‘Number of supports’, ‘Number of loads’ etc, then get the coordinates and connections – in other words, step through your traditional text file input. You will find doing the file in Excel far more effective than that. You don’t need NAMELIST.
The alternative approach (which I use) is to go for the ‘New from template’ approach. In a finite element context, it might start with a single element, with 2 support nodes and the third with a load on it, already supplied with a constitutive law and material properties. You could analyse that, and get an answer, although not very useful, I admit. Then, your user can add to it, maybe stretching and distorting that first element, then adding lines of elements (or areas that the program infills with a mesh), at all times keeping a dataset internally that is fully valid.
If there is only a small set of changes that the user can make, they are selected with appropriate mouse clicks, possibly with SHIFT, CTRL or ALT key modifiers. If there is a bigger set of editing options, the user must select each from a palette of options – in my case, one that pops up with a double click – but which can be on a toolbar. If you save such a dataset, there is no need to use NAMELIST, but you can, of course, because as the programmer you do know what the variable names are. However, it does make ‘File | Open’ and ‘File | Save’ (or ‘Save As’) callbacks far more complicated. I use formatted output, and unformatted input, when saving or opening such files. The big advantage of the ‘New from template’ approach is that you never need to test your dataset for completeness.
My main interest is not FE analysis any more (it has been in the past) but I still dabble. I’m working (intermittently) on a system of creating closed polygons that the program fills with elements and nodes via a mesh generator.
For me, NAMELIST is, and always has been, a facility that I can see might have attractions to someone, although that someone is not me.
Eddie
PS. You can always fill your initial internal dataset with values that scream 'I am not defined'.