Hi @Northern -
In this case, I think I would use the Column Filter node prior to the Column List Loop Start node to determine which columns would have the loop operation performed on them. After the loop, you could then use the Joiner node to put everything back together in the same table. The pseudo-workflow might look something like this:
ColumnListLoopStartExample.knwf (16.4 KB)
This example workflow sets pulls out a particular string column, capitalizes it, and rejoins it to the original table. Note that there are a few changes from the defaults in the Loop End node to make this work properly.
This doesn’t involve use of any flow variables, but in this case I’m not sure you need them - that is, if I am understanding your problem correctly.