calculate all combinations using FOR loop

A slightly shorter version than what Martin posted.  We don't need to loop through one vector if we vectorize.  Not better, just another way to go about it.

Two other ways - taking advantage of a DOE feature. fullfact provides the combinations of subscripts needed for every combination.  Personally I'd just use nested for's since it's easier to understand and more flexible.  But it's fun to explore new ways.  (Note: answer sequence changes from other posts)